P11

84 Home Power #82 • April / May 2001

uch of my early adult life wasspent homesteading in the

Alaskan bush. Winters are thepredominant force there, and like mostothers in a northern or temperate zoneclimate, my main concern was keepingmy living space warm. My locationcolored my entire world view. It neveroccurred to me that in some places inthe world, the problem was to stay cool.Most of the residents of the United States have similarproblems of perception. Because of this necessaryemphasis on heating, there is not a lot of informationavailable on alternative methods of cooling. In 1980, Iwas fortunate enough to “retire” to the Central Americancountry of Belize where I routinely encounteredtemperatures in the eighties and nineties, and humidityin the upper 30 percent of its range. 95°F (35°C) at 95percent humidity will quickly draw your attention to theneed to cool down.

In the United States and most other industrial nations,cooling is dealt with by refrigeration. Air conditioners arepredominantly powered by electricity, which is usuallyproduced by burning fossil fuels. Affluence allows us tocondition our living space using an expensive fuel ofconvenience. Most “third world” nations only allow thisluxury to the very well-off. Where grid power is availablein Belize, it costs 25 cents per kilowatt-hour. This is fartoo expensive for the average person to use for coolingon a regular basis.

Cooling for the Humid TropicsOver the years, I’ve studied the problem of low energyinput cooling in the tropics worldwide. There are twovery different environments that demand solutions tothe cooling problem. Hot, arid landscapes may requirecooling as much as hot, humid areas, but the principlesused to address the two problems are quite specific.

In this series of articles, I will try to pass on what I havelearned about using sun, wind, and the basic principlesof heat transfer to create a comfortable l ivingenvironment. I am specifically targeting the humidtropics, but many of the principles I will discuss arerelevant to arid areas as well. I will emphasize passivetechniques here—things that can be done without usingany technically derived energy to move heat, ortechniques using devices to control heat flowautomatically.

This will be a multi-part article. In the first part, I coverthe basic principles of heat transference, and try toexplain how they interact and what type of effects theyproduce. Later, I wil l discuss materials andenvironmental factors. Also, I will specifically apply thebasic principles to building design and construction.

Heat FundamentalsHeat is the motion of molecules in a substance. Thehotter the temperature, the more energetic the motionbecomes. There is no such thing as “cold”—there isonly more or less heat. Cold is our own subjectivereaction to a condition of too little heat for the body tobe in its comfort zone.

This is an important concept because there is no oneperfect temperature at which we are all comfortable.The human comfort zone depends on several factors,

Cliff Mossberg©2001 Cliff Mossberg

Though the climate of Belize is hot and humid, residents can use various passive techniques to create a cooler, more comfortable living environment.

Part I — Basic Principles

85Home Power #82 • April / May 2001

Cooling

not least of which is the human acclimatization to thespecific environment we live in.

While temperature is proportional to the energy ofvibration in molecules of a substance, heat quantity is ameasure of the numbers of these molecules and thetemperature at which they are vibrating. A large pan ofboiling water has more heat in it than a small one does,even though they are at the same temperature.

As matter heats up, the molecules move farther apart—they expand. Thus for the same volume of matter, thereare fewer molecules if the material is hotter. This meansthat the same volume of our hypothetical materialweighs less per unit volume when it is hot and morewhen it is cold and dense. This is true of solids, liquids,and gasses that are unconfined.

Three Modes of Heat TransferThere are three ways that heat can be transferredbetween a source and a receiver body. They areradiation, conduction, and convection.

They all accomplish the task of imparting heat energy toa receiver body, and they do so in proportion to thedifference in temperature between the sending sourceand the receiving body (called “delta t” and written“∆t”—∆ means “the change in”). The higher thedifference in temperature between a heat source and aheat receiver, the faster heat will flow into the receiverand the faster its temperature will rise.

RadiationWhen we talk about the electromagnetic spectrum, allwe’re talking about is “radio” waves—waves ofmagnetic energy that can propagate through a vacuumin space, thus transferring energy from the sun, stars,and galaxies to our earth. We are familiar with AM radioand the higher frequencies of FM radio and TV, but theradio spectrum contains many other waves of muchhigher frequencies. Visible light is a series of radiowaves that our bodies can detect directly.

Other frequencies such as infrared (lower in frequencythan visible light), ultraviolet (above the frequency ofvisible light), and x-rays (very, very high frequency) are

undetectable by thehuman eye. Yet thesefrequencies transferenergy as surely as thevisible light frequencies,and we are affecteddirectly by them. Infraredradiation from the sunproduces the feeling ofheat on our skin when thesun’s rays hit us.

Ultraviolet radiation causes sunburn, and x-rays can killor mutilate our body’s cells.

Infrared radiation is the vehicle of heat transference thatis most important to life on earth. It is heat radiationtransmitted directly to the earth by the sun. It is one ofthe principles that allows a woodstove or a bonfire toradiate heat that warms at a distance.

Visible wavelengths can be converted to infraredradiation when they fall on an absorptive surface, suchas a roof or a photovoltaic panel. The energy in theselight waves is absorbed by the surface, causingheating. This heating in turn causes re-radiation fromthe absorber as heat, or infrared light. This is thereason hot water collector panels are self limiting intheir efficiency. The collector panel heats up the wateruntil the water re-radiates as much energy back to thesky as it takes in. At this point, there is no further gain incollection of radiant energy possible.

A roof heats up in the sun’s rays until it re-radiatesinfrared heat energy down into the house as well as out

Window

Radiant Heat:Energy in wavelengths that

travel through through glass,air, and even a vacuum

Sunlight

Becomes heat whenenergy excites

molecules of objectin its path

Methods of Heat TransmissionTransmission Transmission Direction of Heat

Method Mechanism Medium MovementRadiation Electromagnetic Vacuum or transparent Any direction, line

radiant energy medium of sight from sourceConduction Molecule to molecule Any substantial Any direction into

mechanical transference material in contact material in contactConvection Physical relocation of Usually movement Usually upward,

a heated substance of a heated fluid unless forced

Heat Transmission through Radiation


Cooling

into the air. If the ceiling has no barrier to radiantenergy, this radiation will heat up the ceiling surface,which in turn will re-radiate the heat directly into theliving area of the structure. Radiant energy is theprinciple vehicle for moving heat in a downwarddirection into a structure.

Effects of Solar IncidenceThere are several factors that affect the ability of asurface to absorb or radiate infrared energy, and one ofthe most important is the angle at which the radiationhits the absorbing surface, known as the angle ofincidence. If you want to absorb energy at themaximum efficiency, radiation should fall on a collectionsurface that is exactly perpendicular to that radiation.

The diagram above shows a variety of panel angles inrelation to the sun's rays. When the panel isperpendicular to the sun's rays, the most energy isintercepted. When the panel is set at 45 degrees to thesun's rays, only about 70 percent of the availableenergy is captured.

Absorption & ReflectanceAnother factor that affects the amount of radiationconverted to thermal energy on a hypothetical earth“panel” is the color and texture of the surface. This is sofundamental to our experience that the concept isunderstood intuitively. Dark surfaces absorb heat andenergy, while light surfaces reflect them. Roughsurfaces absorb energy, while smooth surfaces reflectit. What is not so intuitive is that colors and textures thatabsorb energy well, also radiate energy well.

Reflective metallic foils take advantage of this. They areactually conductors, but when specifically engineeredinto buildings to control radiant energy, they are asmuch as 95 percent effective at blocking radiant energyabsorption. They are also very resistant to re-radiatingabsorbed energy.

To be this effective, a radiant barrier must be installedwith an air space on one or both sides of the material.Its mirror surface will then reflect any infrared energyrather than absorbing it and conducting it as heat.

ConductionConduction is the most intuitively understood mode ofheat flow. For conduction to occur, materials must be incontact with each other. For example, imagine a copperbar one foot long, two inches wide, and half an inchthick (30 x 5 x 1.3 cm)—a rather substantial piece ofcopper. If we support this bar, and place a candle or aBunsen burner under one end, the bar will slowly heatup from one end to the other. Soon the whole bar willbe too hot to touch. Heat is being transmitted byconduction throughout the bar.

