Example

Calculate the heat loss for 10,000 square feet of wall with 4-inch face brick, R-11 insulation and 5/8-inch sheet rock when the outdoor temperature is 20°F and the indoor temperature is 70°F.

Figure 9.3

The thermal resistance, Rt, is the sum of the individual resistances of the various layers of material which comprise the envelope component. Rt is calculated by adding them up as follows:

Figure 9.3

In the ASHRAE Handbook we find that a conservative resistance for brick is 0.10 per inch. Four inches of brick would therefore have a resistance of 0.40. Sheet rock (called gypsum board by ASHRAE) has a resistance of 0.90 per inch, which would be 0.56 for 5/8-inch sheet rock. Batt insulation with a rating of R-11 will have a resistance of 11.0, if expanded to its full rated depth.

The first step is to add all the resistances.

R1, R2 and Rn represent the thermal resistance of each of the elements in the path of the "heat flow." The thermal resistance of common construction materials can be obtained from the ASHRAE Handbook of Fundamentals. Other physical phenomenon, such as convection and radiation, are typically included as well. For instance, free and forced convection are treated as another form of resistance to heat transfer, and the "resistance" values are tabulated in the ASHRAE Fundamentals Manual for various surface orientations and wind velocities. For example, the outdoor resistance due to forced convection (winter) is usually taken as 0.17 hr/(Btu-ft2-°F) and the indoor resistance due to free

Outdoor Air Film (15 m.p.h.) 4-inch Face Brick R-11 Batt Insulation 5/8-inch Gypsum Board Indoor Air Film (still air)

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