Thermal performance: U-factor and R-value
What NFRC U-factors mean, why whole-window numbers matter, and how to read claims.
What U-factor measures
U-factor (U-value) is thermal transmittance: the rate of heat flow through one square foot of window for each degree of temperature difference, in Btu/h·ft²·°F. It is the reciprocal of the R-value used for insulation. Smaller is better. A code-minimum window in a cold climate is around U 0.30 (R-3); a good triple-glazed window reaches U 0.15–0.20; the GlasFëns flagship fiberglass and vacuum-glass system is rated at a whole-window U-value of 0.045 (R-22).
Whole-window, not center-of-glass
Energy codes (IECC, ASHRAE 90.1, Massachusetts Stretch and Specialized codes) and NFRC labels use the whole-window U-factor, which averages the glass, the edge-of-glass zone at the spacer, and the frame. Center-of-glass numbers are always better than whole-window numbers, sometimes dramatically so. When comparing products, compare NFRC whole-window values at the same test size and configuration.
How NFRC values are produced
NFRC 100 defines the simulation and testing procedure: the unit is modelled in LBNL WINDOW and THERM and validated by guarded hot-box testing at an accredited laboratory. NFRC 200 covers solar heat gain coefficient (SHGC) and visible transmittance (VT). Standard NFRC sizes are based on residential windows; large fixed units and window wall perform better than their labelled values suggest.
Claims to be careful with
- Center-of-glass values quoted as if they were whole-window values.
- Values from thin residential glass (3 mm) or from suspended-film units that would not be used on a real project.
- Small differences in the second decimal place: testing and modelling tolerances make them meaningless.
Balanced design
U-factor is one number among several. SHGC, visible transmittance, air infiltration, condensation resistance and durability all matter, and the right balance depends on climate and orientation. In a cold climate a low U-factor with a moderate SHGC on the south façade harvests free heat; in a hot climate a low SHGC matters more than the last hundredth of U-factor. Our KØLD and HEIT glass packages exist for exactly this reason.