Abstract
This paper presents an economic model for calculating the optimum thermal resistance for roof insulation. It applies primarily to reroofing work, because it does not account for heating and cooling equip ment cost savings attributable to increased insulation.
The economic model uses current costs and heat content of fuels, heat ing and cooling degree days, and the installed cost of roof insulation per unit of thermal resistance. It also shows the economic penalty for deviating from the calculated optimum insulation.
Optimum thermal resistance is calculated for twenty representative cities in the United States, and is compared with ASHRAE Standard 90-75 for insulation in commercial and high rise residential buildings.
Optimum thermal resistance calculated with natural gas as the fuel is lower than called for by the ASHRAE standard; the calculated optimum thermal resistance using electricity or oil is about the same as the ASHRAE standards. The scatter of the optimum thermal resistance calculated for each city shows that the thermal resistance should be calculated using local data rather than average values.
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