Heat Loss Calculator

Building heat loss is the steady-state rate at which thermal energy crosses the envelope, and it is dominated by two mechanisms. Conduction through opaque and glazed assemblies follows Q = U x A x Delta-T, where U is thermal transmittance in BTU per hour per square foot per degree F and is simply the reciprocal of the R-value printed on insulation packaging. An R-13 stud wall works out to U = 0.077 and an R-30 attic to U = 0.033, while glazing is far leakier: roughly 1.10 for a single pane, 0.50 for a standard double pane, and 0.25 to 0.30 for triple or low-emissivity units. Delta-T should be a design temperature difference rather than a seasonal average, since ASHRAE tabulates 99 percent and 99.6 percent winter design dry-bulb temperatures by weather station, running near minus 13 degrees F for Minneapolis and near 23 degrees F for Atlanta. The second term, 0.018 x ACH x volume x Delta-T, covers infiltration. The 0.018 constant is the volumetric heat capacity of air at standard conditions, about 0.075 pounds per cubic foot times 0.24 BTU per pound per degree F. Natural air change rates span roughly 0.2 to 0.35 for a tightly built, blower-door tested house up to 1.0 or more in older leaky construction. The main limitations of this model are that it ignores solar gain, internal gains from occupants and appliances (roughly 230 to 400 BTU per hour per person), duct losses, thermal bridging through framing, and wind-driven pressure effects, all of which a full room-by-room load study accounts for.

The mathematics behind the conduction term trace to Joseph Fourier, whose Theorie analytique de la chaleur of 1822 established that heat flux is proportional to the temperature gradient and to a material property, the thermal conductivity. Turning that physics into a usable building quantity was the work of the trade bodies that became ASHRAE: the American Society of Heating and Ventilating Engineers, founded in 1894, and the American Society of Refrigerating Engineers, founded in 1904, which merged in 1959. Their Guide and later Handbook of Fundamentals published tabulated conductivities and assembly transmittances, which is what allowed a designer to look up a U-value rather than measure one. Load calculation nonetheless stayed a rule-of-thumb craft, usually square feet per ton, until the oil shocks of 1973 and 1974 made oversizing expensive. ASHRAE Standard 90-75, issued in 1975, was the first widely adopted energy standard for new building design and the direct ancestor of today's Standard 90.1 and the model energy codes. Over the same period the blower door came into use for measuring envelope leakage directly, replacing guessed air change rates with tested ones. The Air Conditioning Contractors of America codified the room-by-room residential procedure in Manual J, now in its eighth edition, and most North American jurisdictions require it when permitting new heating equipment.