U-Factor for Windows: What Is Good and How Europeans Report Uw

U-factor is the rate at which a window transmits heat, measured in Btu per hour per square foot per degree Fahrenheit (Btu/h-ft2-F), and a lower number means better insulation. The US rating covers the whole window, frame and glass together. European data sheets report the same physical quantity as Uw in W/m2K, and 1 Btu/h-ft2-F equals 5.678 W/m2K, so a Uw of 1.0 W/m2K is roughly 0.18 in US units.

What U-factor measures

ENERGY STAR defines U-factor as heat transfer per unit time, per unit area and per degree of temperature difference. Multiplied by the indoor-outdoor temperature difference and the window area, it gives the total heat flowing through the product by conduction, convection and long-wave infrared radiation. The rating includes the frame, the glass and the spacer at the glass edge. ENERGY STAR notes that published U-factors generally fall between 0.25 and 1.25 Btu/h-ft2-F.

Solar heat gain is a separate quantity. U-factor says how much heat leaks out in winter or in through the glass on a hot day because of the temperature difference. The solar heat gain coefficient (SHGC) says how much sunlight turns into heat. Both appear on the NFRC label.

U-factor and R-value

R-value is the inverse of U-factor: R = 1 / U. A window with a U-factor of 0.30 has an R-value of about 3.3, and one at 0.22 has about R-4.5. These numbers describe the whole window, so they should not be compared with the R-value printed on wall insulation, which describes a layer and not an assembly.

How Europeans report it: Uw, Ug and Uf

European data sheets break the window into three values, all in W/m2K:

The simplified calculation in ISO 10077-1 combines them by area: Uw = (Ag x Ug + Af x Uf + Lg x Psi-g) / Aw, where Ag is the glass area, Af the frame area, Lg the length of the glass perimeter and Aw the total window area. One calculation reference gives typical values for triple low-e glass of 0.6 to 1.1 W/m2K for Ug and 0.04 to 0.06 W/m-K for warm-edge spacers, against 0.08 to 0.12 for aluminum spacers. Those figures are rules of thumb. A certified test or calculation report for the exact product is the only reliable source.

Converting W/m2K to Btu/h-ft2-F

NIST lists the factor as 5.678263 W/m2K per Btu/h-ft2-F. To convert a European value to US units, divide by 5.678; to go the other way, multiply.

Center-of-glass versus whole window

Center-of-glass U-value describes the best part of the window, and the whole-window value is almost always worse. The glass edge loses more heat than the center because of the spacer, and the frame usually conducts more heat than the glass unit. A catalog that leads with Ug can therefore look better than the finished window.

The frame share of a window matters: a typical frame adds 15 to 25 percent of the overall window area according to one calculator reference, and a small or heavily divided window has a larger frame share than a large fixed pane. Aluminum frames conduct heat readily, which is why a thermal break in the profile is central to aluminum window performance. The Efficient Windows Collaborative states that the best source for the thermal properties of an overall window is its NFRC label.

When comparing windows, compare whole-window values at a comparable size and configuration, never a Ug against a whole-window U-factor.

What is a good U-factor by climate zone

Two published sets of limits apply to US residential projects, and they use different zone systems. ENERGY STAR sorts the country into four named zones. The IECC uses numbered climate zones 0 through 8. Both are by county or region, so the first step is to find the project's zone.

ENERGY STAR Version 7.0

ENERGY STAR Version 7.0 for residential windows took effect on October 23, 2023. The prescriptive criteria for windows are:

In the Northern zone, windows may also qualify through an equivalent-performance table that allows a U-factor of 0.23 to 0.26 when SHGC is at least 0.35 to 0.40. The NFRC-certified U-factor and SHGC are what count. ENERGY STAR qualification is based only on U-factor and SHGC. These are program criteria for labeled products, not a building code.

2021 IECC

The 2021 IECC sets a maximum fenestration U-factor in Table R402.1.2 (and an alternative R-value-based table, R402.1.3). Based on the Pacific Northwest National Laboratory summary of the 2021 table:

The 2021 table also sets a maximum SHGC in warmer zones. A county adoption form for the 2021 table shows 0.40 for climate zone 4 except Marine. The 2021 table carries a footnote that allows 0.32 in zones 5 through 8 for buildings above 4,000 feet in elevation or in windborne debris regions. I checked the 2021 edition. The 2024 IECC is published, but I did not verify its fenestration table, and many states and cities are still on 2018 or 2021 editions or have amended them. The building official's adopted edition and local amendments control.

Reading the two together

A whole-window NFRC U-factor of 0.30 or lower satisfies the 2021 IECC maximum in zones 3 through 8, while ENERGY STAR in the Northern zone asks for 0.22. Neither document defines "good" for every project. Passive house targets and cold-climate custom homes often aim well below code. Choose the target from the climate zone, the glazing area and the energy model, and then require a rating at or below it.

Why a converted Uw is not an NFRC U-factor

A Uw converted to US units is an estimate, not a rating. NFRC and the European method calculate the same quantity with different procedures. They assess frame, spacer and glass under different environmental conditions, and they use different reference specimens. NFRC rates the complete product at a standard size under standard conditions, while the ISO 10077 approach combines separately determined frame, glass and edge values. The gap between the two can go either way depending on the product, so there is no safe correction factor.

For ENERGY STAR and for the energy code, the number that counts is the NFRC-certified rating. For how that rating is produced, see the NFRC label explained. For how energy code acceptance works for imported products, see whether European windows meet US code.

How to specify U-factor

• Require a whole-window NFRC U-factor at or below the target for the project's climate zone.
• Ask for the NFRC rating for the exact frame, glass and spacer configuration, not the product family.
• Treat a European Uw as a planning number for comparing systems, and convert it by dividing by 5.678.
• Compare Uw with Uw and Ug with Ug, never one against the other.
• Request the SHGC with the U-factor, because the zone limits pair them.
• Confirm the adopted energy code edition and local amendments with the building official.

For the vocabulary used here, the window and door glossary defines each term. The wider comparison of US and European practice is in European vs American windows.