Window U-Factor Calculator
Compare window efficiencies and find recommended ENERGY STAR U-factors and Solar Heat Gain Coefficients (SHGC) for your region.
Recommended Rating (Lower is Better)
U-Factor ≤ 0.27
Recommended SHGC: Any
Estimated Savings: 12% - 15%
Understanding Window Efficiency Ratings
Replacement windows feature energy rating stickers from the **National Fenestration Rating Council (NFRC)**. The two most critical numbers are the **U-Factor** (which measures heating loss) and the **SHGC** (which measures cooling gain).
U-Factor vs. SHGC Sizing Guidelines
- U-Factor (Conductivity): Measures how well the window prevents heat from escaping. Ratings typically range from **0.20 to 1.20**. The lower the U-factor, the better the insulation. Cold Northern climates require low U-factors (≤ 0.27) to trap winter furnace heat.
- SHGC (Solar Heat Gain Coefficient): Measures how much solar heat radiates through the glass. Ratings range from **0 to 1**. Low SHGC numbers (≤ 0.25) are crucial in Southern hot regions to block direct baking sun and lower summer AC loads.
Frequently Asked Questions
"Low-E" stands for **Low-Emissivity**. It is an microscopically thin, invisible layer of metallic oxide applied to window glass that reflects infrared (heat) waves. Low-E coatings reflect winter heating back into the room and bounce summer outdoor sun heat away.
Triple-pane windows add a third pane of glass and an additional insulating gas chamber (argon or krypton), driving U-factors down to 0.18–0.22. However, triple-panes cost 30% to 50% more than double-panes, resulting in a very long return on investment unless you live in extremely cold Northern regions (Canada / Northern US).