How to Read an NFRC / ENERGY STAR Window Label
The four NFRC numbers
U-factor measures heat loss (lower is better, ~0.17-0.30 for efficient windows). Solar Heat Gain Coefficient (SHGC) measures solar heat let in (lower for hot climates, higher for cold). Visible Transmittance is how much light comes through. Air Leakage shows draftiness (lower is better).
Match the numbers to your climate
In cold northern zones prioritize a low U-factor and a moderate-to-higher SHGC for passive solar warmth. In hot southern zones prioritize a low SHGC to keep heat out. ENERGY STAR sets different thresholds per zone.
Why it matters
Every certified window carries an NFRC label, so two windows marketed as 'energy efficient' can perform very differently. The label is your apples-to-apples comparison.