How to Calculate Heat Loss: Step-by-Step Walkthrough
What You Need Before You Start
- Tape measure for room dimensions
- Your local ASHRAE 99% winter design temperature
- U-values for each material (walls, windows, roof, floor)
- An ACH estimate: 0.5 tight, 1.0 average, 2.0 leaky construction
Step 1 — Measure Each Surface Area
Walls = perimeter × ceiling height − window area. Windows = glazed area. Roof and floor = building footprint. Record everything in square feet.
Step 2 — Look Up U-Values
| Element | U-value |
|---|---|
| R-13 frame wall | 0.077 |
| R-19 frame wall | 0.053 |
| Double-pane window | 0.50 |
| Single-pane window | 1.10 |
| Attic R-38 | 0.026 |
| Floor / basement | 0.04 |
Step 3 — Find Your Design Temperature (ΔT)
ΔT = 70°F − local 99% winter design temperature. Examples: New York City 14°F → ΔT 56; Chicago 3°F → ΔT 67; Miami 42°F → ΔT 28.
Step 4 — Calculate Each Component
For each surface: Q = U × A × ΔT. Compute walls, windows, roof, and floor separately, then add them.
Step 5 — Add Infiltration
Qinf = 0.018 × ACH × Volume × ΔT, where Volume = length × width × height in cubic feet.
Worked Example: A 1,500 sq ft Home
| Surface | Area (ft²) | U | ΔT | Q (BTU/h) |
|---|---|---|---|---|
| Walls | 1,200 | 0.077 | 56 | 5,178 |
| Windows | 180 | 0.50 | 56 | 5,040 |
| Roof (R-38) | 1,500 | 0.026 | 56 | 2,184 |
| Floor | 1,500 | 0.040 | 56 | 3,360 |
| Infiltration (0.7 ACH) | Vol 12,000 | — | 56 | 8,467 |
| Total | — | — | — | 24,229 |
Mistakes That Skew Your Result
- Counting basement floors as outdoor-exposed when they are not
- Using the average temperature instead of the 99% design temperature
- Guessing ACH instead of measuring with a blower door
- Ignoring thermal bridges (window frames, rim joists, steel beams)
Frequently Asked Questions
How do you calculate heat loss for a building?
Measure each surface area, multiply by its U-value and the design temperature difference (ΔT), sum all surfaces, then add infiltration (0.018 × ACH × volume × ΔT). The total in BTU/h is the heating loss.
What is the formula for heat loss through a wall?
Qwall = Uwall × Awall × ΔT. For an R-13 wall (U=0.077) of 200 ft² at ΔT 56°F: 0.077 × 200 × 56 ≈ 862 BTU/h.
How much heat loss is normal for a house?
A reasonably insulated home runs about 20–30 BTU/h per ft² of floor area. Our 1,500 ft² example at 24,229 BTU/h is ~16 BTU/h per ft² — tight, because it assumes good insulation and low ACH.
Does insulation reduce heat loss?
Yes. Insulation lowers the U-value, so Q = U × A × ΔT drops directly. Going from R-13 to R-19 walls cuts wall loss by about 31%.