Vapor Retarders in the South

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Vapor Retarders in The Hot and Humid Climate ‐ Dispelling Myths.

On several occasions, I was made acutely aware how terribly misunderstood the application of this

material is in the hot and humid climate. Here are just few examples from my practice:

1. Moisture, mold, and mildew discovered under a peeling wallpaper. Moisture plus extensive pest damage

discovered under flooring placed over crawlspace. Both the the wallpaper and flooring materials were produced of

an impermeable plastic.

2. Technical representative of a reputable material manufacturer claimed vapor retarders are necessary on the

interior side of thermal insulation in order to avoid condensation in South Florida at his educational seminar.

3. Director of operations of the Northeast American branch of the largest facade engineering firms in the world,

claimed during his presentation in Miami Beach last year, there is no risk of building envelope condensation in hot

and humid climates such as South Florida. In response to my clarifying question, he further explained that it almost

never gets cold enough in the winter in Miami to worry about condensation.

4. Every now and then I stumble upon architectural drawings, and textbook drawings exhibiting vapor retarder on

the wrong side. Below is the reproduction of the CBS system popular in South Florida (CBS stands for Concrete

Block Stucco).

(Bad) example from the Internet. CBS system popular in South Florida.

Vapor retarder pictured on the wrong side of the interior thermal insulation.

5. A project manager of the South Floridian branch of one of the "Top 10 Largest" architectural firms in the U.S.

explained to me the policy of his company is to never place an impermeable material on the exterior of any facade.

It came up when I submitted my forensic report showing this practice as one of the sources of failures observed in

the local building his company designed. He strongly believes this policy is flawless; therefore, our observations

and findings must be false.

6. A building technologist of a Northwestern branch of one of the "Top 10 Best" architectural companies in the

North America denied any water vapor diffusion would occur through the polyurethane foam sprayed on a bare

gypsum sheathing of an exterior stud wall covered with an open‐joint metal cladding rainscreen, and located in hot

and humid climate. He explained he was advised so by his building envelope consultant. He even drew a nice

sketch illustrating his belief, which I thankfully saved in my growing collection of architectural goofs.


As illustrated by those examples, not only architects are misinformed, but also some of their role

models. Therefore, even though the subject is so well described in the available literature, I decided to

add to the information overflow, by writing this short explanatory article.

What is Vapor Retarder?

Vapor retarders control the diffusion of water vapor; they are characterized by permeability (the rate of

of water vapor flow) and they need to be installed in a continuous manner. Sounds simple?

The Florida Building Code gives the following definition, which seems to be good enough for our

purpose:

VAPOR RETARDER. A vapor‐resistant material, membrane or covering such as foil, plastic sheeting or

insulation facing having a permeance rating of 1 perm (5.7 ´ 10‐11 kg/Pa × s × m2) or less, when tested

in accordance with the dessicant method using Procedure A of ASTM E 96. Vapor retarders limit the

amount of moisture vapor that passes through a material or wall assembly.

This definition also covers mechanical conduits, concrete floors, wood protection, and other uses, but

we limit ourselves to the general bulk building envelope applications in this article.

Why Vapor Retarder?

Two chief reasons: 1) To control condensation, 2) To control humidity in the building.

Where Vapor Retarder Should Go?

The rule of thumb is assemblies should be layered in an increasing order of permeability, starting

from the prevailing high‐latent‐energy side. It's so simple. This would assure the drying potential in

every climate. The tough part is to figure out where the loads are. For example, the high latent energy is

represented by the boiling kettle on the pictures below. As you can see, it's very easy to design in the

hot and humid climate, because energy flows the same direction most of the time.

and

Cold and mixed climates are trickier because the energy flow changes direction every season. Energy

may also change direction every day (think morning dew), but in this article we will conveniently

overlook the diurnal heat cycles for sake of clarity. Therefore, placing the vapor retarder on the exterior


should be no brainer in the hot and humid climate, because you can integrate it with the layers

controlling air and water, without the moisture entrapment risk inherent in other climates. It also saves

the energy, which otherwise would need to be used to dehumidify the building from the additional

latent load coming via the diffusion.

