Glossary · Insulation & windows
U-factor (U-value)
Also called: thermal transmittance
How fast heat passes through a window, door or wall assembly; lower is better, the opposite of R-value.
What it means
U-factor is the rate of heat flow through a whole product, such as a window including its frame and edges. It's roughly the inverse of total R-value, so a lower number means better insulation. Compare whole-window ratings rather than center-of-glass numbers, which look better because they leave out the frame.
Why it matters in an audit
Windows and doors are often the weakest part of the building envelope, so U-factor is the key number when deciding whether replacement is worth it and checking rebate eligibility.
Typical numbers
Reference values with their sources. Your auditor’s report gives the figure for your own house.
| What | Value | Source |
|---|---|---|
| ENERGY STAR windows, US (Version 7.0, since Oct 23, 2023) | Northern ≤ 0.22; North-Central ≤ 0.25; South-Central ≤ 0.28; Southern ≤ 0.32 (Btu/h·ft²·°F) | energystar.gov ↗ |
| Typical good windows (NRCan) | Better double-glazed units U-0.21 or lower; triple-glazed as low as U-0.14 (Btu/h·ft²·°F) | natural-resources.canada.ca ↗ |
| Single glazing (NRCan) | Loses roughly 10–20 times as much heat as the same area of insulated wall | natural-resources.canada.ca ↗ |
| Unit conversion | U (W/m²·K) = U (Btu/h·ft²·°F) × 5.678 | natural-resources.canada.ca ↗ |
In the United States
US windows carry an NFRC label with U-factor in Btu/h·ft²·°F, and ENERGY STAR criteria differ across four US climate zones.