Solar Heat Gain Coefficient, usually written SHGC, measures the fraction of solar radiation that passes through a window and becomes heat inside the room.
It is a decimal between 0 and 1. An SHGC of 0.25 admits 25 percent of the sun's heat energy. An SHGC of 0.60 admits 60 percent.
Unlike U-factor, lower is not automatically better. SHGC is the one window number where the correct target genuinely depends on where you live and which direction the window faces.
Which direction you want it to go
In hot climates, you want SHGC low. Solar gain is the enemy. It is what makes a west-facing room unusable from three in the afternoon, and it is what your air conditioner spends the summer fighting.
In cold climates, moderate solar gain is free heating. A south-facing window with a higher SHGC collects winter sun and reduces what your furnace has to supply. This is why ENERGY STAR Version 7.0 introduced a minimum SHGC of 0.17 for the Northern zone rather than only a maximum. Buying the lowest-gain glass available in Minnesota throws away a genuine benefit you already paid for.
In the South-Central zone, the requirement tightened to 0.23 or lower.
| Climate | Sensible SHGC target |
|---|---|
| Hot, cooling-dominated | 0.25 or lower |
| Mixed | 0.25 to 0.40 |
| Cold, heating-dominated | 0.30 to 0.60 on south-facing glass, 0.17 minimum under ENERGY STAR |
Where buyers get hurt
One spec for the whole house. The most common expensive mistake is ordering identical windows for every opening. The north side of a house and the west side have opposite requirements. A quote that lists one SHGC for twenty windows was not designed, it was priced.
Confusing SHGC with tint. Modern low-E coatings control solar gain without making the glass look dark. A salesperson demonstrating heat rejection with a heat lamp and a tinted sample is selling a feeling, not a specification.
Optimizing the wrong number. Buyers who learn that lower U-factor is better often assume the same about SHGC, then spend thousands and end up with rooms that run cold in winter because they blocked the free heat.
Both numbers appear together on the NFRC label of every window you are quoted. They have to be read together, against your climate and your floor plan, or the label is just decoration.