Vapor retarder on exterior, allowing for all‐in‐one integration of many functions in one layer.

What Happens If There is no Vapor Retarder?

What happens if you do not design the vapor retarder at all in the hot and humid climate? (Remember,

one of the leading architectural firms in this country made it its official policy.) Either of two things:

1) Murphy's Law says "If anything can go

wrong, it will." If your (or your tenant's)

interior designer decides later to embellish

the interiors with something impermeable,

than you may expect condensation and the

associated damage to adjacent, moisture‐

sensitive materials, as well as the microbial

growth.

It's not a question if it happens, it's a question

where; Many materials used in the interior

design are impermeable. (think vinyl wall

paper, ceramic tiles, epoxy flooring, etc.) it's a

safe bet to assume there would be a e.g. a

kitchen, bathroom, or simply a piece of

furniture placed next to the building

Miami. WUFI simulation showing water content in an exterior concrete

wall assembly with an impermeable coating on the interior surface.


envelope, which may lead to this condition.

This is one of the most typical condensation problems in the South, caused by a vapor retarder on the

"wrong side." How do you know there is a problem? Your nose would probably tell you first, by the

unmistakable musty odor (In the new construction you may need to use your eyes first, because new

materials are often treated with biocides, which would wear after several months.) Blistering, peeling,

and delamination would further indicate the issue.

Solution? Lawyers would advise

architects to set up a sacrificial

corporation to isolate the

architectural malpractice risk.

Seriously, you may want to

address the core issue and place

the vapor retarder where it's

missing. It's very easy in the

typical barrier wall construction:

just coat the exterior with an

impermeable paint and you are

done. It's trickier in rainscreen

and drainable veneer

arrangements.

Interior workarounds include the removal of offending materials, ventilation of cavities behind them, or

heating these cavities to depart from Dew Point. Needless to say, these methods would also increase air

conditioning energy costs.

2) Keeping The Chopper in the Sky. At the remaining locations, where the vapor retarder was not placed

on the wrong side, the only downside of water vapor diffusing into the interior spaces would be the

increased operational cost; the interior would be more expensive and challenging to dehumidify.

The phenomena responsible for this symptom is the water vapor pressure differential, which in the

South may exceed 1 inch Hg (13.6 inches of water) in the summer. For comparison, the worst‐case

winter condensation in the cold climate is typically driven by less than 0.5 inch Hg (~6.8 inches of H2O).

This is probably why the South beats North in both quality and quantity of bad cases of moisture

condensation. The pressure differential would be even higher across exterior partitions of a mechanical

duct, around 1.5 inch Hg (~20 inches of H20) because air in a duct has a lower water vapor pressure. We

often see failures in mechanical plenums designed by architects, regardless of climate.

Also, exterior absorptive materials, e.g. brick veneer create water reservoirs, which further increase

exterior vapor pressures to insane levels, when sun heats them.

Alabama: Restroom in a Hospital. Microbial growth behind the impermeable interior wall

coating


The most typical assembly of this kind in the South is the 6 inch thick concrete precast panel. Its

permeability is around 0.7 perm at best. Such an assembly would allow a steady flow of water vapor

through the assembly, which would produce high air humidity readings inside, which in turn would need

to be removed by the air conditioning system, adding to the operating costs and depletion of

environment.

To illustrate, such a wall measuring 10ft x 10ft would transfer ~2‐3oz of water on a hot summer day. The

additional cost of dehumidification would depend on the type and efficiency of the system.

Another good example of such a wall is a polyurethane spray foam installed on a gypsum sheathing

pictured on the illustration reproduced on the next page

Example of a rainscreen wall designed without a vapor retarder.

