TL;DR
U-factor is the rate at which a window, door, or skylight assembly conducts heat, expressed in BTU per hour per square foot per degree Fahrenheit, where lower numbers mean better insulation; it is the inverse of R-value. The figure on the NFRC label covers the whole unit, glass, spacers, and frame together, which is why two windows with identical glass can rate differently.
What it means
U-factor is the rate at which a window, door, or skylight assembly conducts heat, expressed in BTU per hour per square foot per degree Fahrenheit, where lower numbers mean better insulation; it is the inverse of R-value. The figure on the NFRC label covers the whole unit, glass, spacers, and frame together, which is why two windows with identical glass can rate differently. Current ENERGY STAR criteria push northern-zone windows to 0.22 or better, and energy codes set maximums by climate zone through the IECC.
Where it sits in the glossary
U-factor is part of the Trade jargon group inside the ProFix Directory glossary. Browse every term in this category from the glossary index.
Why Ohio homeowners should know it
This is a term Ohio homeowners encounter when reading contractor quotes, hiring paperwork, or inspection reports. Understanding it well enough to ask one good follow-up question is usually all the protection a homeowner needs.
ProFix Directory keeps definitions short on the index page and saves the longer context — Ohio-specific rules, where the term comes from, and which ProFix tools touch it — for these per-term pages so the term is easy to cite and easy to share.
ProFix tools that touch this term
See also
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