Thermal Conductance

8/12/2019 Thermal Conductance

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thermal conductanceA measure of the ability of a material to transfer heat per unit time, given one unit area of the material anda temperature gradient through the thickness of the material. It is measured in watts per meter per degreeKelvin.

Thermal resistanceis a heat property and a measurement of a temperature difference by which anobject or material resists aheat flow(heat per time unit or thermal resistance). Thermal resistance is

thereciprocalofthermal conductance.

Inphysics,thermal conductivity(often denoted k, , or ) is thepropertyof a material toconductheat.It

is evaluated primarily in terms ofFourier's Lawforheat conduction.

Heat transfer occurs at a higher rate across materials of high thermal conductivity than across materials

of low thermal conductivity. Correspondingly materials of high thermal conductivity are widely used

inheat sinkapplications and materials of low thermal conductivity are used asthermal insulation.Thermal

conductivity of materials is temperature dependent. The reciprocal of thermal conductivity is called

thermal resistivity.

The R-valueis a measure ofthermal resistance[1]

used in the building andconstructionindustry. Under

uniform conditions it is the ratio of the temperature difference across an insulator and theheat flux(heat

transfer per unit area per unit time, ) through it or . The R-value being discussed is

the unit thermal resistance. This is used for a unit value of any particular material. It is expressed as the

thickness of the material divided by thethermal conductivity.For the thermal resistance of an entire

section of material, instead of the unit resistance, divide the unit thermal resistance by the area of the

material. For example, if you have the unit thermal resistance of a wall, divide it by the cross-sectional

area of the depth of the wall to compute the thermal resistance. The unit thermal conductance of a

material is denoted as C and is the reciprocal of the unit thermal resistance. This can also be called theunit surface conductance, commonly denoted by h.

[2]The higher the number, the better thebuilding

insulation's effectiveness. R-value is thereciprocalof U-value.

A thermal breakor thermal barrieris an element of low thermal conductivity placed in an assembly to

reduce or prevent the flow of thermal energy between conductive materials.

Inarchitectureandbuilding constructionsome examples include the following:

a thermal break is also a load-bearing thermal insulation system used inreinforced

concretestructures to form a thermal break betweencantileverstructures and internal floor.

Insulated glazingthe air or gas between the panes stops the conductive thermal energy from

passing through the glass.

Metal window or curtain wall framinga separator material is used between the inner and outer

frames to prevent the temperature transfer through the frame andcondensationon the inside frame.

Concrete worka single row ofconcrete masonry units(CMU block) is commonly set between the

inner concrete slab and exterior concrete work to prevent the transfer of heat or cold through the

slab.
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Garage doorsin some doors that have high R-rating insulation, avinylthermal break is used along

the edges of each segment instead of rolled steel.

Metal and wood framed buildings - an insulation material installed on the roof, walls and floor

prevents thermal short circuit creating the heat transfer through the framing material and controls

when desired (winter/summer)resulting in energy savings.

Metal windows and doors - separating the frame into two separate interior and exterior pieces joinedwith a less conductive material reduces temperature transfer. Thermal breaks (made of substantially

rigid, low thermal conductivepolyamide or polyurethanewhich is mechanically locked in aluminum

window framing can be more than a thousand times less conductive than aluminum and a hundred

times less than steel.

Windows and doors - separating the frame into 2 separate interior and exterior pieces joined with a

less conductive material between reduces temperature transfer. In addition thermal breaks can have

the added benefit of reducing sound transmittance by dampening vibration

Infiltrationis the process by which water on the ground surface enters the soil.Infiltration rateinsoil

scienceis a measure of the rate at which soil is able to absorb rainfallorirrigation.It is measured in

inches per hour or millimeters per hour. The rate decreases as the soil becomes saturated. If the

precipitation rate exceeds the infiltration rate,runoffwill usually occur unless there is some physical

barrier. It is related to the saturatedhydraulic conductivityof the near-surface soil. The rate of infiltration

can be measured using aninfiltrometer.

