What U-value is a window?

Single glazing 5.7; double 2.8; low-e double with thermal break 1.4–1.8 W/m²K.

What air change rate?

0.3–0.6 ACH for tight homes; 1 or more for older houses.

What outdoor design temperature?

The ASHRAE 99% heating design temperature for your location.

How do I get a wall's U-value?

U = 1 ÷ total R of all layers including air films.

What is infiltration?

Uncontrolled air leaking through gaps, outlets and windows.

BTU heat load calculator

Areas and U-values of walls, windows and roof, volume, air changes and temperatures: you get heat loss by component and the total heating load.

Total heat loss

3.62 kW

Total heat loss3.62 kW
Wall losses1.06 kW
Window losses0.74 kW
Roof / ceiling losses0.79 kW
Ventilation and infiltration losses1.03 kW

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How it's calculated (visible formula)

Q = Σ U × A × ΔT + 0.34 × ACH × volume × ΔT (W); 1 W = 3.412 BTU/h

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Measure wall, window and roof areas exposed to outside.
Assign each a U-value.
Set indoor and outdoor design temperatures.
Transmission loss = U × A × ΔT.
Add ventilation and infiltration losses.

Worked example: 970 sq ft home, ΔT 40 °F (22 °C)

Walls: 80 m² × 0.6 × 22 = 1,056 W
Windows: 739 W; roof: 792 W
Air: 0.34 × 0.6 × 230 × 22 = 1,032 W

→ 3.62 kW (about 12,400 BTU/h) of heat loss.

Common mistakes

Using the insulation U-value instead of the whole wall assembly.
Ignoring infiltration around old windows.
Using average winter temperature instead of design temperature.

Pro tips

Use whole-assembly U-values, not just the insulation.
Add 10–15% for thermal bridges in older homes.
Windows are often the weakest link.
Include slab-on-grade losses with an equivalent U.

From calculation to quote

Add labour, overhead and your margin, and generate the professional PDF.

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FAQs

How do I convert R-value to U?: U (W/m²K) = 5.678 ÷ R (US).
What U-value is a window?: Single glazing 5.7; double 2.8; low-e double with thermal break 1.4–1.8 W/m²K.
What air change rate?: 0.3–0.6 ACH for tight homes; 1 or more for older houses.
What outdoor design temperature?: The ASHRAE 99% heating design temperature for your location.
How do I get a wall's U-value?: U = 1 ÷ total R of all layers including air films.
What is infiltration?: Uncontrolled air leaking through gaps, outlets and windows.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.