Window Heat Loss Calculator (U-Factor)
Calculate heat loss through a window using U-factor, window area, and indoor/outdoor temperature difference.
Useful for energy audits and window upgrades.
What Is Window U-Factor?
The U-factor (or U-value) measures how well a window prevents heat from escaping. It is the rate of heat transfer per unit area per degree of temperature difference.
Lower U-factor = better insulation = less heat loss.
This is the opposite of R-value (which measures insulation resistance). U-factor = 1 ÷ R-value.
Heat Loss Formula
Heat Loss (BTU/hr) = U-Factor × Area (ft²) × ΔT (°F)
Heat Loss (Watts) = U-Factor (W/m²·K) × Area (m²) × ΔT (°C)
Where ΔT is the difference between indoor and outdoor temperature.
Typical U-Factor Values
| Window Type | U-Factor (Imperial, BTU/hr·ft²·°F) | U-Factor (SI, W/m²·K) |
|---|---|---|
| Single pane, clear glass | 1.10 | 6.24 |
| Single pane with storm window | 0.50 | 2.84 |
| Double pane, clear glass | 0.49 | 2.78 |
| Double pane, Low-E coating | 0.32 | 1.82 |
| Triple pane, Low-E | 0.20 | 1.14 |
| Best available (triple, gas fill) | 0.10 | 0.57 |
ENERGY STAR requirement for most US climate zones: U-factor of 0.30 or lower.
Practical Example (Imperial)
A 24 sq ft double-pane Low-E window (U = 0.32) on a day where it is 70°F inside and 20°F outside:
- ΔT = 70 − 20 = 50°F
- Heat loss = 0.32 × 24 × 50 = 384 BTU/hr
Compare to a single-pane window (U = 1.10):
- Heat loss = 1.10 × 24 × 50 = 1,320 BTU/hr — 3.4× more heat loss
Practical Example (Metric)
A 2.5 m² double-pane Low-E window (U = 1.82 W/m²·K), indoor 21°C, outdoor −5°C:
- ΔT = 21 − (−5) = 26°C
- Heat loss = 1.82 × 2.5 × 26 = 118 Watts
Over a heating season, upgrading windows can save hundreds of dollars in energy costs.