Section C402.4/Fenestration
IECC C402.4 covers fenestration performance requirements including maximum U-factor, SHGC limits, and window-to-wall ratio restrictions.
Fenestration products must have a U-factor and solar heat gain coefficient (SHGC) not exceeding the values in Table C402.4 for the applicable climate zone. The total vertical fenestration area must not exceed 30 percent of the gross above-grade wall area (40 percent with additional energy-offsetting measures). Skylights must not exceed 5 percent of the gross roof area. All fenestration products must be rated and labeled by NFRC.
Why this section exists
Windows and curtain walls are the weakest thermal link in the building envelope. Even high-performance glazing has a much higher heat transfer rate than insulated opaque walls. The U-factor limit controls heat loss in heating climates. The SHGC limit controls solar heat gain in cooling climates. The window-to-wall ratio cap prevents designers from using excessive glazing that overwhelms the HVAC system regardless of the glass performance.
What plan reviewers look for
Plan reviewers check the fenestration schedule for U-factor and SHGC values against Table C402.4. They calculate the window-to-wall ratio from the elevations or area takeoff and verify it does not exceed 30%. They check that the values are NFRC-rated (not center-of-glass values, which are lower than whole-assembly values).
Common violations
Related IECC requirements
Section C402.1 covers the overall envelope requirements. Section C402.5covers air leakage of fenestration assemblies. Section C406 covers additional efficiency measures that can offset fenestration area overages. ASHRAE 90.1 Section 5.5.4 provides equivalent fenestration requirements.