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Choosing

Two numbers, and one of them reverses

Every certified window carries a U-factor and a solar heat gain coefficient. The first means the same thing everywhere. The second is wanted high in the north and low in the south.

What the two numbers are

U-factor is how fast heat escapes through the whole window, frame included. Lower is better, everywhere, without exception.

Solar heat gain coefficient is the share of the sun's heat that gets through the glass. In a heating climate that is free warmth in winter, so the northern criterion sets a minimum. In a cooling climate it is load on the air conditioning, so the southern criterion sets a maximum. This is the number salespeople skip, and it is the reason a window sold as efficient in Minnesota is the wrong window in Florida.

Both are certified by the National Fenestration Rating Council and printed on a label on the glass. Ask for them in writing on the quote; a product line name on its own tells you nothing.

A certification label on the glass of a new window
The label is on the glass when the unit arrives. The two numbers on it are the ones worth checking against what was quoted.

The four zones

Northern
U-factor 0.22 or lower · solar heat gain 0.17 or higher. Heating dominates. Solar heat through the glass is heat you did not pay for, so the criterion is a floor rather than a ceiling.
North-Central
U-factor 0.25 or lower · solar heat gain 0.40 or lower. Cold winters with real summers. Heat loss still leads, but the gain is capped so the south side does not overheat in August.
South-Central
U-factor 0.28 or lower · solar heat gain 0.23 or lower. Cooling begins to decide. The U-factor allowance loosens; the solar number tightens and becomes the one that matters.
Southern
U-factor 0.32 or lower · solar heat gain 0.23 or lower. Cooling decides everything. Keeping the sun out of the room is worth more than keeping heat in it.

Criteria are ENERGY STAR version 7.0 for residential windows, in effect since 23 October 2023. Zones are assigned by county.

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