SHGC (Solar Heat Gain Coefficient)
SHGC (Solar Heat Gain Coefficient) is the fraction of the sun's heat that passes through a window into the room, on a scale from 0 to 1. Lower means less solar heat gets in - a cooler room in summer and a smaller air conditioning load.
Every window lets some of the sun's heat into the house. SHGC tells you how much. A value of 0.25 means about a quarter of the solar heat hitting the glass ends up inside. The other three quarters is reflected or re-radiated back out.
SHGC and U-factor are the two numbers an energy code checks on every window. They answer different questions. U-factor is about heat moving through the window because of the temperature difference between inside and outside. SHGC is about sunshine. A window can have an excellent U-factor and still overheat a west-facing room if its SHGC is high.
What the code asks for
The code limit changes with the climate. These are typical 2021 IECC residential numbers. Your local jurisdiction may have its own amendment, so check it before you lock the spec.
| IECC climate zone | Example areas | Typical SHGC max |
|---|---|---|
| 1-2 | Miami, Houston | 0.25 |
| 3 | Dallas-Fort Worth, Atlanta | 0.25 |
| 4 | New York, Denver | 0.40 (not required in marine 4C) |
| 5-8 | Chicago, Minneapolis, Alaska | not required |
In the south, SHGC is usually the number that fails plan review, and U-factor is the easy one. In the north it works the other way around. The climate zone guide covers both sides.
What actually sets it
The low-E coating on the glass does most of the work. The number of panes matters much less. A double-glazed European unit with a good solar-control coating can get down to about 0.20-0.25 and still look clear, not dark or mirrored. Keep an eye on visible transmittance (VT) at the same time. The coatings that cut SHGC the hardest also cut daylight.
One thing to watch on European spec sheets: the solar number is often given as g-value, and it is often for the glass alone. The whole-window SHGC comes out lower, because the frame blocks sun too. I convert every figure to whole-window SHGC before it goes on a submittal.
The estimator prices the frame system and double or triple glazing. The coating that sets SHGC is chosen at quote review, for each elevation if needed.
Common questions
- What is a good SHGC for windows?
- It depends on the climate. In hot IECC zones 1 to 3 the residential code caps SHGC at 0.25, and a good hot-climate spec lands at 0.20 to 0.25. In zone 4 the cap is 0.40. From zone 5 north there is no code cap, and a moderate SHGC on south-facing glass is often the better choice because it lets in free winter heat.
- Is SHGC the same as the European g-value?
- They measure the same thing, but usually not for the same part of the window. A European g-value normally describes the glass alone. US SHGC describes the whole window, frame included, so it comes out lower than the g-value of the same glass. Always compare whole-window SHGC to the code number.
Used in