Window Heat Loss Calculator
Calculate how much heat your windows lose and how much you'd save by upgrading to better glazing.
Window Type Comparison
200 sq ft windows, 4,800 HDD (NYC), electric heat at $0.16/kWh
| Window Type | U-Factor | Heat Cost/Yr | vs. Single-Pane |
|---|---|---|---|
| Single-Pane | 1.1 | $1,190 | baseline |
| Double-Pane (clear) | 0.5 | $541 | -$649/yr |
| Double Low-E (ENERGY STAR) | 0.3 | $324 | -$865/yr |
| Premium Double Low-E | 0.27 | $292 | -$898/yr |
| Triple-Pane Low-E | 0.2 | $216 | -$973/yr |
| Premium Triple-Pane | 0.15 | $162 | -$1,027/yr |
Frequently Asked Questions
How much heat is lost through windows?
Windows typically account for 25–30% of residential heating and cooling losses, according to the DOE. A single-pane window has a U-factor of 1.20–1.30 (R-value ~0.9), while a good double-pane Low-E window has a U-factor of 0.25–0.30 (R-value ~3.5–4). Replacing single-pane with double-pane Low-E windows reduces window heat loss by 50–70%.
What is U-factor and how is it different from R-value?
U-factor (or U-value) measures heat transfer rate through a material — lower is better. R-value measures thermal resistance — higher is better. They are reciprocals: U = 1/R. A window with U-factor 0.30 has an R-value of 3.33. Windows are rated by U-factor rather than R-value because they involve different heat transfer mechanisms (radiation, convection, conduction).
Are triple-pane windows worth the extra cost?
In very cold climates (Climate Zones 6–8, e.g. Minnesota, Wisconsin, Canada), triple-pane windows (U-factor 0.15–0.20) can save an additional $50–$150/year per home vs. double-pane Low-E, with payback in 15–30 years. In moderate climates, double-pane Low-E is usually the cost-optimal choice. Triple-pane is most justified for new construction where the incremental cost over double-pane is smaller.
Should I replace windows or add interior storm windows?
Interior storm windows or window insulation film ($5–$20/window) improve U-factor from ~1.1 (single-pane) to ~0.5–0.7, saving 30–50% of heat loss at a fraction of the replacement cost. Full window replacement ($300–$1,000/window installed) achieves better performance (U-factor 0.20–0.30) but has longer payback. Storm windows are great for historic homes or tight budgets.
How do I find my heating degree days?
Heating degree days (HDD) measure how cold a location is during the heating season. Chicago averages ~6,500 HDD; New York ~4,800; Atlanta ~3,000; Miami ~200; Minneapolis ~8,500. The NOAA website provides historical HDD data by city. Your gas utility also publishes local HDD. This calculator uses HDD to estimate annual heat loss through your windows.