0° (parallel) tosun’s rays:

No raysintercepted

45° to sun’s rays:71% of raysintercepted

15° to sun’s rays:26% of rays intercepted



30° to sun’s rays:50% of rays intercepted

90°

45°

Panel width

71% ofpanel width

Sun

90° (perpendicular) tosun’s rays:

100% of rays intercepted

Solar Incidence at Various Angles Absorbtance Characteristicsfor Common Building Materials

Asphalt Shingles

Surface Solar Absorptance

Dark 95%White 75%

Rough Wood

Dark 95%White 60%

Smooth Wood

Dark 90%White 50%

Glazed or Enameled Surfaces

Dark 87%White 37%

Stucco

Dark 90%White 50%

Unpainted Brick

Dark 85%White 65%

Concrete BlockDark 95%Unpainted 77%White 55%


Cooling

What is happening here is that the heat source isexciting the molecules in the copper to vibrate moreenthusiastically, becoming more and more energetic asthe temperature increases. As these copper moleculespick up physical motion from the heat energy, theycontinuously “bump” into the molecules next door.

This physical disturbance imparts energy to theadjacent molecules, causing them to increase theirvibrational energy—they warm up. Heating progressesdown the bar, away from the heat source, until thewhole bar has reached a state of equilibrium based onthe amount of heat supplied by the source.

Conductive Heat FlowRadiant energy is one of the loss factors that drawsheat from the bar. Another factor that allows the bar tolose heat is conduction to the medium surrounding it.This is a loss by physical contact with the fluid—air—surrounding the bar.

Different materials will move heat at different rates.Based on these rates, materials are classified as“insulators” if they retard the flow of heat, or“conductors” if they facilitate the movement of heat.These are far from absolute definitions. Most insulatorsare designed to retard heat flow in conduction, but there

are some exceptions such as metallic foil radiantbarriers.

Air can be either an insulator or a conductor. Forexample, air is used as an insulator to slow down thetransmission of heat in homes. It is the “dead” air spacein fiberglass batt insulation that does the work. But air isalso a cheap and relatively effective conductor of heatin electric motors, vehicle cooling systems, and manyother applications. So while it is important tounderstand how material properties affect heat flow,you should also realize that these properties can beapplied in many ways to achieve an engineering goal.

Boundary LayerIn conduction, heat flows through a substance becauseof tangible physical interaction between molecules.These same forces allow heat to flow between anysubstances that are in contact with one another. Theboundary where one substance stops and anotherbegins (between the copper bar and the air, forexample) is known as the interface. Heat flow across aninterface can be complicated by factors that are notobvious. The first of these factors is the variable rate ofconduction by different materials. The second factor isthe mobility that a fluid has, which results in convectiveflow.

Conductive heat flow is impeded when a fluid such asair is in contact with a heated surface such as a wall.This impediment is caused when a layer of stagnant airis changed in temperature and density by heat movingacross the interface. The air in the layer next to the wall

Conductive Heat:Excited (hotter) molecules heat the molecules in contact with them

Heat Source

Warmer Air

Bou

ndar

y La

yer

Bou

ndar

y La

yer

Colder AirWall

th(indoorambient

temperature)

tc(outdoorambient

temperature)

tw(wall

temp.)

∆t = th - tcTem

pera

ture

Col

der

War

mer

Thermal Conduction

Conduction through a Boundary Layer

Materials and their ConductivityMaterial Conductivity (Conductance)*

Copper 220.000Aluminum 122.000Steel 25.000Concrete 0.600Water 0.350Brick, red 0.270Rubber, soft 0.100Wood, pine 0.070Corkboard 0.025Rock wool 0.023Air 0.014Vacuum 0.000*BTU per hour per sq. ft. per degree per foot thickness


Cooling

will heat up more that the air some distance away.When this situation exists, the change in temperature(∆t) between the warm wall and the warm layer of air isreduced. This cuts back on heat flow.

The existence of the boundary layer and its removal isthe essence of “wind chill.” This is when it feels colderthan the real ambient temperature because of the extraheat loss when the wind blows away the boundary layeraround our bodies. This is undesirable when we aretrying to keep warm, but very desirable when we aretrying to cool down.

The conductivity of any material can be measured andquantified so that the relative qualities that make it aninsulator or conductor can be examined in absoluteterms. The conductivity table lists some materials andtheir conductivity. Even without knowing how to use the“soup” of units with which these materials are labeled, itis obvious that copper has a very high conductancevalue (220), while air is very low (0.014).

ConvectionIn its most generic form, convection involves themovement of heat by transporting some hot substance.Convective heat movement is usually associated withthe movement of fluids. There are two common formsof convection—”forced” and “free.” In forced convection,power is used to move a heated fluid from the source ofheat to the heat destination. Vehicle radiator typecooling systems and hot water or hot air home heatingsystems are common examples of this.

Since we are interested in heat flow that occurs withoutany energy input from us, we will be concentrating onfree convection to move our heat. Free unpoweredconvection happens due to the difference in density ormolecular concentration per unit volume that occurswhen a fluid is heated.

Molecular Density & WeightThe same volume of material weighs less per unitvolume when it is hot and more when it is cold. Thus a“cold” (less hot) fluid packs more matter into the samevolume than the same amount and type of fluid when itis heated.

The practical result of this change in density is that ahot fluid, being lighter, will “float” on a colder fluid.Conversely, a cold fluid will move downward under thepull of gravity until it finds the lowest level possible.These are dynamic processes. The fluid actuallyphysically flows from one position to the next as itsthermal status changes. Such flow results in themovement of heat.

If you put your hand over a heated stove burner, youcan feel air rising off the burner. A hot air balloon

depends on the change in density between the hot airinside the balloon and the cooler air outside it to riseinto the sky. On a warm summer day, the lake you swimin will have a warm layer at the top and cooler waterunderneath. These are all examples of fluid movementcaused by a change in density that causes convectiveheat to rise.

Convection is the movement of the heat rather than themovement of the fluid. But the two are inexorablyintertwined, so much so that we also call the fluidmovement convective flow.

Stratification & the Greenhouse EffectHot air flows up; cold air flows down. This causesseveral familiar effects such as stratification. The warmwater on the lake surface in the example above is acase of stratification. Water in the lake is heated bysunlight and rises to the top level, where it cannot go upany farther. Here it forms a layer. It gives off some ofthe sun-induced heat to the air above it, becomes moredense, and eventually sinks again.

Depending on the amount of solar energy available, thisconvection loop will stabilize so that approximately thesame amount of water is constantly heated, rises, givesoff its heat, and sinks back into the cold depths. Thussolar heat is moved from the lake to the air.

The conversion of visible light energy into re-radiatedradiant energy contributes to what is called the“greenhouse effect.” That’s the label for the tendency ofheat to build up in a greenhouse so that the air inside ismuch warmer than ambient outside temperature.

Warmer Air

Colder Air

ColdWindow

DowndraftUpdraft

HeatSource

Thermal Convection in a Fluid


Cooling

This happens because glass that is transparent tovisible light waves impedes the re-radiation of infraredwavelengths. The trapped radiation heats the structure,fixtures, and air inside the building. This heated air istrapped inside the greenhouse by the glass (probablycausing stratification), and cannot move the heat awayby convection.

Chimney Effect & Boundary Layer DisturbanceConvection directly affects the comfort of our livingspace, and even the clothes that keep us warm. It alsoaffects the boundary layer, which is made up ofstagnant air that acts like an insulator.

If the ∆t between the ambient air and the boundarylayer is anything but zero, the boundary layer willattempt to rise or fall of its own accord, inducingconvective heat flow. This can be the boundary layeraround our own warm bodies on a cool day, chilled airflowing down a cold windowpane to create floor draftsin a dwelling, or heat rising off the inside of a solarheated wall.

Another convective phenomena commonlyencountered is the “chimney effect.” In most furnaces,exhaust gasses exit the combustion process under theinfluence of convection. The heated gasses are lighterthan the ambient air, so they rise up the chimney,pulling air into the furnace or stove through cracks orthrough a controlled draft regulator. The hotter the fluegasses and the longer the chimney (within limitsimposed by conductive heat loss), the faster the gasseswill exit, so the stronger the gas column flowing up thechimney will be. Most stoves and furnaces would simplynot work if this convective flow was not possible.

This chimney effect is not limited to chimney flues. Itcan be used in a building as a tool to move hot air outof the living space. The rising hot air can be supplied bysolar energy. The resultant air movement is used toinduce whole house ventilation where it might otherwisebe difficult to achieve passively.

Wind as a Heat MoverUnder the right circumstances, warm lake water willheat the cooler air above it, inducing another fluidconvection cell in the air. This air is heated, rises, cools,and circulates back down to the surface to be heatedagain. This process is much the same as the draftssettling off a cold window. It is much greater in volume,and we call this movement wind. Anything that canaffect the heating of the air mass is important.

