HEAT LOSS
About this calculator
This heat loss calculator computes the conductive heat loss through a building envelope using Q = U × A × ΔT per surface, where U-value is the inverse of R-value. Enter the area and U-value for walls, windows, ceiling/roof, and floor, then the indoor design temp and outdoor 99% design temperature for your location. Add infiltration losses (typical 0.35 ACH × volume × 0.018) for a full envelope figure. The result is BTU/hr at design conditions — multiply by hours of heating season for annual energy. ESTIMATE ONLY — full Manual J adds duct losses, internal gains, and solar to get installed equipment size.
How to use this calculator
Enter each surface's area and R-value: walls (above grade, minus windows/doors), window/door glass area with U-value (single pane = 1.0, double low-E = 0.30, triple = 0.18), ceiling/roof area with R-value, and floor over unconditioned space (slab on grade or basement on conditioned space = 0).
Set your indoor design temp (typically 70°F) and the 99% outdoor design temp from ACCA Manual J Table 1 — the temperature your area is colder than only 1% of winter hours. Chicago: -3°F, NYC: 11°F, Atlanta: 23°F. The calculator returns BTU/hr loss per surface and total — your conductive heating load at design conditions.
Worked example
For a 2,000 ft² house in Chicago (99% outdoor = -3°F, indoor = 70°F, ΔT = 73°F): walls 1,600 ft² at R-19, windows 240 ft² at U=0.32, ceiling 1,500 ft² at R-38, floor 1,500 ft² at R-19 (over unheated crawlspace).
Wall loss: (1,600 ÷ 19) × 73 = 6,147 BTU/hr. Window loss: 240 × 0.32 × 73 = 5,606 BTU/hr. Ceiling loss: (1,500 ÷ 38) × 73 = 2,882 BTU/hr. Floor loss: (1,500 ÷ 19) × 73 = 5,763 BTU/hr.
Total conductive loss: 20,398 BTU/hr ≈ 1.7 tons.
Notice windows are 28% of total loss despite being only 6% of envelope area — they're the biggest single line item per ft². Upgrading windows to U=0.18 (triple pane) drops window loss to 3,154 — saves 2,452 BTU/hr.
Add infiltration (~30% of conductive for typical homes) to estimate full envelope load: 20,398 × 1.3 = 26,517 BTU/hr ≈ 2.2 tons heating equipment needed at design conditions.
Common mistakes & waste factors
Using R-value as U-value. R = 19 means U = 1/19 = 0.053. Mixing them up gives nonsensical results.
Forgetting infiltration. Conductive loss is only the heat moving through walls. Air leaks through cracks, outlets, attic hatches add 20-50% on top — old homes can hit 100%+.
Using a milder design temp than your area's 99%. The point of design temp is to size for the cold weather you actually see. Using an annual-average temp undersizes the system 2-3×.
Ignoring duct losses for ducted systems. Ducts in unconditioned attics or crawlspaces lose 15-30% of heat — adds another big bucket on top of envelope loss.
Rules of thumb
Q = U × A × ΔT (or A/R × ΔT for R-value).
U = 1/R for walls, ceilings, floors. Windows are quoted directly in U.
Window U: single 1.0, double 0.40, double low-E 0.30, triple 0.18.
Wall R: 2x4 batt 13, 2x6 batt 19-21, sprayfoam 6/inch (R-23 for 2x6).
Ceiling R: code minimum 30-60 by climate zone. R-38 is the typical residential default.
Infiltration: ~30% of conductive for code-built homes; 50%+ for pre-1980 leaky homes.
Duct losses: 15-30% of envelope load for ducts in unconditioned space.
For Manual J equipment sizing, total envelope + infiltration + duct + internal gains.