Why calculate heat loss
A house loses heat through every surface that separates warm inside air from cold outside air, and through the outside air that leaks in. On the coldest days, the heating system must replace all of that loss. Knowing the design heat loss helps you size a furnace or heat pump correctly and shows which improvements will save the most energy.
The formula
For each building element:
Heat loss = U × A × ΔT
- U is the U-factor in BTU/h·ft²·°F, the inverse of the assembly’s total R-value.
- A is the area in square feet.
- ΔT is the design temperature difference between inside and outside.
Air leakage adds:
Infiltration loss = 0.018 × volume × ACH × ΔT
where 0.018 BTU/ft³·°F is the heat capacity of air, the volume is floor area times ceiling height and ACH is the number of air changes per hour.
Worked example
A 1,500 sq ft single-story house with 8 ft ceilings, a 60 °F design difference and 0.5 air changes per hour:
- Walls: 1,200 sq ft × 0.06 = 72 BTU/h·°F
- Windows and doors: 180 sq ft × 0.30 = 54
- Attic ceiling: 1,500 sq ft × 0.03 = 45
- Floor: 1,500 sq ft × 0.05 = 75
- Air leakage: 0.018 × 12,000 ft³ × 0.5 = 108
The total coefficient is 354 BTU/h·°F. Multiplied by 60 °F, the heat loss is 21,240 BTU/h, about 6.2 kW, or 14.2 BTU/h per sq ft. Air leakage accounts for 31% of it, a sign that air sealing would pay off.
Choosing the inputs
- U-factors: a 2×6 wall with R-19 batts is about 0.06 overall; an R-38 attic about 0.03; ENERGY STAR windows 0.30 or less; old single-pane windows about 1.0.
- ACH: a tight new house may be 0.2 to 0.35, an average house 0.5, an old leaky house 1.0 or more. A blower-door test measures it.
- ΔT: indoor temperature (often 70 °F) minus your local 99% heating design temperature, which ranges from above 30 °F along the Gulf Coast to −10 °F or lower in the northern states.
From heat loss to equipment
A furnace or heat pump should cover the design heat loss without a large margin. Oversized equipment cycles on and off, wastes energy and wears out faster. Heat pumps lose capacity as it gets colder, so check the manufacturer’s output at your design temperature.
This is a simplified estimate. A full ACCA Manual J calculation, required by many codes for new equipment, also accounts for duct losses, basement and slab edges, thermal bridges and internal gains. Floors over a crawl space or on a slab lose less than the full ΔT suggests, so the example is slightly conservative.