U-factor
U-factor is the rate at which a whole window, door or skylight lets heat through, measured in Btu/(h.ft2.F) in the US. Lower means better insulation. The same quantity in metric units is the U-value, in W/(m2.K).
U-factor is the rate at which heat flows through a whole window, door or skylight, measured in Btu/(h.ft2.F); the lower the number, the better the window insulates. It covers the entire assembly - glass, spacer and frame - and describes heat moving from the warm side to the cold side. You meet it in two places: on the NFRC label of a US window and as the Uw line on a European data sheet.
What is the difference between U-factor and U-value?
They measure the same thing; the difference is the unit system and the name. In practice, US sources say U-factor for the Btu/(h.ft2.F) figure, and a European data sheet says U-value for the W/(m2.K) figure. People use the two words loosely, so I always check the unit next to the number before I trust the word.
What units is U-factor measured in?
U-factor is measured in Btu/(h.ft2.F) in the US, and the equivalent metric U-value is in W/(m2.K). The table shows how the two systems line up.
| Item | US | Metric |
|---|---|---|
| Name | U-factor | U-value |
| Unit | Btu/(h.ft2.F) | W/(m2.K) |
| Whole window | U-factor on the label | Uw |
| Glass center only | not a label figure | Ug |
| Frame only | not a label figure | Uf |
Only Uw lines up with the whole-window U-factor. Ug and Uf describe parts of the window, so they do not equal a whole-window rating.
How do you convert a European Uw to a US U-factor?
Multiply the Uw by 0.17611; to go the other way, multiply the US U-factor by 5.678. The factor comes from the NIST conversion 1 Btu/(h.ft2.F) = 5.678 W/(m2.K). Here are three worked examples.
| Uw (W/m2K) | Calculation | US U-factor |
|---|---|---|
| 1.7 | 1.7 x 0.17611 | about 0.30 |
| 1.1 | 1.1 x 0.17611 | about 0.19 |
| 0.8 | 0.8 x 0.17611 | about 0.14 |
Checking the other direction, U-0.30 x 5.678 is about 1.70 W/m2K and U-0.20 x 5.678 is about 1.14 W/m2K. For any other number, use the metric U-value to U-factor converter, and the U-factor to R-value calculator turns the result into R = 1/U.
Does a lower U-factor mean a better window?
Yes for insulation: a lower U-factor means less heat passes through. ENERGY STAR puts typical window U-factors at roughly 0.25 to 1.25. Compare like with like, meaning whole-window values for the same type of assembly. A glass-only figure looks better than the whole window, because the frame and edge of glass lose more heat than the center of the glass.
What does the U-factor on a window label include?
The U-factor on an NFRC label is an area-weighted average of three zones, calculated under NFRC 100. These are the zones:
- Center of glass.
- Edge of glass, where the spacer sits.
- Frame.
A European Uw is calculated under a different standard, so it is the same quantity but not the same rating procedure, and I do not treat the two as equivalent. For what the label does and does not prove, see my guide to the NFRC rating.
Where do I find the number I need for my project?
The number you need is set by your climate zone and the energy code your jurisdiction adopted, so start with the code table. These pointers cover the three places the number comes from:
- For the code target by climate zone, read the good U-factor guide and the climate zone breakdown.
- For the figure on a window you are buying, look at the NFRC label or the whole-window rating in the documentation.
- For a European data sheet, take the Uw and convert it; the steps are in reading a European quote in US units.
When I quote, I give the whole-window U-factor in US units and convert the Uw myself, so you never compare a glass-only number with a code target. Try a configuration in the estimator. Related terms: SHGC and thermal break.
Common questions
- Is U-factor the same as U-value?
- They describe the same physical quantity: the rate of heat transfer through a window per unit of area per degree of temperature difference. The units differ. The US U-factor is in Btu/(h.ft2.F) and the metric U-value is in W/(m2.K), so multiply the metric number by 0.17611 to get the US figure.
- What is the U-factor for 1.0 W/m2K?
- 1.0 W/(m2.K) x 0.17611 is about 0.176 Btu/(h.ft2.F). The matching R-value is the reciprocal, 1 / 0.176, which is about R-5.7.
- Is a lower U-factor always better?
- Lower means less heat passes through, so the window insulates better. Compare whole-window values for the same kind of assembly, and never set a glass-only (Ug) figure against a whole-window rating.
- What U-factor do I need for my project?
- It depends on the climate zone and the energy code your jurisdiction has adopted. The code table sets the maximum, not a single number that fits every house, so check the table for your zone before choosing glazing.
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