Wind is our ally. We have limited ourself by definition tocreating comfort passively in our living environment. Wehave cut ourselves off (or been cut off bycircumstances) from the use of highly concentratedfossil fuel derived energy. Yet to move heat around to

our advantage, it takes energy—sometimes largeamounts of it. Wind is the one source of energy readilyavailable to us that can do this job.

The differences in reflectance of the earth’s surface isimportant to heat absorption wherever we are. Blackbasalt rock will absorb more solar energy than lightsilica sand. A farmer’s pasture will absorb less heatenergy than the concrete streets and building walls in acity. This brings us back to the basics of material,surface texture, and color.

We don’t usually think of something like a parking lotaffecting natural breezes. Yet such a man-made featurecan have a vast effect on the microclimate that we aresubjected to in our living spaces. A large black parkinglot will absorb a lot of solar energy. This solar energywill be transmitted into the soil through conduction, re-radiated into the surrounding environment as radiantheat, and will heat the air above it, which can then riseconvectivly.

This convective flow may induce local breezes wherethere would be none, or it may disrupt natural wind flow.The radiant energy will distribute itself outward from itssource to all the surrounding areas adjacent to the lot,causing local heating and possibly destroying anybenefits a locally induced breeze might produce.Conductive heating of soil will create a reservoir of heatthat will continue to radiate to the surroundings longafter the ambient air temperature should have becomenaturally cooler. All three factors as well as terrain andvegetative cover are interactive and each affects theother.

Humidity & EvaporationNo discussion of wind and weather would becomprehensive without understanding the role ofhumidity and evaporation. Wind and weather areformed as part of a large heat cycle driven by solarenergy. One of the principle forces acting on this cycleis the addition or subtraction of heat throughevaporation.

It takes one BTU (British thermal unit) to raise thetemperature of one pound of water from 211 to 212°F(99.4 to 100°C), but 970.4 BTUs are needed to turn it tosteam at 212°F. Those 970.4 BTUs are known as latentheat, measured under standard conditions of oneatmosphere of pressure at sea level.

Water does not have to boil to absorb this latent heat. Itwill slowly evaporate at room temperature, requiring thesame latent heat. Evaporation requires heat, and thisheat, coming from surroundings, cools the environmentconsiderably. The heat taken in or given off as thisprocess occurs creates a very complicated thermaldance in everything from deserts to hurricanes.


Cooling

Unless it is artificially dried, air contains water vaporsuspended in molecule-sized droplets. The amount ofwater air can hold is determined by its temperature anddensity. Hot air can hold more moisture than cold air.So that there will be some common point of referencewhen talking about air moisture or humidity, figures forthe water content are given as “relative humidity.”

Relative humidity measurements are given in thepercent of moisture that air holds relative to itsmaximum possible moisture content at a giventemperature. The range runs from 0 percent forabsolutely dry air to 100 percent for air that holds asmuch moisture as is physically possible. This is knownas the saturation point. Anything greater than 100percent relative humidity will lead to free watercondensing out of the air as mist, fog, clouds, rain, orsnow.

The amount of moisture that air can absorb under anycondition is dependent on temperature and the amountof moisture it already contains. Thus air measuring 70percent humidity can only absorb the equivalent of theremaining 30 percent moisture capacity. The lower theair humidity, the more potential moisture the air can stillabsorb.

The more moisture that can still be absorbed, the morepotential there is for heat removal through evaporation.By evaporating moisture into the air as humidity, coolingcan be produced. And the more moisture that can beabsorbed, the more efficiently you can cool withevaporation. Humidity bears directly on the creation ofthe human comfort zone, since the body depends onevaporation through perspiration to rid it of excess heat.

Vegetative CoverThe black surface of an asphalt parking lot is a verygood absorber of thermal energy. The dark greensurface of vegetation is also a good absorber of thermalenergy, yet the plants cool their microenvironment. Howcan this be?

Plants are designed to effectively trap solar energy. Butinstead of absorbing light and producing heat, theyproduce plant sugars through photosynthesis. Much ofthis solar energy has no chance to be turned intoexcess heat. It is directed to the plants’ needs instead.Because of this, the use of green foliage to blocksunlight striking a building is very effective. Theadvantage of such shade is obvious when it comesfrom trees, but the use of vining plants on trellisescovering roofs and walls also works effectively to lowertemperatures.

One of the products plant leaves give off is water vapor,a vegetative “breath” that is transpired from pores in the

leaves. Transpiration is the process of taking in gasses(mostly CO2) and sunlight, and giving off oxygen andwater vapor. This evaporating water absorbs heat fromthe leaves and the surrounding air, cooling the localmicroclimate. The combination of transpiration andevaporation is called “evapotranspiration.”

Local BreezesTranspiration can also play a significant role in localbreeze generation. The figure on the facing page is ascale cross section of the Barton Creek valley where Ilived in Belize. The east side of the valley and theadjacent hill was cleared for pasture when the originalsettlers moved in. It is covered with low bushes and adense fern covering that is locally called “tiger bush.” Itfaces squarely into the afternoon sun, and the rate ofvegetative transpiration is poor.

The west side of the valley was too steep to be cleared,so it is mostly covered with undisturbed jungle canopy.Direct morning sun hits this slope and is cooled by thevegetation, but late in the afternoon when the eastslope is hottest, this west slope is taking the indirect(non perpendicular) sun’s rays and is cooled still further.Air is heated on the east slope and rises, while it iscooled on the west slope by the tree canopy and sinksdown into the valley.

The net result of this differential movement is a strongafternoon breeze that blows straight across the valley inthe hot dry season, contrary to the direction of theprevailing Caribbean trade winds. The existence ofsuch a wind is completely counterintuitive, but verymuch appreciated because it is much more local andintense than the prevailing breeze. This illustrates howmuch significance local and regional factors, bothnatural and man-made, can have on ventilation andheat flow.

Terrain such as hills or mountains can act as deflectorsto re-route prevailing winds, either creating a windshadow or augmenting wind velocity. Up to a point,when you are trying to get cool, more is better, soastute selection of a house site with an emphasis onmaximizing (or minimizing) local wind is important. Agood site for wind will provide the energy needed todeal with uncomfortable temperatures either passivelyor actively.

Human Heat PhysiologyIn spite of any surplus heat from the environment, thebody must maintain an internal temperature very closeto 98.6°F (37°C). There are a great many mechanismsthat we have evolved to effect this precise temperatureregulation. All three mechanisms for transferring heatare at work—radiation, conduction, and convection. Inaddition to those three, the body also uses


Cooling

perspiration—shedding excess heat through the latentheat of evaporation.

The high relative humidity typically encountered in thehumid tropics (around 95–98%), will severely interferewith the ability to lose heat through evaporation. The airis already saturated and cannot hold more moisture.This is the great difference between a hot humidenvironment and a hot arid environment.

Where the humidity is low, the body has the coolingmechanism of evaporation at its disposal and the lowair moisture considerably increases the efficiency of theprocess. This makes the designer’s job much easier insuch an environment.

Since I am targeting passive cooling in a hot humidclimate, my emphasis will be on techniques for thehumid tropics and subtropics. For those readers whoare fortunate enough to have dry desert conditions astheir design criteria, I direct you to two books by theEgyptian architect Hassan Fathy. These books aresuperb, clear, well illustrated, and relatively non-technical.

AcclimatizationWhen I moved from Alaska to Belize in 1980, I wasadapted to the subarctic environment of interior Alaska.Winter temperatures plunged to -60°F (-51°C) routinely,while summers were “oppressively hot” at 80°F (27°C).I could work in shirtsleeves at 35 to 40°F (1.6–4.4°C)and be comfortable.

In five years in Belize, the coldest temperature I everencountered was around 55°F (13°C). The typical hightemperatures were 75 to 80°F (24–27°C) in winter, 90to 95°F (32–35°C) in the wet season, and 95 to 108°F(35–42°C) in the hot, dry season. Getting used to thesetemperatures so that my body could regulate itself wasdifficult. I acclimated about 80 percent in the first year,and by the end of year two, I was 90 to 95 percentacclimated. I never reached 100 percent in the fiveyears I lived there full time.

