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

IECC Climate Zone

Window U-Factor

SHGC (Solar Heat Gain)

Total Window Area sq ft

Replacing Old Windows?

Energy Performance

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.