Estimate conductive heat loss through a home's envelope
Heat escapes through exterior walls, the insulated ceiling, windows and doors whenever indoor air is warmer than the surroundings. For each surface, the simplest steady-state model multiplies its area by heat conductance and the temperature difference. The calculation uses effective assembly R-values for insulated walls and ceilings, and whole-unit U-factors for windows and doors. R and U have an inverse relationship when measured in compatible imperial units: lower U or higher R means less heat transfer at the same area and temperature difference.
The four surface-area fields should describe distinct areas. When entering opaque wall square footage, subtract the window and exterior door areas that are already counted separately, or the model will include those openings twice. For insulated ceilings below an attic, use the relevant ceiling boundary rather than total roof slope unless the thermal enclosure follows the roof deck. Manufacturer window U-factor ratings must apply to the full window assembly, not merely the center of the glass.
Building-envelope heat-loss formulas
Window or door heat loss = area × whole-unit U × temperature difference (BTU/h)
Optional sensible infiltration loss ≈ 0.018 × indoor cubic feet × air changes per hour × °F
Modeled total = wall + ceiling + window + door + optional air-exchange heat loss
Worked example: an insulated house on a cold day
Consider a house with 1,200 square feet of net walls at R-13, 900 square feet of ceiling at R-38, 180 square feet of windows with U-0.30, and 40 square feet of doors with U-0.50. With an indoor-outdoor temperature difference of 40°F, wall conduction is about 3,692 BTU/h, ceiling conduction about 947, window conduction 2,160 and door conduction 800. These figures show why window performance and exposed area matter alongside nominal insulation levels.
For a conditioned volume of 7,200 cubic feet and an assumed 0.35 air changes per hour, the optional simplified sensible air-exchange term adds about 1,814 BTU/h. The modeled total is approximately 9,414 BTU/h. This is not a furnace or heat-pump size recommendation; real design-load calculations include spatial rooms, thermal bridging, ventilation equipment, duct distribution and additional loads that are not represented here.
When a simple envelope model is insufficient
Effective R-values differ from package labels. A wall rated R-13 based on cavity insulation alone also contains framing members, junctions, sheathing and air pathways. Continuous exterior insulation, wood or steel stud spacing and workmanship change whole-wall performance. Similarly, attic insulation performance can be weakened by uncovered corners, compressed batts and air leakage. Using an effective assembly figure gives more meaningful comparisons than inserting a product-only insulation rating without qualification.
Air changes per hour are not automatically equivalent to a blower-door ACH50 test result. ACH50 is measured under an artificial pressure difference and cannot be entered as a normal infiltration rate without an appropriate conversion method. Infiltration also varies with wind and stack effect, while a balanced mechanical ventilation system introduces separate heating considerations. Use the air-exchange field only when you have a defensible operating assumption; entering zero leaves the simple conductive calculation intact. For equipment sizing, use an appropriate room-by-room design method and a qualified HVAC professional.
Home heat-loss questions
Should window area be included in the wall square footage?
No. Enter opaque exterior wall area after subtracting windows and doors that are described in their own fields. Otherwise heat through those openings will be counted once as if it traveled through insulated wall and again at the actual window or door U-factor.
Can I use a blower-door ACH50 number for air changes per hour?
Not directly. ACH50 describes building leakage during a standardized depressurization test, and natural operating air exchange is usually much lower and weather-dependent. The optional field expects an assumption for actual hourly air exchange under the modeled conditions, not the raw pressure-test result.
Is the BTU per hour total the correct furnace capacity?
No. It is a limited heat-loss estimate at one specified temperature difference. Heat-pump and furnace design requires appropriate weather design temperatures, room distribution, duct effects, ventilation and performance at design conditions, as well as specialized sizing procedures.