If you live in Phoenix, Arizona where the temperaturesgo to 125°F (52°C) in August, and you are used to a72°F (22°C) air-conditioned environment, you will neveracclimate to the heat because you are not forced into it.But if you are out in the heat as it gradually increasesover the spring and summer, you will find yourselfgrowing accustomed to an environment that would haveseemed impossibly hostile before. If you are acclimatedto the local climate, whether hot or cold, it will takemuch less energy input to remain in the comfort zoneunder adverse conditions.

The Comfort ZoneThe comfort zone is defined as those combinations ofconditions of humidity, temperature, and air motionunder which 80 percent of the population experiences afeeling of thermal comfort. In temperate zones, this isfrom 68 to 80°F (20–27°C), and 20 to 80 percenthumidity.

House Site

Barton Creek:Elevation 246 feet (75 m)

AfternoonSun

0100200300400500600 100 200 300 400 500 600 700 800 900 1000 1100 1200

500 1000 1500 2000 2500 3000 3500500100015000

200

400

600

800

0

100

200

300

Elevation 640 feet(195 m)



Tree Covered Slope:Cools air

Shrub Covered Slope:Heats air

Prevailing Winds

Ele

vatio

n in

Fee

t

Ele

vatio

n in

Met

ers

Distance in Meters

Distance in Feet

Downdraft:Cooling air

Updraft:Heating air

Localized Winds in the Barton Creek Valley of Belize


Cooling

Different conditions can redefine this zone of comfort.Air motion or breeze can extend it to almost 98 percenthumidity and 90°F (32°C). Evaporative cooling canextend the highest comfort temperature up to 105°F(41°C) at lower humidities. High thermal mass (such asrock or concrete) acts like a thermal flywheel, remainingcool into the day, and warmer at night than ambient air.Thermal mass alone can extend the comfort zone up to95°F (35°C), while thermal mass cooled by nighttimeventilation can extend this zone all the way up to 110°F(43°C). Combinations of techniques are even moreeffective.

Evaporation (Perspiration) & Air MotionAt higher humidity and temperature, most of the excessbody heat is lost through perspiration. Air motion canincrease the boundaries of the comfort zone up to 98percent humidity. This boundary would be 80 percent instill air.

Research with a large sample of people shows thatcomfort can be maintained at 100 percent humidity and82°F (28°C), if air velocity across the skin is maintainedat around 300 feet per minute. This is the approximatevelocity of a good ceiling fan on high speed. At lowerhumidities (50 percent or less), temperatures of around90°F (32°C) are comfortable at this velocity. Because ofthis relationship, the designer’s goal is to create orpreserve air velocity in the dwelling whenever possible.

A breeze blowing against our bodies removes heatthrough two mechanisms—convection and latent heattransfer. When convection occurs, the skin heats the airand this heated air is carried away by the breeze. Withlatent heat transfer, perspiration evaporates, soaking upheat from the skin in the process. Moving air aids theprocess of evaporation at higher humidities, as well asremoving the boundary layer on the skin. This dead airlayer acts as an insulator to block thermal transfer fromthe skin to the air.

The boundary layer also blocks evaporative transferfrom the skin to the air. This layer heats up and reducesthe ∆t between the skin and the air, slowing down heatexchange. It also absorbs moisture from the skin, but isunable to immediately pass this on to the surroundingair. The boundary layer thus rises in humidity, reducingthe difference in humidity between the skin and the air.This slows down skin evaporation and the exchange ofheat to the air. Air movement shifts this boundary layerof warm, moist air, allowing the skin to come in contactwith drier, cooler air that can cool more efficiently.

SummaryIn Part 1, I’ve taken a look at the basic principlesgoverning the movement of heat, and tried to give you afeel for the way these forces interact with the

environment. We’ve looked at comfort, and found thatthe experience of thermal comfort is largely subjectiveto the individual.

In the next article, I will move from the general to thespecific. I’ll try to apply these principles of thermaldesign to the goal of creating a comfortable, passivelycooled house in the Barton Creek valley of tropicalBelize.

AccessCliff Mossberg, PO Box 16, Kasilof, AK 99610907-262-6098 • [email protected]

Resources for Further Study:Building for the Caribbean Basin and Latin America;Energy-Efficient Building Strategies for Hot, HumidClimates, Kenneth Sheinkopf, 1989, Solar EnergyResearch and Education Foundation, 4733 BethesdaAve. #608, Bethesda, MD 20814 • 301-951-3231Fax: 301-654-7832 • [email protected]

Air Conditioning: Home and Commercial, Edwin P.Anderson and Roland E. Palmquist, Theodore Audel &Co., a division of Howard W. Sams & Co., Inc.,Indianapolis, Indiana, 1978. Any library should have acomparable book on air conditioning that will treat thissubject thoroughly.

Architecture For the Poor, 1973; and Natural Energyand Vernacular Architecture, Principles and Exampleswith Reference to Hot Arid Climates, 1986, HassanFathy, both published by The University of ChicagoPress, Chicago. These books can be hard to find. I wasable to locate them through my regional inter-libraryloan program and have them brought to my local library.

66 Home Power #83 • June / July 2001

In Part 1 of this series (see HP82,page 84), I outlined the three modesof heat transfer, and tried to integrate

this knowledge with the factors ofhuman comfort. This information isbasic to any design or constructiongeared towards minimizing thediscomfort of living in the tropics.In Part 2, I would like to show some of the designtechniques for dealing with the effects of heat andhumidity in a dwelling located in what we know as “thehumid tropics.” This label differentiates this climate fromthat of a hot, arid, desert type of environment. Thedesert might ultimately be hotter than the conditionsfound in the humid tropics. But the low humidity foundin the desert makes it practical to use some techniquesof dealing with the heat that we cannot use in morehumid locations.

Solar Incidence as a Design ElementThe sun is the primary engine of heat gain in a tropicaldwelling. It is not usually ambient air temperature thatcauses heat discomfort, but the radiant energy ofsunlight, either directly or re-radiated in long waveinfrared. The first line of defense against heat buildup ina building is to minimize the surfaces that sunlight canfall on.

It is obvious that the building’s roof is going to be themain absorber of solar energy. If the roof is designed toblock heat flow down into the dwelling, and made largeenough to cover and shade the walls, the builder shouldbe successful at reducing unwanted heat. This simpleconcept is more difficult to accomplish that it seems atfirst.

If the sun was always in the high-noon position, the jobwould be simple, but it’s not. In the morning, it starts outshining low in the eastern sky. It can heat up abuilding’s walls for many hours before it rises highenough for the roof’s shadow to shield the east wallfrom radiant energy. In the afternoon, the sinking sunhas the same effect on the western wall.

Orientation for Minimum IncidenceSomething can be done at the design stage to reducethis wall heating. The very first effective step is todesign and orient the structure on the building site sothat the areas of the east and west walls are minimized.

Long, unshaded walls on the east and west sides ofa building can significantly contribute to the heating

problem.

This problem is not as severe on the northand south walls. The sun will be lower in the

southern sky in winter when wall heating isnot as big a problem. But the sun willnever be as low in the southern sky as itis near sunrise and sunset in the east

and west, so engineering roofoverhangs to block the southern sunis much easier.

Roof OverhangsIn Figure 2, angle A represents

directly overhead. Angle Bhas its pivot point at the baseof the south wall. It is plotted

at the local angle of north latitude. Atthat angle, the sun would appear

Part II — Applied Construction Cliff Mossberg©2001 Cliff Mossberg

S

NE

W

84°50°

Sunrise

Sunrise

Sunset

Sunset

Solar noon, approx.

December 21,shortest day

Solar noon,approx.June 21,

longest day

For Barton Creek, Belize, approximately 17° North Latitude

17° North

Figure 1: Seasonal Variation of the Solar Path

67Home Power #83 • June / July 2001

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directly overhead at the equator on the days of the solarequinox. A is easy to find—it is straight up. B is easy tocompute graphically once local latitude is known. Onceyou have B, you have a baseline.

If we swing an angle north 23° from B, we will have thenorthernmost angle of the sun’s travel in the sky inBelize. In this case, it is an angle of 6° north of vertical,or 84° vertical declination from level ground, pointingnorth (Figure 1). I have labeled this line C1. If C2 isdrawn at the exact same angle as C1, but touching theedge of the roof overhang on the north wall, the lowerextension of C2 will indicate the path of the sun’s rayson the north side of this building.

In this case, the sun will not ever touch the base of thenorth wall. C3 is the position the sun would have totravel to for it to begin to heat the base of the wall. C3 isan imaginary angle, since the sun is never that far downin the northern sky at this time of day and this locationin Belize. This shows us that a standard 2 foot (0.6 m)overhang on the north edge of the roof is sufficient toshade this north wall at all times of the year at thislocation.

