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.
References & Methodology
- National Fenestration Rating Council (NFRC), "Window Energy Performance Ratings," nfrc.org. — U-factor, Solar Heat Gain Coefficient (SHGC), and Visible Transmittance rating definitions used in energy savings calculations.
- U.S. Department of Energy (DOE), "Energy Star Windows, Doors, and Skylights," energystar.gov. — Climate-zone-specific window efficiency requirements and estimated annual energy savings ($125–$465 per home).
- International Residential Code (IRC), Section R308, "Glazing," 2021. — Safety glazing requirements, egress window minimum dimensions, and fenestration energy compliance criteria.
References
- 28 CFR Part 35 — Nondiscrimination on the Basis of Disability in State and Local Government Services
- ASHRAE Climate Zone Map — U.S. Department of Energy Building America Program
- Advisory Council on Historic Preservation — Section 106 of the National Historic Preservation Act
- ADA Standards for Accessible Design — U.S. Department of Justice
- 2010 ADA Standards for Accessible Design — U.S. Department of Justice
- 21 CFR Part 177 — Indirect Food Additives: Polymers, U.S. FDA / Electronic Code of Federal Regulatio
- 24 CFR Part 3280 — Manufactured Home Construction and Safety Standards
- 29 CFR Part 1926 — Safety and Health Regulations for Construction