building science- physics of heat

8/8/2019 Building Science- Physics of Heat

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Thermal performance of buildings

This session provides a much neededanalysis of the thermal behaviour of buildings, considering the means bywhich heat flows into and out of astructure as well as a summary of

relevant control mechanisms..


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What does this mean to ArchitectsImproving thermal performance of buildings

through energy efficient design is animportant program to reduce greenhousegas emissions and global climate change

towards sustainable design.In other words:

Through design, reduce the amount of energy used to achieve

comfortable levels of temperature and humidity, and adequatelevels of lighting in buildings.

(Australian Building Codes Board, 2001)


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Thermal performance of buildingsUnder all situations, heat flows either

into or out of a building.

ConductionConductionConductionConduction

ConvectionConvectionConvectionConvection

RadiationRadiationRadiationRadiation


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What is Conduction ? Heat energy travels through/transfer between

bodies in direct contact. Heated excited molecules bump into and

transfer some of their energy into adjacent,cooler ones.

The faster the rate of heat flow, or molecular interaction at a given temperature through amaterial, the higher the conductivity.


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What is Convection ? Heat is transferred by the bodily movement of

a carrying medium (gas or liquid). Movementmaybe self-generating due to temperaturedifferences or propelled by an applied force.

The rate of heat transfer in convectiondepends on:

Temperature difference The rate of movement of carrying medium

Specific heat of the carrying medium.


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What is Radiation ? Heat energy transmitted from the source

through the space to the bodily contactwithout a medium.

Radiant energy is transmitted aselectromagnetic waves.

Rate of heat flow depends on: Temperature of emitting and receiving surfaces. Certain qualities of these surfaces.

Radiation received by a surface can be partlyabsorbed and partly reflected (a + r =1).


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What is Radiation ? Light coloured, smooth & shiny surfaces

tend to have higher reflectance. A perfect reflective surface : r=1, a=0 A perfect absorber (black surface : r=0,

a=1 Measurement of radiation = W/m2.


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Thermal balance

Heat may be lost outwards flow, or Heat may be gained inwards flow.

The net result may be either too hot, toocold, or just right.


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Heat Flow

Heat energy tends to distribute itself evenly until a perfectly diffused uniformthermal field is achieved.

It tends to flow from high temperature tolower temperature zones by conduction,convection and radiation.

The greater the temperature difference

the faster the rate of heat flow.


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Heat Flow RateHeat Flow RateHeat Flow RateHeat Flow Rate

It is equivalent to Power the ability to carry outcertain work (energy to

carry out the work) inunit time.

J/s = Watt (can bemeasured in kilowatt(1kW = 1000 W)

Density Of Heat FlowDensity Of Heat FlowDensity Of Heat FlowDensity Of Heat Flow

RateRateRateRate

In heat calculations theterm is Intensity (whichis heat flow in relation tounit area) is used.

I = W/m2


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Modifications to adjust thermal comfort

If it is likely to be too hot or too cold, then:

Design the building to control heatflow, andandandand

Add heating or cooling to modify

temperature.


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Three basic considerations of design tocontrol heat flow

Consider thermal properties of materials(insulation or heat storage)

Consider solar radiation (shading) Consider airtightness (ventilation)


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Thermal properties of materials

Insulation

Thermal mass heat storage


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Insulation

Insulation is the use of a material with alow overall conductance to reduce theenergy flow across another material.

The insulation acts to retard and/or

reduce the flow of heat, thus it musthave a high resistance (resistancebeing the inverse of conductance).


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Thermal Mass Heat Storage

Solid masselements such as

concrete, solid brick,stone, earth,rammed earth,absorb and releaseheat slowly.

The effect is tostabilize the effectsof diurnaltemperaturechanges.


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Airtightness and Ventilation

Heat may be lost (exfiltration), or gained(infiltration)

Control of airflow in and out of abuilding is an essential design

consideration.- Window design,- vent and opening location- avoid uncontrollable gaps and cracks


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Conductivity, k-value (M&E Book pg 179) In order to calculate heat transfer and to

compare different materials it is necessary toquantify just how well a material conductsheat.

k = rate of heat flow in watts across athickness of 1m for a temperature differenceof 1 degree C.

Or a measure of the rate at which heat isconducted through a particular material under specified conditions

Unit measurement is W/m deg C. The lower the k-value, the better the

insulation (good insulator = 0.03 W/m deg.C) (tables M&E page 1547 and Metric Handbook page 38-3)


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Resistance (R) and Resistivity

R = t / k R = t / k R = t / k R = t / k mmmm 2222 C/WC/WC/WC/W

where: R is the resistance of the material (mC/W),t is the thickness of the material (m), andk is the conductivity of the material (W/mC).

