Window SHGC Requirements
SHGC (Solar Heat Gain Coefficient) measures how much solar radiation passes through a window as heat. It ranges from 0 to 1 — lower values mean less solar heat enters the building. The IECC regulates maximum SHGC values in heating-dominated climates zones 1–3, where cooling loads are the primary concern.
Why SHGC Matters
In hot climates (Zones 1–3), solar heat gain through windows is a major contributor to cooling energy use. A window with SHGC 0.40 admits 40% of incident solar radiation as heat. A window with SHGC 0.25 admits only 25% — meaning your air conditioner works less hard. In southern states, specifying low-SHGC windows is one of the most cost-effective energy improvements available.
SHGC Requirements by Zone (IECC 2021)
| Zone | Max SHGC | Why |
|---|---|---|
| 1–3 (hot climates) | 0.25 | Cooling dominated; solar gain drives energy costs |
| 4–8 (mixed/cold) | No requirement | Solar gain in winter is beneficial; no maximum set |
Low-E Coatings and SHGC
Low-emissivity (Low-E) coatings are the primary way manufacturers achieve low SHGC values. There are two types: hard-coat Low-E (pyrolytic, applied during manufacturing) and soft-coat Low-E (sputtered, applied after). Soft-coat Low-E generally achieves lower SHGC values and is more common in windows targeting Zone 1–3 markets.
SHGC and Visible Light
Lower SHGC coatings sometimes reduce visible light transmission (VT). This is not always the case — modern selective coatings can achieve SHGC 0.20 while maintaining VT above 0.55. When comparing windows, check both SHGC and VT on the NFRC label.