Returning to our baseline B, we need to turn another23° angle, south from B this time, just as we turnednorth before. This will produce line D1, the angle of thesun’s rays at its extreme southern sky position. It isimmediately obvious that D1 does not touch both the

base of the south wall and the edge of the roofoverhang. We know from this that the roof overhang isinsufficient, even at 3 feet (0.9 m), to completely shadethe south wall.

The south roof overhang would have to be extended allthe way out to 5 feet 2 inches (1.6 m) to completelyshade the wall. This large overhang would bestructurally weak in high winds, and would also hangdown far enough to block the view out of windows onthe south wall. A compromise between 100 percentshade, vision, and structural rigidity will be necessary.

There are at least two possible solutions to this need forcompromise. In Figure 2, I have chosen to construct D2as a line parallel to D1 but moved over enough so that ittouches the south roof overhang. If it is extended downto intersect the wall, the lower projection of D2represents the limit of the south wall shading. Above theintersection with the wall will be shaded; below will seedirect sun at this time of the year. The line of shadeappears here to be sufficient to keep the sun’s rays outof the window openings.

VegetationTrees and shrubs that shade the structure are oneapproach to blocking sunlight. From a practicalstandpoint, it is difficult and extremely expensive to addmature trees of any size to a building design. The usualprocedure is to plant smaller ones and tolerate the sun

Figure 2: Angles of the Sun and Cast ShadowsLocal latittude: 17° 10'

(rounded off to 17°)Sun directly overhead

two times a year Projected line of the eave’sshadow on the south wall atnoon, on the shortest day

of the year (winter solstice)

Position of the sun inthe sky at noon on

winter solstice,lowest seasonal travel

Lines D1 & D2 are parallel

2' 0"

Position of the sunat northern extreme of travel

on summer solstice

Sun'sposition atequinox

Maximum amount of south wall exposed towinter sun. This exposure is a compromise toallow for a reasonable (3 ft.) roof overhang,rather than the 5 ft. 2 inches necessary toshade the entire wall. A 3 ft. overhang willblock sun from shining in the windows at noonduring the warmest time of the year.

40°

North porch isshaded from thesun at its most

extreme northernposition

Angle of the sun in the northern skyon the day of summer solstice

6°

16°

AB

C2

C1

D3

17°

Projected line of the eave’s shadowat the angle of sun that willcompletely shade the wall

6°40°

3' 0"

5' 2"N

S

Base of wallAll angles are drawn from this point

4"

12"

3:1 slope

Tilt of the Earth's axis in relation to the sun is 23° 27'(rounded off to 23° for our purposes)

23°

23°

D2

D1

C3


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until the smaller trees are big enough to produce shade.Unfortunately this can take ten years or longer. Wherepossible, keep what you have.

Vining plants are a good alternative to trees, with oneserious caveat. One of the goals of a tropical housedesign is the exclusion of termites from the woodenparts of the structure. This can be done by buildingelevated columns with termite collars on top. Anyvegetation planted on the ground and close enough tothe structure to touch it will provide a path for termitesto circumvent the exclusion features of the design.Without the termite problem, it would be effective to usea trellis on the east and west walls. Vining plants suchas passion fruit can intercept the sunshine and put it togood use growing flowers or edibles.

Wall Shading with Architectural ElementsIt is possible to use architectural elements to moderatedirect sun on the walls. Properly designed architecturalscreens can be made to block and modulate sunlight togood advantage. The photo above illustrates the use ofsuch a screen, here composed of simple decorativeconcrete blocks placed together into a pleasing texture.This very effectively opens up a whole wall to air andmuted sunlight.

This screen can conceal wooden or metal louvers fittedwith insect screens. These can be opened for the warmdry weather, but closed for storms. In this design, theconcrete screen is integrated as part of an upscale-style Belizian house. It will take a substantial foundationto support such a screen. Such massive architecture isnot necessary.

Hassan Fathy describes a traditionalscreen used throughout the MiddleEast that is made up of round turnedspindles arranged into a rectangulargrid. It is known as a mashrabiya.The same term is used to describevertical louvered blinds that can beadjusted to shade an entire wall.

Both of these devices allowconditioned light to enter thebuilding for i l lumination, whileblocking the strong exterior sunlight.The harsh contrast of the sunbeating on the outside of the screenblocks outsiders from seeingthrough the screen to the inside. Butit allows someone on the inside toeasily see out into the brightexterior.

Window Shading DevicesThere are two problems to deal with

if you wind up with sunshine on your outer walls. Thereis the re-radiation of the solar energy into the interiorfrom the walls. I’ll deal with that next. But first I want todeal more thoroughly with the problem of solar energydirectly heating the interior space through the windowopenings. Where this is a problem, the windowsthemselves can be constructed to block the sun’s raysthrough reflective glass coatings and through the use ofsolar screens.

Jalousie windows are commonly used in the tropics.They use single panes of glass to form the louvers.These single panes have virtually no insulation value. Incontrast, double and triple pane argon-filled glass usedin the colder regions are designed primarily to blockconductive and radiant heat flow outward, not tofacilitate natural ventilation inward. They would bevaluable in an air conditioned house.

While air conditioning has a role in tropical cooling, it isnot going to be a factor in our passive design focus. Wewant to foster good air circulation and a design thatexcludes solar radiation. Jalousie windows glazed withglass that uses reflective films can do this.

Glass can be made with a permanent reflective coatingdeposited on one face. This is conventionally eitherbronze or aluminum in color. This coated glass canblock up to 80 percent of the heat energy in incomingsunshine. Films that can be applied to uncoated glassare also available for this purpose, and provideapproximately the same excellent result. The downsideto reflective coatings is a reduction in the amount ofvisible light entering a house for general illumination.

A wall of decorative concrete block allows ventilation and provides shade, transmitting only muted light.


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For a radiant barrier to be effective, itmust have an air space on one orboth sides. Aluminum is a very goodconductor of heat. Without this airspace, the foil would simply moveheat from whatever substance is onone side of it to whatever is on theother. It would do this very efficiently.When it is installed with an adjacentair space, the air (which is a goodinsulator for heat transfer in theconduction mode) blocks conductionof heat from the foil, while the pooremissivity of the foil blocks heattransfer through the process ofradiation.

Roof Design & Radiant BarriersThe roof is the most critical heatblocking device in your arsenal. Itcan operate passively, blockingradiant energy from movingdownward into the house using a

radiant barrier. It restricts conductive flow of heatthrough the roofing materials. And it can be designed touse thermal convective flow to carry off air heated bythe roofing.

The roof design I prefer is actually two roofssandwiched together. The upper roof blocks wind andrain. It also contains convective air channels (seeFigure 3) between spacers over the structural joists.These cavities form ducts so that air heated by the hotroofing can rise and exhaust at the high point throughthermal convection. Below these vent channels is alayer of radiant barrier material. This barrier blocks theheat that is radiated by the metal roofing, keeping it outof the dwelling.

Solar screens that go on the outside of the windows inplace of conventional insect screens are also veryeffective, reducing the incoming heat energy by up to60 percent. Using both of these strategies produces atropical window that is extremely effective at blockinginvading radiant energy, while still providing excellentventilation. The cost is higher than uncoated glass andnormal screening, but it is worth the money.

There are many traditional methods available forblocking solar heat from infiltrating the inside of a housethrough the window openings. As a general rule,external devices such as awnings, louvers, and rollshades are more effective than inside devices such asvenetian blinds and roll shades. The efficiency of eachdevice is a function of its material, color, and texture.

Radiant BarriersThe material of choice for blocking both visible light andinfrared is a shiny sheet of polished metal. Aluminumfoil is one of the best materials, reflecting up to 95percent of both wavelengths. This foil is a very goodconductor of heat energy, but it is a very poor radiator ofradiant heat energy. It has a maximum emissioninversely proportional to its reflectance.

In English, that means that a highly polished aluminumfoil might only re-radiate 5 percent of the radiant heatenergy falling on it. It is an ideal blocker of radiantenergy. Used in this way, these foils are known asradiant barriers. Under peak sunshine conditions, aradiant barrier can reduce heat inflow by as much as 40percent or more.

Louvered “jalousie” windows are coated with a reflective surface to block sun.They also readily facilitate ventilation.

