5. Thermal DesignObjective: Control heat flow to:2. Maintain comfortable indoor

conditions3. Reduce heating/cooling loads, which

reduces operating costs4. Control vapor

movement/condensation5. Design to accommodate

contraction/expansion of building materials and sealant joints

Heat Flow

Warmer area Cooler areaHeat flow

Temp gradientT1

T2

T

Factors affecting thermal energy flow:

2. Solar radiation

3. Air temperature

4. Wind/air movement

5. Humidity

Flow of Thermal Energy Conduction : direct

transfer by contact of solid, liquid or gas

Flow of thermal Energy- Continue

Convection: transfer of heat by the movement of air or water

Flow of thermal Energy- Continue

Radiation: Flow of energy in the form of electromagnatic waves ( light, ultraviolet, infrared heat)

Solar radiation is a function of latitude, time of the year, slope/orientation of the surface

Solar EffectAbsorptance0.25 ---> 0.95Reflectance0.1 ---> 0.95Emittance 0.08 ---> 0.95Transmittance (calculated)Note that Reflection +

emittance + transmittance is equal to 100%

Thermal Performance Factors affecting thermal performance:

Air space Thermal mass Thermal resistance

Thermal Mass Heat migrates through solid materials

from the hot side to the cooler side. The time delay involving absorption of the heat is called thermal lag

The amount of energy necessary to raise material temp is proportional to the wt of the material

Thermal Mass- Continue

Heavy materials like concrete and masonry absorb and store a significant amount of heat and substantially retard its migration. This characteristic is called thermal storage capacity. It affects the rate of conductive heat transfer and is a critical consideration in passive solar heating and cooling strategies

Thermal ResistanceThermal conductance, C, is the time

rate of heat flow through ft2 of area at a temp difference of 1°F for a specific thickness of material.

Thermal conductivity, k, is the conductance for a standard unit thickness.

Thermal Resistance- continued

C = k n

R = l c

Btu/h.ft2.°F

Btu/h.ft2.°F.in.

Thickness in inches

Thermal resistance °F/Btu/h.ft2

1/R range 0.2 ---> 8.0 (per t = 1n)

The thermal efficiency of a building’s component/assembly are normally judged by its accumulative thermal resistance.

RT = ? Ri of each layer

U-valueOverall heat transfer coefficient:U = I thermal transmittance

RT Btu/h.ft2.°F It is useful in determining the overall thermal

performance of a bldg envelope that includes different construction assemblies in parallel heat flow.

Some bldg energy codes give allowable u-value based on a weighted average for the coefficients for opaque walls, windows (fenestration) and doors.

Thermal Gradient

Effect of Insulation on Thermal Gradient

Caculation of Thermal Gradient

Effect of Thermal Inertia

Effect of Thermal Inertia- Continue

Effect of Moisture Content on Thermal resistance

Thermal EnergyThermal energy can flow from one object to another by:2. Conduction - direct transfer by contact of solid, liquid or gas.3. Convection - transfer of heat by the movement of air or

water.1. Natural convection - movement of fluid air by difference in

temp/density.2. Forced convection - movement of fluid by a pump or fan.

4. Radiation - flow of energy in the form of electromagnetic waves (light, ultraviolet, infrared heat).

1. Direct2. Diffuse3. Reflected (see figure)

Solar radiation is a function of latitude, time of the year, slope/orientation of the surface.

6. Evaporating - function of temp, RH, air movement.

Factors affecting thermal performanceAir space - affects both conductive and

convective heat flow.

Wide/long air space ---> reduce conductive HT.

Narrow air space ---> reduce convective HT.

For LV4” air space ---> diminish convective heat flow and increases conductive heat flow.

Factors affecting thermal performance- continued

Mass Heat migrates through solid materials from the hot

side to the cooler side. The time of delay involving absorption of the heat is called thermal log.

The amount of energy necessary to raise material temp is proportional to the wt of the material.

Heave materials like concrete and masonry absorb and store a significant amount of heat and substantially retard its migration. This characteristic is called thermal storage capacity. It affects the rate of conductive heat transfer and is a critical consideration in passive solar heating and cooling strategies.

U-value continuedU0 = (UwAw)+(UfAf)+(UdAd)

Aw+Af+Ad

Overall heat gain/loss per ft2 = U0 (t0 - ti) Where t0 - ti is the Temperature difference is between outside and

inside.

Af

Aw

Ad

Types of Insulation1. Loose

(fibers,chips) - fill insulation (poured, blown)

2. Flexible and semi-rigid (batt, blanket)

3. Rigid (wood, fiberglass board)

4. Formed-in-place (urethane foam)

Effects of InsulationImprove the thermal performances of building walls and

roofs by reducing both conductive heat flow through the section and corrective heat flow in air spaces:

2. Results in more comfortable indoor air temp and less fluctuation

3. Reduces cooling/heating loads

Thermal efficiency of insulation depends on:6. Thermal resistance R7. Stability over time (R value dimensional stability)8. Resistance to deterioration9. Securing attachments

Effect of Insulation- Examples

Effect of Thermal Insulation- Continue

Effects of Thermal BridgingA thermal bridge occurs when a subject of high

thermal conductivity penetrates a material of low thermal conductivity (insulation) increasing the rate of heat flow at the penetration.

To account for thermal bridging correction factors (<1.0) should be used. Example 1 - Use table 3.9 in text for correction of R value (0.5 ---> 0.38).

Thermal Bridging- Examples

Insulation RequirementsState and local codes specify

requirements for minimum thermal resistance of bldg components to save energy, see map.

Recommended Minimum Thermal resistance of BE in the US

Loss of Thermal ResistanceIt is recommended to have TRR be greater than

80%. Less than 80% insulation is considered wet. See table 3.13 in text.

Thermal Resistance Ratio:TRR = wet thermal resistivity

dry thermal resistivity