U-Factor
A rating (0.15–1.20) of how much heat passes through a window assembly — lower is better insulating.
Quick answer
U-Factor: A rating (0.15–1.20) of how much heat passes through a window assembly — lower is better insulating. Typical cost: No direct cost — drives window selection..
Why it matters
U-factor is the single most important number on the NFRC label for cold climates; ENERGY STAR northern-zone windows require ≤0.27.
Typical cost
No direct cost — drives window selection.
Pros
- • Apples-to-apples efficiency comparison
Cons
- • Doesn't capture solar heat gain (see SHGC)
Common uses
- • Window shopping in cold or mixed climates
Alternatives
R-value (reciprocal of U-factor)
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Frequently asked questions
- What U-factor is good?
- ≤0.30 for cold climates, ≤0.40 for southern zones.
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Estimates and guidance are educational. Always confirm with a licensed local professional before making decisions.