Low-E Coatings Explained: Reading a Window Sticker
How to interpret U-factor, SHGC, VT, and air leakage ratings.
Every Energy Star window carries an NFRC sticker with four key numbers: U-factor, SHGC, VT, and Air Leakage. Knowing what each means is the difference between buying the right window for your climate and overpaying for the wrong one.
The four numbers
U-factor: how well the window insulates. Lower is better. Range: 0.15 (best) to 1.20 (worst). Target 0.30 or lower for heating-dominated climates.
SHGC (Solar Heat Gain Coefficient): how much solar heat passes through. Range 0-1. Lower is better in hot climates; higher is better in cold climates wanting solar gain in winter.
VT (Visible Transmittance): how much visible light passes through. Range 0-1. Higher is better for daylighting.
Air Leakage: how leaky the window is. Lower is better. Target 0.3 or lower (cubic feet per minute per sq ft).
Climate-appropriate targets
Northern climates: low U-factor (0.27 or below), moderate-to-high SHGC (0.40+) to capture winter solar gain.
Southern climates: low U-factor (0.30 or below), low SHGC (0.25 or below) to block summer solar heat.
Mixed climates: balance both — typically U-factor 0.30, SHGC 0.30.