Do you feel uncomfortable, your printers and copiers fail to print, envelopes glued themselves together,

steel is covered with a thin layer of rust, millwork delaminated and bowed, pests overrun the place, and

the musty odor explains why your employees are on a sick leave? The trouble is the mechanical design

is seldom coordinated with the architectural design; therefore, more likely than not, it did not address

the loads properly, resulting in the interior spaces located too close to Dew Point. This is often

manifested in old buildings retrofitted with air conditioning, but new buildings are prone as well.

Solutions? Installation of a vapor retarder on the exterior side of your envelope would address the core

issue. As above, it's very easy in the typical barrier wall construction: just coat the exterior with an

impermeable paint and you are done. Older concrete buildings, such as the one I am sitting in while

writing these words, are often covered with numerous layers of inexpensive paint, which typically acts

as a very effective vapor retarder. It'd be trickier in rainscreen and drainable veneer arrangements,

where the building envelope may be inaccessible.

Workaround include dehumidification of the supply air. However, you would need to prepare for the #1

above, because as soon as you do it, moisture would be encouraged to condense in your building

envelope, attracted by the larger water vapor pressure differential. Also, any interior dead spots in air


circulation would need to be eliminated (which may require costly CFD analysis) to assure the consistent

moisture removal.

What Happens If The Vapor Retarder is Placed On the Interior?

There is nothing wrong with placing the vapor retarder on the interior side of the thermal insulation, if

one knows what one is doing.

Exterior Insulation System relies on the integrity of the weather resistive barrier (WRB) behind, which should be impermeable.

A simple field observation or a hygrothermal computer simulation (e.g. WUFI) would tell you where you

should expect water condensation:


So as long, as you can keep all moisture‐sensitive materials out of harms way and drain the water

toward the exterior, you would devise a good, functional assembly. This is exemplified by the exterior

insulation systems, utilizing a moisture‐insensitive thermal insulation, a drainage interlayer, and a

waterproofing layer applied on the backing wall acting also as the vapor retarder.

Specifications Division 7.

The only one physical characteristic that describes vapor retarders is its permeability. Therefore, one

would reasonably expect the permeability to be placed in the relevant section of architectural

specifications. However, regardless of the climate, the universal mistake made by specification writers is

omitting the only one requirement that actually matters. (Vapor retarder is often incorporated in the

thermal insulation section; the most important physical characteristic that describes thermal insulation

is its thermal resistance, and this requirement is also universally omitted from the specifications, but this

would be a subject for another article.) Specifications seldom actually tell the installer which side the

vapor retarding facing should face, but the spatial relationships should be described on drawings.

What Happens if Both Sides are Exterior?

Dr. Joe Lstiburek said a parapet walls' function is

to confuse architects, while describing the

common air leakage at the roof‐to‐wall interface

stemming from architectural errors and

omissions. However, we specifically avoided

discussing air barriers and air leakage in this

article, to avoid further confusion. (We would

cover the air leakage soon.)

Another interesting aspect of parapet walls is

they are often covered with impermeable

roofing membrane on the side facing a roof. Also, freestanding walls, due to their high visibility, tempt

architects to experiment with different combinations of exterior finishes, such as stucco and paint.

Sometimes, the inconspicuous side is left

unfinished. Due to their double exposure they

are subjected to the much higher heat loads

than the average building envelope. The water

vapor pressures generated in these assemblies

are insane (think steam engines).

The ultimate result is similar: unequal drying

capacity with one side more permeable than the

other side, resulting in all kinds of interesting

symptoms, such as peeling paint, efflorescence,

and discoloration as primary examples.

South Florida. A stained parapet wall covered with asphaltic

membrane on the other (roof) side.

Alabama. An exterior wall around an upscale shopping mall exhibiting

peeling paint in a spectacular way.


Dispelling The Most Common Myths.

Below are listed the most common architectural misconceptions, accompanied by explanations:

‐You must place a vapor retarder in your design to avoid condensation.

