Thermal Design Original Material by A.

Hamid Modified by J. Mitchell 10/03

Purpose of Thermal Design? Why do we undertake thermal design?

Write your own reasons We’ll discuss them.

Thermal DesignObjective: Control heat flow to:2. Design Proper HVAC systems

1. Maintain comfortable indoor conditions2. Reduce heating/cooling loads, which reduces

operating costs3. Understand Building Envelope Loads

1. Control vapor movement/condensation2. Design to accommodate contraction/expansion of

building materials and sealant joints

An Example

How does Heat Transfer affect this building?

How we Control Heat Transfer Roof – Insulate & Reflect Walls – Insulate, Reflective, seal gaps,

control vapor movement Windows – Insulate, Emissivity, seal

gaps, control vapor Doors – Insulate, Seal gaps Foundations – Insulate, avoid water

The Mechanisms of Heat Transfer Now we’ll look at how heat actually

moves. This is almost all in Beall – it should be

a review.

How Heat Moves

Cooler areaHeat flow

Temperature

High Pressure Low Pressure

Warmer area

Mass flowPressure

Physical Mechanism There are four ways heat moves Define and explain them to a partner

Conduction Conduction : direct

transfer by contact of solid, liquid or gas

Q=A*Delta-T / R Analog to Electrical

Resistance Current = Voltage/R Circuits follow directly

Linear if constant and in plane. Very complex if have

time variance or complex shapes

Convection Convection: transfer

of heat by the movement of air or water Heat moves with the

mass of fluid. Warmer or colder replaces original.

Complex - Simplify

RadiationNon-Linear

•Highly Complex

•We must simplify

•Do so with “equivalent temperatures” – empirically derived

4TQ

Radiation BehaviorAbsorptance0.25 ---> 0.95Reflectance0.1 ---> 0.95Emittance 0.08 ---> 0.95Transmittance (calculated)Note that Reflection +

emittance + transmittance is equal to 100%

Evaporation Phase change from solid or fluid to gas

Takes energy to do so, thus cools the materials it’s on

Complex calculation, dependent on temperature, Rh, material properties, air flow etc.

Usually simplify – look at long term.

Simplifications of Complexity Simplify to linear behavior Consider one-dimensional situation Ignore time variation Use “effective” properties

Emperically derived simplifications Convective behavior converted to R Surface Temperature Air gap behavior

More Complex 3-D complexity – usually try to ignore it

Beall does deal with it slightly with ties analysis Time effects

Daily Sun motion

Yearly Sun Path variation Shading variation from vegetation

Material Properties Thermal Mass Time variation – degradation Temperature & Humidity variation

Calculations Wall Gradient Calculator Spreadsheet

Thermal gradient across a wall Saturation vapor pressure across a wall Actual vapor pressure across a wall Joint width necessary to address component

movements and construction tolerances. Thermal Bridging Wind pressure on a wall in both PSF and inches of

water

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.

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.

Insulation Low density material Many types

Beall Describes them

Major Insulation Types1. 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)

Effect of Moisture Content on Thermal resistance

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

Recommended Minimum Thermal resistance of BE in the US

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

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).

Better yet, calculate it.

Below Grade Soil Temperature

Varies daily 1-2’ Varies yearly 20-30’

Take 3D Effects into account Water is critical issue