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Glossary

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A I B I C I D I E I F I G I H I I I J I K I L I M I N

O I P I Q I R I S I T I U I V I W I X I Y I Z

Thermal conductance (C)

The rate of heat flow through a defined thickness of a homogenous material or a specific nonhomogenous object, such as a concrete masonry unit (CMU).

I-P units: Btu/(h•ft²•°F)    SI units: W/(m² °C)

Thermal conductivity (k)

The rate of heat flow through a unit thickness of a homogenous material.

I-P units: Btu-in/(h•ft²•°F)    SI units: W/(m °C)

Thermal resistance (R)

A measure of resistance to heat flow; the reciprocal of conductivity (or conductance). Essentially the force required to cause a unit flow of heat.

I-P units: (h•ft²•°F)/Btu    SI units: (m² °C)/W

Thermal resistivity (r)

The reciprocal of thermal conductivity (1/k).

Thermal storage capacity

The product of density, specific heat, and conductivity.

I-P units: Btu²/(h•ft⁴•°F²)    SI units: W²/(m⁴ °C²)

Thermal storage wall

A passive solar heating strategy consisting of of south-facing wall composed of thermally-massive materials. In the case of the Trombé-wall (TW) strategy this mass is a type of heavy masonry. In the case of water-wall (WW) strategy, of course, water-filled containers constitute the thermally-massive wall. The exterior of the south-facing mass is glazed to admit incident solar radiation and reduce night heat losses.

Thermal transmittance (U)

Also known as the overall coefficient of heat flow, U-factor, and U-value, thermal transmittance refers to the rate of heat flow through an assembly (window, wall, etc.) per unit of time per unit of area, bounded by air on both sides. It includes the effects of all materials, air films, and air spaces.

I-P units: Btu/(h•ft²•°F)    SI units: W/(m² °C)

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Definitions for C, k, R, and U were reproduced from:

Stein, B., Reynolds, J.S., Kwok, A., Grondzik, W., 2006. Mechanical and
Electrical Equipment for Buildings, 10th ed. John Wiley and Sons Inc., New York.

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