Not really, unless the makeup of your assembly is already screwed up to begin with. (See the rule of

thumb above for reference.) Although the inclusion of vapor retarder is required by building codes, the

requirements apply only to the avoidance of condensation, and it's possible to rationally engineer such

an envelope without a vapor retarder. (However, you would still need the vapor retarder in thehot and

humid climate to limit the interior air humidity level and the energy use, unless your wall is already

impermeable enough.)

‐ Vapor retarder is just another name for air barrier.

No. The two perform different functions and have different physical requirements. E.g. air barriers need

to be structurally strong enough to control air movement. Some materials (e.g. single ply roofing

membranes) can perform both functions, but don't let it fool you. Air leakage typically contributes to the

condensation issues experienced in buildings, but the two would typically require different remedial

measures.

‐Vapor retarder should be placed inside.

There are some projects on which vapor retarder should be placed inside, but it should not confuse you.

It proved devastating in the South over and over again‐ see the photographs of the mold and mildew

three pages up. You can also download the free demo of the intuitive WUFI software from Fraunhofer

Institute's webpage, and ten minutes later you should know the answer.

‐Vapor retarder should not be placed outside.

This is how architects contribute in their modest way to the depletion of the natural environment and

the operational budgets of their clients (unless their client happens to be an oil sheikh).It proves to be

expensive in the hot and humid climate, as illustrated on the examples cited above.

‐ Roofing membrane is a sufficient vapor retarder.

The vast majority of roofing felts are impermeable to water vapor, but don't let it confuse you. There are

exceptions to the rule. Ironically, popular asphalt shingle assemblies are vapor permeable, leading to the

widespread decay observed in many roofing assemblies in the South.

‐ Vapor retarder should be placed on the warmer side.

It's almost right, but like many other rules of thumb, there are exceptions to this rule. The good example

is a typical basement wall, where you may need vapor retarder on the colder side, because this is where

the prevailing moisture load may come from, depending on your specific project conditions.

About the author Karol Kazmierczak (Kaz), CSI, CDT, AIA, ASHRAE, NCARB, LEED AP, is the Senior Building Science Architect and President at Building Enclosure Consulting, LLC and the leader of the BEC Miami. He has 15 years of experience in building enclosure technical design, engineering, consulting, and inspection.He can be contacted via e-mail at [email protected].


Small Residential Construction

The above examples came from large buildings, where disinterested agents of owners tend to hire

architects to design their buildings. In such a scale, the waste is easily written off, even if it's measured

in hundreds of thousands of dollars.

How an average contractor and DIY are doing, in the more cost‐conscious small residential

environment? Frankly I had no idea, until recently I was remodeling my house, and I visited two major

home‐improvement chains to pick some thermal insulation. Imagine my surprise when I discovered

I could not buy unfaced batt insulation because it was unavailable in the local stores in Miami.

Shelves were full of the (more expensive) faced insulation. So I purchased it and I read the label.

The instructions printed on the label did not help me to figure out which side the facing should go.

I guess if I were the average contractor or a homeowner, I would install it facing the interior because it

would hurt the least (glass fibers have a nasty tendency to get into your skin, so it's easier to push it via

the protective facing). The only reasonable use for this insulation I can think of is in unconditioned attics,

facing up, so the vapor retarder would protect the insulation from the high latent load in the attics.

Otherwise, the best I could do, was to install it with the vapor retarder facing the exterior, using gloves.

The typical batt insulation purchased at a store in Miami. The label does not say which side the facing should go. The Building Code neither.

Air Leakage.

The combination of high temperature and humidity experienced in the hot and humid climate this time

of a year puts the mechanism of water vapor diffusion in focus and emphasizes the importance of the

proper vapor retarder design; exemplified on numerous lessons learned. This is why this article was

written. However, ventilation is a much more potent source of moisture. The uncontrolled ventilation,

or simply air leakage can dump gallons of water inside a building in a single minute (think open doors

after rain), while the diffusion gradually deposits ounces of moisture. We will explore the subject of air

leakage in the future articles.

The subject of vapor retarders in the South is explored at more depth in the "Hot and Humid" seminar available on DVD.

Documents

Vapor Retarders in the South