Envelope Components

We have seen from Equation (9.1) that the heat loss through a component, such as a wall, is proportional to the area of the component, the indoor-outdoor temperature difference, and U, the proportionality constant which describes the temperature-dependent heat transfer through that component. U, currently described as the "U-factor" by ASHRAE2, is the reciprocal of the total thermal resistance of the component of interest. If the thermal resistance of the component is known, U can be calculated by dividing the total thermal resistance into "1" as shown:

convection of a vertical surface is usually taken to be 0.68 hr/(Btu-ft2-°F).

To calculate the overall U-factor, one typically draws a cross-sectional sketch of the building component of interest, assigns resistance values to the various material layers, sums the resistances and uses the reciprocal of that sum to represent U. The total calculation of a wall U-factor is demonstrated in the example below.

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