The Solar Heat Gain Coefficient (SHGC) measures the fraction of solar radiation — both directly transmitted and absorbed then re-emitted inward — that passes through a window as heat. It ranges from 0 (no solar heat passes through) to 1 (all solar heat passes through). A window with SHGC 0.25 admits 25% of the solar heat that strikes it.
SHGC and Climate
The ideal SHGC depends on your climate and window orientation:
- Cold climates (Ohio winters) — higher SHGC (0.40–0.60) on south-facing windows captures passive solar heat and reduces heating costs
- Hot climates (summer cooling) — lower SHGC (0.20–0.30) blocks solar heat and reduces air conditioning load
- Mixed climate (Ohio) — Energy Star recommends SHGC ≤ 0.40 for the North-Central zone, balancing winter gain and summer blocking
SHGC vs. U-Factor
U-factor and SHGC measure different things. U-factor measures heat loss through the glass (conduction/convection — relevant year-round). SHGC measures solar heat gain (radiation — relevant when the sun shines on the glass). Both appear on the NFRC label. For Ohio, prioritize a good U-factor (≤ 0.27) first; then choose SHGC based on window orientation.
How Low-E Coatings Affect SHGC
Low-E coatings are the primary tool for controlling SHGC. There are two types:
- Low solar gain Low-E — blocks most infrared and UV; SHGC ≈ 0.20–0.27. Best for east/west/south windows in cooling-dominated climates.
- High solar gain Low-E — blocks infrared but allows more visible solar energy; SHGC ≈ 0.40–0.55. Best for south-facing windows in heating-dominated climates.
Finding SHGC on Your Windows
The NFRC (National Fenestration Rating Council) label on new windows shows both U-factor and SHGC. For existing windows, you can look up the original window model on the manufacturer’s website or on the NFRC Certified Products Directory.