Reciprocal of conductivity is resistivity = 1/k Resistivity is a material property and refers to

that material's ability to resist the flow of heat Better insulators will have higher resistivity

values


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Conductance, C

C = is the heat flow rate through unit area of body(density of heat flow rate) when the temperature

difference between 2 surfaces is 1 degree C.C =C =C =C = 1111 ==== 1111 = W/ m2 deg C= W/ m2 deg C= W/ m2 deg C= W/ m2 deg CRRRR m2 deg Cm2 deg Cm2 deg Cm2 deg C

WWWW

Conductivity values are more readily available for

most building materials than resistivity Conductance unit is - (W/mC ). Conductance is the inverse of resistance,

C =C =C =C = 1111 ==== kkkk W/mW/mW/mW/mCCCCR tR tR tR t


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R t-value and U-value Resistance is usually given as an "R" value

which is given as the resistance of one squaremetre of the structural element subject to aone degree temperature difference. Rincludes surface air resistances.

RRRR tttt ==== RRRR sosososo ++++ RRRR nnnn ++++ RRRR sisisisi mmmm2222C/WC/WC/WC/W

The U-Value is the overall heat transfer property of a structural element (W/m K) andis the reciprocal of its total resistance.

U =U =U =U = 1 111 W/mW/mW/mW/m CCCCRRRR tttt


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Transmittance, U-value

The reciprocal of the air to air resistance

is the air to air transmittance or U-value(use for heat gain/loss calculation) U = 1/Rt (total thermal resistance) or U

= 1/Rsi+1/R 1 +1/R 2 +.+1/Ra+Rso unit is the same as conductance ie.

W/m2

deg C except that the differenceis the air temperature difference and notthe surface temperature difference.


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U = 1/Rsi+1/R 1+1/R 2+.+1/Ra+Rso

U = U value Rsi= standard inside surface resistance R 1 , R 2 = Resistance of that particular

material Ra = Standard resistance of air space

Rso= standard outside surfaceresistance

Overall heat transfer property


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Transmittance of someconstructions (Book: Manual of Tropical Housingand Building page 287)


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REFERENCES

Introduction to Architectural Science:Steven V Szokolay

Metric Handbook: David Adler Manual of Tropical Housing & Building:

Koenigsberger Mechanical & Electrical Equipment for

Buildings: Benjamin Stein


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Worked example 1

Calculate the U value of a cavity wall with a 105mm thk brickouter leaf, a 75mm unventilated cavity containing 50mmfiberglass quilt then a 100mm light weight concrete block innerleaf with a 15mm layer of light weight plaster.

Thermal conductivity, in W/mK, are: brickwork 0.84, light weightconcrete block 0.19, fibreglass 0.04 and lightweight plaster0.16. The standard thermal resistances, in m2K/W are: outsidesurface 0.055, inside surface 0.123, cavity 0.18


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Layers Thickness

Thermalconductivity

Wm/KResistance

(m2K/W)

outside surface - - standard 0.055

light weight plaster 15mm 0.015 0.16 0.0105/0.16 0.09375

cavity 25mm 0.025 n/a standard 0.18

fibregalss quilt 50mm 0.05 0.04 0.05/0.04 1.250

lighweight concreteblock inner leaf100mm 0.1 0.19 0.1/0.19 0.526

Exposed brickwork 105mm 0.105 0.84 0.105/0.84 0.125

inside surface standard 0.123

Total 2.353

Using U-value = 1 / Rt

= 1/2.353

= 0.43 W/m2K


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Worked Example 2

A certain uninsulated cavity wall has a U-value of 0.91W/m2K. If expanded polyurethane board is included in the construction whatminimum thickness of this board is needed to reduce the U-value to0.45W/m2K? Given that the thermal conductivity of expanded

polyurethene = 0.025W/mK.

Target U value U2 = 0.45

Target Total resistance (1/U) R2 = 1/0.45 = 2.222

Existing U-Value U1 = 0.91

Existing total resistance v(1/U) R1 = 1/0.91

Extra Resistance Required R2 - R1 = 2.222 - 1.099 1.123


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The k-value of the proposed insulating material k= 0.025, so using formula

R = d / k Thickness of material d = R x k

= 1.123 x 0.025= 0.028meters

So minimum thickness of insulating board needed to give 0.45 U-value is 28mm


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Tutorial 1

A cavity wall is constructed as follows brickwork outer leaf 105mm, air gap 25mm, expanded polyurethene

board 25mm, lightweight concrete block inner leaf 100mm, plasterboard 10mm. The relevant values of thermal conductivity, in W/mK, are: brickwork 0.88,polystyrene 0.035, concrete block 0.19, plasterboard0.16. The standard thermal resistances, in m 2 K/W are:outside surface 0.055, inside surface 0.123, air gap 0.18.

a) calculate the U-value of this wallb) calculate the U-value of the same wall sited in a position

of severe exposure for which the outside surfaceresistance is 0.03m 2 K/W

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