Weatherstrippingis the process of sealing openings such as doors, windows, and trunks from the

elements. The term can also refer to the materials used to carry out such sealing processes. The goal of

weatherstripping is to prevent rain and water from entering entirely or partially and accomplishes this by

either returning or rerouting water. A secondary goal of weatherstripping is to keep interior air in, thus

saving energy on heating and air conditioning.

utomotive weatherstripping is used extensively aboard automobiles, and can be found anywhere the

interior compartment must be sealed from the environment. It must be both functional and cohesive with

the body design of the vehicle. In addition to factors standard to weatherstripping, additional factors must

be considered for vehicles, specifically in the engineering of the parts. For example, the weatherstripping

must function the same while the vehicle is parked and at full speed; be flexible to accommodate motion

vibrations; endure extreme temperatures of hot and cold; withstand long periods of sun exposure; and

resist automotive liquids such as oil, gasoline, and windshield washer fluid (methanol). Weatherstripping

also plays a part in maintaining satisfactory ride quality in the vehicle, being partially responsible for

sealing noise out from the passenger compartment.

Automobile flex when going over bumps, and vibrations cause relative motions between the relatively

fixed body and movable parts like doors, windows, and sunroofs. This movement could allow water in thevehicle so the weatherstrip must compensate by filling the gap. Furthermore, this relative movement can

cause noises such as squeaks, rattles, and creaks to be heard within the vehicle.

Considering a standard four-door vehicle, the doors require 20 feet (6 meters) or more of material per

door, windows require upwards of 10 feet (3 meters), and trunks require large amounts.

Automotive weatherstripping can fail because of age or use.[1]
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Poorly performing weatherstripping should be reported to the car dealership if the vehicle is under

warranty, as fixes may be known.

The efficacy of weatherstripping can be significantly increased by specialty coatings during manufacture.

Coatings for weatherstripping must adhere to all of these weatherstrip materials. Like other paints and

coatings, a large variety of weatherstrip coatings are commonly available, with a large variety of coating

performances. Silicone is the most difficult to adhere to, but at least one coating is commerciallyavailable. After bonding to the weatherstrip, these coatings provide chemical andultravioletresistance,

decrease the static coefficient of friction (thereby reducing the force required to open or close

doors),[2]

and reduce or eliminate noise. In vehicles without coated weatherstripping, the weatherstripping

is much more likely to cause the above-mentioned issues along with others such as rust following

premature failure of the paint. That some cars have non-coated weatherstripping is surprising, given that

the cost of the coating is less than US$1 to US$3 per vehicle, whereas the rubber and steel is tens of

dollars; an entire car set of weatherstripping may be worth US$100 to US$300 in the total cost of the

vehicle, which includes all labor and costs of the manufacturing equipment.

Radiant barriersor reflective barriers inhibitheat transferbythermal radiation.Thermal energy may alsobe transferred viaconductionorconvection,however, and radiant barriers do not necessarily protect

against heat transfer via conduction or convection. To improve the resistance to convection and

conduction, a reflective insulation may be used by having the radiant barrier installed facing a dead air

space or with a layer of traditional insulation like foam, fiberglass, or bubblepack.

How to Decide Between Batt Insulation and

Loose-Fill InsulationWhen it comes to deciding betweenbattinsulationandloose-fillinsulation,its important to keep in mind what you

intend to insulate and what kind of insulation method you want to use. Both batt insulation and loose-fill insulation

work very well you just need to decide which one you prefer.

Loose-fill insulation:Made out of small chunks of fibers, this insulation type is also known as blown

insulation because its installed with a giant blower hose. Loose-fill insulation is also available in bags for filling

gaps.

When loose-fill insulation settles and compacts, you should vacuum out the old and start fresh.

Batt insulation:Made out of insulating fibers that are woven together to create a continuous blanket of material

with a moisture barrier (either paper or foil) that is glued to one side. Batt insulation is available in 16 and 24

inch wide rolls (or 8 foot strips) to fit between the framing in ceilings and walls.
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Thermal Conductance