Window Energy Performance Calculator
Window selection significantly impacts heating and cooling costs. This calculator compares U-factor and SHGC values against IECC requirements for your climate zone and estimates annual energy impact.
Window Specifications
Energy savings estimates use simplified degree-day calculations and average energy costs. Actual savings depend on home orientation, shading, HVAC efficiency, and local utility rates. ENERGY STAR certification provides verified ratings.
IECC Window Requirements
| Zone | Max U-Factor | Max SHGC | ENERGY STAR |
|---|---|---|---|
| 1 | 0.50 | 0.25 | U ≤ 0.40, SHGC ≤ 0.25 |
| 2 | 0.40 | 0.25 | U ≤ 0.40, SHGC ≤ 0.25 |
| 3 | 0.35 | 0.25 | U ≤ 0.30, SHGC ≤ 0.25 |
| 4 | 0.32 | 0.40 | U ≤ 0.27, SHGC ≥ 0.32 |
| 5 | 0.30 | 0.40 | U ≤ 0.27, SHGC any |
| 6 | 0.30 | 0.40 | U ≤ 0.27, SHGC any |
| 7–8 | 0.30 | 0.40 | U ≤ 0.27, SHGC any |
Understanding Window Ratings
U-Factor
Measures how well a window prevents heat from escaping. Lower is better. A U-factor of 0.25 means the window conducts 0.25 BTU per hour per square foot per degree F of temperature difference. Range: 0.15 (triple-pane) to 1.0+ (single-pane).
SHGC (Solar Heat Gain Coefficient)
Measures how much solar radiation passes through the window as heat. Range 0 to 1. In hot climates, lower SHGC is better (blocks solar heat). In cold climates, higher SHGC can be beneficial (free solar heating).
Frequently Asked Questions
Are triple-pane windows worth it?
Triple-pane windows (U = 0.15–0.22) cost 25–40% more than double-pane low-E (U = 0.25–0.30). In climate zones 5–8, the energy savings often justify the cost within 10–15 years. In zones 1–3, the payback period is usually too long.