"Galalume"corrugated aluminumplated steel roofing

2 x 3 inch wood spacer creating aventilation chamber between eavesand roof ridge. Heat buildup between

inner and outer roofs rises andexhausts at ridge

1 x 4 inch purlins on2 foot centers

Reflective mylar radiantbarrier with air space

on both sides

Wall framing

Single layer of 15 lb. roofing felton top of 1/2 inch plywood

1/2 inch hardwoodplywood finished on

one side

Fiberglass battinsulation Three 1 x 8 inch

boards laminated intoa continuous rafter

1 x 4 inch nailer oneach side of rafter

1 x 2 inch wood spacer

Figure 3: Cross Section of a Roof in the Tropics


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The lower sandwich contains the structure as well asfiberglass batt insulation to block conductive heatflowing downward into the living area. As mentioned inPart I, radiant heating is the principal mode of heat flowdownward. Conductive heat does not move as readilydownward through materials.

Where the roof is built of standard sheet roofing overrafters, installing the radiant barrier is quite simple. Itcan be tacked to the underside of the rafters, above theceiling joists. For this use, radiant barrier is available inseveral different designs.

Radiant Barrier in the WallsWalls can also easily incorporate a radiant barrier.Where double-wall construction is used, the barriermaterial can be installed on the inside with the foilmaterial facing the outer wall. In areas where insulationis to be used in the wall, more care must be taken sothat there is an air space between the insulation andthe barrier material.

One method of utilizing the radiant barrier materialrequires that it be installed on the outside of thesheathing. Spacers are then nailed over the barriermaterial, and a second, vented skin is installed on theoutside. Vents at the top and bottom of this secondbuilding skin form a solar chimney, allowing heated airto exhaust from the wall by convection. This tacticworks with either open single-wall construction orinsulated double-wall construction.

Building InsulationMany materials have been developed to do the job ofholding air as an insulator. From sawdust, thatch, andstraw, to high tech materials such as aero-gells andceramic foams, all materials have pros and cons. Thefirst materials I’ve mentioned are organic, and subjectto biological degradation. The second two areridiculously expensive for home use. Good homeinsulating materials should be cheap, effective, andstable.

The ideal building insulation is nothing. Thenothingness of the vacuum in space is a case in point.Heat flow due to conduction or convection simplycannot occur in a vacuum because it depends on theinteraction between molecules of a substance to movethe heat. No substance equals no heat movement. Buta vacuum is not easily maintained.

Among commonly available materials, air is a very goodinsulator. It is cheap and efficient, but air has atendency not to stay in one place when it is heated. Weneed to stop convective air movements by trapping it.

Stability in Insulation MaterialsMany insulating materials are available that do this

successfully. Sawdust is one of the earliest andcheapest insulators. One of the great drawbacks ofusing sawdust is that it can absorb water from rain ormoisture in the air, or even from the building interior.Water absorption will degrade the insulation value, andmay lead to bacterial, fungal, or insect damage.

Sawdust is also subject to settl ing. Even themechanical vibrations a building may be subject to cancause settling of the sawdust, opening up large cavitiesabove the insulating material where convective heatflow can occur. A good insulator must be more thanefficient; it must be stable too, maintaining its originalvolume and material properties.

Insulation ToxicityTo be a stable building insulator, a material mustcontain as much air as possible, trapped in a matrix ofinert material. Rock wool is one of the oldestcommercial insulators available in batt form. It is stillused around heating systems where resistance to flameor high heat is desirable.

Rock wool is manufactured from inert materials thathave been heated and spun out into fine fibers. It isthen fabricated into batts containing innumerable smallair spaces. It is a brittle material with friable fibers thatcan break down easily during handling. These fiberscan be a severe irritant to the human body, both to thelungs and to the skin.

So besides being stable, a good building insulatorshould be benign to the people who must install it andlive around it. Asbestos is the classic example of theperfect insulation material that is also supremely toxic.

Materials such as glass wool—fiberglass—and severaltypes of closed-cell foams are non-toxic and non-irritating to a greater or lesser degree. Fiberglass is lessbenign than other materials, but not nearly as irritatingas rock wool.

Fire Retardant QualitiesAnother material that is common in the residentialbuilding trades is cellulose insulation. This ismanufactured out of ground-up paper, frequentlynewspaper. It has fire retardant added, and sometimesmaterials to make it resistant to insect damage.Cellulose is a very efficient, non-toxic insulator, but ithas a tendency to settle in vertical cavities, just assawdust does. Because of this, it is primarily used asloose fill above ceilings. If it is kept dry, it works verywell.

Foam boards and foamed-in-place urethanes areexcellent insulators, but they do not like heat. Underhigh heat conditions, they can produce toxic gases thatare lethal. Under sustained heat conditions such as

R-Values of Common Building Materials

Material R-ValueInsulationPolyurethane, per inch 7.00Polystyrene, extruded (blue board), per inch 5.00Polystyrene (bead board), per inch 3.85Rock wool, per inch 3.45Fiberglass batt, per inch 3.35

MasonryConcrete blocks, 8 inches 1.11Brick, common, 4 inch 0.80Concrete blocks, 4 inches 0.71Stucco, 1 inch 0.20Concrete, per inch 0.08

SidingWood bevel siding, 3/4 inch 1.05Wood shingles 0.87Wood bevel siding, 1/2 inch 0.81Aluminum siding 0.61

RoofingWood shingles 0.94Asphalt shingles 0.44Felt paper, 12 lb. 0.06

Wall CoveringInsulation board sheathing 1.32Cement board, 1/4 inch 0.94Gypsum board (drywall), 5/8 inch 0.56Gypsum board (drywall), 1/2 inch 0.45

WindowsSealed double glazing 1.92Single thickness glazing 0.91

Wood

Common construction softwoods, 3-1/2 inches 4.35Common construction softwoods, 1-1/2 inches 1.89Common construction softwoods, 3/4 inch 0.94Plywood, construction grade, 3/4 inch 0.93Maple, oak, or tropical hardwoods, 1 inch 0.91Particleboard, 5/8 inch 0.82Plywood, construction grade, 5/8 inch 0.78Hardwood finished floor, 3/4 inch 0.68Plywood, construction grade, 1/2 inch 0.62Plywood, construction grade, 1/4 inch 0.31Tempered hardboard, 1/4 inch 0.31Regular hardboard, 1/4 inch 0.25


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those found under a tropical roof, they can break downand outgas, losing their closed-cell foam structure, andseriously degrading their insulation ability.

The material I prefer for insulation in the tropics is glasswool, most commonly known as fiberglass. It isavailable in the U.S. in either batts or loose fill that canbe blown into place. Fiberglass is similar to rock wool inits physical construction. Since it is “spun” out of finestrands of real glass, it is inert to heat, resistant toairborne moisture in the form of high humidity, and is avery effective insulation. It is slightly more physicallyirritating to handle than some other insulations, but newmaterials are better than aged materials in this respect.

Shipping CostSince all of the commonly accepted thermal insulationsare light and bulky, they are expensive to ship longdistances. The cost of shipping this type of product isbased on its volume rather than its weight. That can besubstantial.

Fiberglass suffers from the same drawback that otherinsulating materials do. It is difficult to obtain in Belizeand other tropical areas because it frequently must beshipped in from more developed nations. Many nationsplace a high customs duty on imported goods such asthese.

Where it is available, two-component urethane foaminsulation is very convenient because the resin tomanufacture it can be shipped by the barrel, inconcentrated form. With modest equipment, the two-part resin can be combined and applied directly. It willthen expand in place. Keep in mind that this foam doesnot like high heat.

How Much Insulation?Some people define “R” values as “resistance” to theflow of heat. This is a good way to think of R-values. R-values can be added together, and they are a directlyproportional measure of heat resistance. The chart atright l ists common building materials, includinginsulation materials, and their associated R-values.

Where there is little difference between inside andambient temperatures, and where air movementthrough natural ventilation is the goal, uninsulated wallsand floors are acceptable. In a temperate climate,where winter heat and summer air conditioningexpense is an important factor, a well-insulated houseenvelope is required. R-values in the floors, walls, andceilings are specified by the location of the house inspecific climate zones.

The type of energy used to heat or cool a buildingaffects recommendations too, with higher R-valuesspecified for electric heat than for fossil fuels, for


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example. Additionally, fiberglass batts are only availablein certain thicknesses, so recommendations usuallyadhere to what is available. 3-1/2 inch (9 cm) thick battsare rated R-11, 5-1/2 inches (14 cm) at R-19, etc.

