Section 5.5.4/Fenestration Requirements
ASHRAE 90.1 Section 5.5.4 covers maximum fenestration U-factor, SHGC, and window-to-wall ratio by climate zone.
Vertical fenestration U-factors and solar heat gain coefficients (SHGC) must not exceed the values in Table 5.5-2 for the applicable climate zone. The vertical fenestration area must not exceed 40 percent of the gross above-grade wall area. Skylight area must not exceed 5 percent of the gross roof area. Fenestration products must be rated and labeled by the National Fenestration Rating Council (NFRC). The U-factor and SHGC must be whole-assembly rated values, not center-of- glass values.
Why this section exists
Fenestration is the weakest thermal element in most building envelopes. Even high-performance glazing transfers heat 5-10 times faster than an insulated opaque wall. The U-factor limit controls conductive heat loss (critical in cold climates), while the SHGC limit controls solar heat gain (critical in warm climates). The 40% window-to-wall ratio cap prevents designs where excessive glazing overwhelms the HVAC system.
What plan reviewers look for
Plan reviewers check the fenestration schedule for NFRC-rated U-factor and SHGC values. They calculate the window-to-wall ratio from the building elevations. They verify that the correct climate zone table column is used and that the values are whole-assembly ratings. Reviewers also check skylight U-factor and SHGC against the separate skylight requirements.
Common violations
Related ASHRAE 90.1 requirements
Section 5.5.3 covers opaque envelope requirements. Section 5.6 covers the envelope trade-off option. Section 11 covers the energy cost budget method for buildings that exceed prescriptive limits. IECC Section C402.4 provides equivalent fenestration requirements.