Guide
U-factor and SHGC, explained in plain language
Two numbers appear on every window label and neither one is intuitive. Here is what each measures and which direction is good.
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U-factor measures heat escaping through the window — lower is better. SHGC measures solar heat coming in through the glass — lower blocks more sun. Both are chosen together for the climate and the elevation.
U-factor: heat leaving
U-factor is how readily heat passes through the whole window assembly — glass, frame, spacer, and all. Lower is better. A number like 0.30 means less heat is escaping than a 0.50 window in the same opening.
The important word is assembly. A great piece of glass in a frame with no thermal break still gives a poor whole-window U-factor, which is exactly why old aluminum frames get replaced rather than reglazed.
SHGC: sun coming in
Solar Heat Gain Coefficient is the fraction of solar energy that gets through the glass. It runs from 0 to 1, and lower means more solar heat is rejected.
Lower is not automatically better. A low SHGC keeps a south or west elevation cooler in August; on a north elevation it mostly just costs you free winter heat. On larger buildings the SHGC is frequently varied by elevation for this reason.
How the two get chosen together
New Jersey falls in a climate zone where the energy code sets a maximum U-factor and, for most fenestration, a maximum SHGC for the project scope. Those are ceilings, not targets — the specification usually sits below them where the payback justifies it.
- Low U, low SHGC
- Cooling-driven elevations and heavily glazed west exposures.
- Low U, moderate SHGC
- Typical northern-corridor residential work.
- Visible transmittance
- The third number: how much daylight gets through. Worth watching if the low-E coating starts darkening the room.
Ready to put numbers to it
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