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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