Dead Air Spaces Used as Thermal BlocksThere are other ways of designing to resist heat flowfrom solar-heated walls besides radiant barriers andinsulated surfaces. In new construction, during thedesign phase, it’s necessary to be aware of potentialheating problems. It is cost effective to design cabinets,closets, garages, or other unoccupied or infrequentlyoccupied spaces along those walls that are sources ofinterior heating due to exterior solar radiation. Thispractice creates a double wall with an interior dead airspace to resist heat moving across that space into theliving environment.

Mass Used as a Thermal FlywheelFrom the adobe pueblos of the southwest Indians to therock walls in the ancient stone city of Great Zimbabwe,many indigenous forms of architecture have takenadvantage of the thermal storage inherent in largemass. This mass can store heat, and can also even outthe temperature fluctuations in a hostile l ivingenvironment. The modern equivalent of these classicexamples is the Trombe wall.

Using mass to mitigate temperature swings in adwelling only works well where the temperaturedifferential between the mass and the tempering heatsource is fairly large. If you try to adapt the Trombe wallor other less passive applications of thermal massstorage to cooling in the humid tropics, you are limitedby environmental factors.

Solar-driven temperatures inside a poorly designedbuilding can go up to 125°F (52°C) in the heat of theday. This gives you a nice temperature differential todrive heat exchange, but such a gain is never desirable!But you do need a significant temperature difference tomove much heat from the hot interior mass to thecooler outside nighttime air. In a passively cooled housein the humid tropics, there is no concentrated source of“cold” that can drive such a cooling heat flow the waythere is with solar heating.

ConvectionIn Passive Cooling—Part 1, I covered the theory ofconvection, the movement of heat carried by the flow ofa fluid such as air or water. Here I will try to explain howthe designer or builder can use the building envelope toforce convective flow to occur passively—without anyinput of energy other than what is applied to the fluidthrough natural influences.

I should say here that I do not personally subscribe tothe need for entirely passive designs. Where the energy

is available or where you can create it efficiently, thereare good arguments for the use of active designs. Lowvoltage DC ceiling fans are a good example.

The key to good design is the word “efficiently.” Bothpassive and active cooling systems can be designedthat are so expensive to install that it could well bemore efficient, all things considered, to run a generatorand an air conditioner. So when I talk about efficiency ofdesign, I am factoring in the overall cost of the design,not just operating costs.

Chimney Effect VentilationHot air is less dense and therefore lighter than cool air.It rises or floats on the heavier, cooler air. As with allforms of heat flow, “hot” and “cold” are qualities that arerelative to the temperature of a human body—98.6°F(37°C). There is no absolute quantity known as “hot” or“cold.”

The important consideration is the difference intemperature between one heat source and another, notwhether it is hot or cold. This concept is knowntechnically as ∆t (delta t), shorthand for the change in,or the difference in temperature.

∆t governs all things thermal, including radiation ofenergy from one hot body to another, conductionthrough a substance, or how easily hot air will float oncooler air. If ∆t is high, hot air is more buoyant and willrise faster. If ∆t is small, there is less tendency for aheated mass of air to move upwards. I am using airhere as a familiar example, but technically, any fluidfrom air to water to molten metals will supportconvective heat flow.

Solar chimneys are structures designed to heat air withsolar energy. This heated air then rises in a duct, just asfurnace-heated air in a stovepipe rises. Under mostconditions, stand-alone solar chimneys cannot justifytheir cost with their performance. Solar-enhancedventilators (roof panels that are designed into newconstruction) may have a slightly better cost/benefitratio, but as a general rule, their performance isdisappointing. They are especially ill-suited to the humidtropics.

Roof VentingIn Part 1, I described heat buildup in the attic air spaceunder a hot roof, and I showed how this builduptransfers heat to the ceiling and then down into theliving space. If we return to that example, we can nowdiscuss the role convection will play.

In the example above, the hot roof will attaintemperatures of around 140°F (60°C) maximum. In theliving space, the desirable temperature is around 72°F(22°C). There is a ∆t between the roof heat source and


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the ceiling of 68°F (20°C). That issizeable.

Suppose now that we open the roofup and allow the hot air, which hasrisen to the highest point of the roof,to keep rising and escape? This airremoval technique is known as roofventing, and it is highlyrecommended for any enclosed roofor attic space.

Of course, for air to flow out of acavity, there must be provision forreplacement air to flow in. The hot air flows out, creatinga very slight vacuum, which draws cooler air in fromsome other place, usually around the roof eaves orgables.

As this replaces the hot air with much cooler air, the ∆tbetween the attic air space and the ceiling membrane isconsiderably reduced. The ∆t between the roof and theattic air is increased, allowing more heat to transferfrom the roof surface to the attic air, which is ventedoutside to the ambient air. This reduces the rooftemperature. Clearly, convection can be useful.

Whole House VentingThe type of convective heat removal described above isnot just useful in attics and roofs. It is also useful forwhole house ventilating under certain conditions. Thepoint of whole house ventilation is to completely changethe air inside the living envelope periodically.

Large fans are typically used for whole houseventilation in hot climates. These are installed in theceiling, and thermostatically controlled to respond tooverheating of the living space. This is typical forhouses without refrigeration-type air conditioning thatencounter seasonal high temperatures. For ourpurposes, we must try to accomplish the same endgoal, but without the fan. (When practical, a wholehouse system is an excellent application for a solar-powered fan.)

Whether you employ a fan or rely only on convection forwhole house ventilation, it is desirable to achieve abouttwenty air changes per hour, or 0.33 air changes eachminute. The volume of the structure can be found bymultiplying the floor area by the wall height. For thehouse in Figure 4, it works out to about 6,850 cubic feet(194 m3). So the resulting airflow desired is around2,260 cubic feet (64 m3) per minute (0.33 x 6,850 =2,260.5).

Disadvantages of Convection AloneIn Figure 4, the outer wall of the house is around 8 feet(2.4 m) tall, while the roof over the clerestory windows

in the center is over 14 feet (4.3 m) tall. The interior ofthis house has a cathedral ceiling that rises to a highpoint above the clerestory. Hot air can flow out at thishigh point to drive whole house venting. Calculating theventilating airflow under the best of conditions givesabout 300 cfm—not very good! Here we are assumingno wind augmentation, just the induced circulation dueto hot air rising and exhausting.

The reality is that we are not going to be able toventilate this dwelling without the help of solar energy.Either we will need it to run an active fan system, or at aminimum to heat up the building so there is differentialtemperature gain that can be put to work moving air.But the last thing you want to do is introduce hot air justto get rid of the hot air! Convective cooling alone is notpossible in this house under these rainy-seasonconditions. During the dry season, some air exchangeis possible using convection.

Buried Cooling TubesAnother idea that frequently creeps into conversationsabout passive cooling is the use of earth tubes as airintakes for solar chimney driven ventilation. Theprinciple here is that pipes are buried in the cooler earthto draw air into the structure. The intake air cools downto earth temperature as it is drawn in, cooling thebuilding.

Where a source of forced ventilation is available, suchas an electrically driven blower, this can be made towork. Even then, there are potential problems withmoisture build-up in the tubes, which can lead tointroducing mold and mildew into the structure. Withoutusing a powered blower to force air through the coolingtubes, non-circulation or even reverse circulation(pulling heated air into the structure from a hot source)is a possibility.

It is important to remember that stack-effect ventilationrequires that the average temperature in the air columnbe higher than the cooler surrounding air. If the aircolumn is 85°F (29°C) in the dwelling, 100°F (38°C) in

PrevailingWind

A low pressure area is developedas wind passes over the top edge

of the clerestory roof.This vacuum draws hot insideair, which has risen into the

clerestory, out of the building.

High pressure on this side forces airthrough windows and also up intocooling ducts under the roofing.

Figure 4: Ventilation Paths


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the stack 10 feet (3 m) above, and 70°F (21°C) 10 feetbelow, down inside the cooling tubes, we have anaverage column temperature of 85°F. Ambient airtemperatures outside would have to be lower than 85°Ffor upward movement of the air column to occur.

Wind Used for VentilationWind is a form of convective air movement driven bythe sun. It is a concentrated form of energy. Every timeyou double wind velocity, you increase wind energyeight times, because wind energy is a cubic function ofvelocity.

Wind will act on a building, whether we intend it to ornot. Contrary winds can and do drive heated airbackwards in solar ventilating ducts. They can allowcold air infiltration into a heatedbuilding envelope, and theygenerally do unexpected things in astructure not well thought out toresist wind dynamics.

Where a reliable breeze is available,you can use it to good advantage todrive air exchange through theenvelope of a building. Aconsiderable amount of informationis available about how wind interactswith the planes and curves of abuilding structure.

The U.S. Federal EmergencyManagement Administration (FEMA)has thoroughly explored thedynamics of wind/structureinteraction, seeking a betterunderstanding of hurricane damageto buildings. Figure 5 and 6 aretaken from FEMA course material,and illustrate the envelope dynamicsof a building very well. Thisinformation is basic tounderstanding how the forcesdeveloped by wind can be used tofoster local area and whole houseventilation.

Wind blowing against the walls androof of a building is forced along theplanes of the surfaces. When itreaches the limit of a surface—thecorner of the wall or the edge orpeak of the roof—it continues toblow in the direction in which it hasbeen flowing. This is a property ofthe inertia of the mass of the air inthe wind current.

As it passes the edge of a building panel, wind does notturn the corner and follow the building planes. Instead,it lifts away from those flat sides, creating an area oflower pressure just past the edge. Technically, it makesa transition from smooth laminar flow along the panel toturbulent flow away from the second panel.

Air Flow Around WallsFigure 5 illustrates wind flow as if we were lookingdown from above on the floor plan of a rectangularbuilding. On the left is a pictorial schematic of the pathof the wind flow. On the right is a schematic diagram ofthe vector forces of pressure and vacuum induced bythe wind pattern on the left. Arrows pointing inward atthe wall represent pressure. Arrows pointing outward,

WindFlow

Building envelopeclosed

Wind forces onoutside of building

envelope onlyA C

D

B

WindFlow

Building envelopeopened on

windward side

Wind forcespenetrating building

produce insidepressure on wallequal to dynamic

wind pressure

A C

D

B

WindFlow

Building envelopeopened on

leeward side

Wind forces createslight vacuum

on lee of building;negative pressureis transferred to

inside walls

A C

D

B

I-a I-b

II-a II-b

III-a III-b

Figure 5: Wind-Induced Pressure Vectors Under Different Conditions of Building Ventillation


Cooling

away from the wall, represent vacuum. The curved linesare a rough representation of a graph of thepressure/vacuum forces, showing how they vary indifferent locations.

Illustrations I-a and I-b in Figure 5 show a building withsealed walls and roof. In this theoretical illustration,there are no paths for pressure to be transmitted intothe envelope. Every time a building design creates animpediment to smooth airflow, it will induce a highpressure area. And every time airflow is forced over ahard edge with nothing behind it, a modest vacuum willbe created.

Figure 5, illustrations II-a and II-b show a building with abreach in its windward wall. This can be a door,window, or just siding torn off under high wind forces.There are no other openings in the walls, so pressurebuilds up inside the building until it exactly equals thedynamic force of the wind entering the windward wall.The air is actually compressed somewhat, causing arise in the static pressure against all of the inside walls.

Once the inside static pressure and the dynamicwind pressure equalize, it is just like a balloonthat’s been blown up. No more air can blow intothe building because it is balanced by the forcepushing out by pressure of compression. As II-billustrates, the pressure inside this building isexactly equal to the highest pressure developedon the windward wall. This is because the wallopening is located in the area of highestpressure.

If the wall opening were moved over to an areawith less pressure (near the corner), that lesserpressure would be what is transmitted to theinside of the building. Wall C is subject not onlyto the force developed by the mild vacuumpulling on the outside, but also to the force ofthe static pressure inside. These forces add up.In hurricane winds, a building can explode underthese forces.

The designer should be sensitive to the areas ofwind-induced high and low pressure in astructure. Maximum interior air flow value canbe achieved by allowing pressure into a buildingenvelope at points of highest dynamic windpressure. Conversely, air can be drawn frominside a building most efficiently by strategicplacement of exit venting at points where thewind has developed negative pressure.Combining both strategies gives a very effectivepush/pull effect. It is desirable to have theoutflow openings larger than the inflow. A six toone ratio of outflow to inflow area is optimum.

Air Flow Over RoofsFigure 6 shows buildings in cross section to illustratethe dynamics of airflow over different roofs. The flat roofand the low-pitched gable roof are subject to negativeforces trying to lift up on them. Roofs with pitches over40° do not have sufficiently sharp eaves to cause theflowing air to pull away from the roof as it moves along.Consequently, the windward side of the roof receives asubstantial impact from the direct wind. So there arepositive pressures on one side of this building, includingthe roof, and negative pressures on the downwind side.

These are the notes of the tune we want to play. Nowwe must put the notes together into a melody. If youopen a building up to ventilation on the windward sideonly, you will have no ventilation. The inside andoutside pressures cancel each other out, and that’sthat. This illustrates that you must have both an inletand an exit for air to flow. Air must flow out of theenvelope as fast as it can flow in or there will bepressure build-up that will restrict inflow.

WindFlow

Flat Roof

A C

D

B

I-a I-b

Floor

WindFlow

Gabled roofless than 40°

A C

D

B

II-a II-b

Floor

WindFlow

Gabled roofgreater than 40°

A C

D

B

III-a III-b

Floor

Figure 6: Wind-Induced Pressure Vectors Over Different Roof Configuartions


Cooling

Direct Action on BuildingFigure 7 illustrates some specific building treatmentsthat are effective in fostering passive buildingventilation. The important concept here is that evenunder moderate wind loads, there are pressuredifferentials on the outside walls and roof of thestructure. If the designer takes advantage of these, it is

possible to induce significant forced ventilationcirculation through a structure. Under the rightconditions, this ventilation will equal or exceed what youcould obtain with an electric fan.

ConclusionIn Passive Cooling Part 1—Basic Principles, I describedthe three basic mechanisms of heat transfer. I alsorelated the transfer of heat to the sensation of comfortthat people seek.

In Passive Cooling Part 2—Applied Construction, I havetried to relate the basic information in Part I to the worldof wood, concrete, and glass. Here, to a limited degree,I have shown specific building techniques for thwartingthe penetration of heat into a living environment. I’vecovered the principles and pitfalls of using naturalforces to create that comfort envelope we seek in orderto keep the effects of excessive heat at bay.

I have also tried to list other, more extensive sources ofinformation on the subject of passive cooling, for theenthusiastic reader. I hope this has been of some helpto people trying to live comfortably in the humid tropics.It can be done.

AccessCliff Mossberg, PO Box 16, Kasilof, AK [email protected]

Resources for further study:Architecture For the Poor, 1973, and Natural Energyand Vernacular Architecture, Principles and Exampleswith Reference to Hot Arid Climates, 1986, by Hassan

WindFlow

Roof overhang createspressure under

windward eaves, andvacuum above

overhang.

WindFlow

Cupola on roofridge has movableopenings on bothsides to controlvariable winds.

WindFlow

Clerestory windowswork well when winds

are constant.

WindFlow

Wind creates localizedareas of high pressure

where its flow is restrictedby the structure.

WindFlow

Placement of wallsand windows can

control air flowand venting.

WindFlow

Vegetation canbe used toinduce high

and lowpressures

when buildingdesign cannot.

Elevation Models

Floor Plan Models

Figure 7: Strategies for Passively Inducing Ventilation

Vent windows placed high on the downwind wall allownegative pressure to extract hot air from inside.


Cooling

Fathy, both published by The University of ChicagoPress, Chicago. These books can be hard to find. I wasable to locate them through my regional inter-libraryloan program.

Building for the Caribbean Basin and Latin America;Energy-Efficient Building Strategies for Hot, HumidClimates, Kenneth Sheinkopf, 1989, Solar EnergyResearch and Education Foundation, 4733 BethesdaAve., #608, Bethesda, MD 20814 • 301-951-3231Fax: 301-654-7832 • [email protected]

Radiant Barriers: A Question and Answer Primer, byIngrid Melody • Florida Solar Energy Centerwww.fsec.ucf.edu/Pubs/EnergyNotes/En-15.htm

Low Energy Cooling; A Guide to the PracticalApplication of Passive Cooling and Cooling EnergyConservation Measures, Donald W. Abrams, VanNostrand Reinhold Company, New York, 1986Apparently no longer in print, but check with usedbookstores and inter-library loan.

Passive Cooling, 1989, edited by Jeffrey Cook, US$55from The MIT Press, 5 Cambridge Center, Cambridge,MA 02142 • 800-356-0343 or 617-625-8569Fax: 6l7-625-6660 • [email protected]://mitpress.mit.edu

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