Section C402.4.3/Maximum Fenestration Area
IECC C402.4.3 limits the maximum vertical fenestration area to 30% of the gross above-grade wall area (40% with daylighting controls) under the prescriptive compliance path.
Under the prescriptive compliance path, the total vertical fenestration area (windows, curtain wall, storefront, glass doors) must not exceed 30% of the gross above-grade wall area. This percentage may be increased to 40% if automatic daylighting controls are provided in daylight zones adjacent to the fenestration per Section C405.2.3. Skylight area is limited separately: the total skylight area must not exceed 3% of the gross roof area for the prescriptive path, or 5% with daylighting controls. These limits apply to the entire building, not to individual facades: a building could have 60% glass on the south facade and no glass on the north facade and still comply if the total is within 30%. The fenestration U-factor and solar heat gain coefficient (SHGC) requirements from Table C402.4 apply to every window regardless of the percentage. Buildings that exceed the 30% (or 40%) limit cannot use the prescriptive path and must demonstrate compliance through the total building performance path (energy modeling) or the prescriptive envelope trade-off (COMcheck).
Why this section exists
Windows and curtain walls are the weakest thermal element in the building envelope. Even high-performance glazing has a U-factor 5 to 10 times worse than an insulated wall, and solar heat gain through glass is the primary driver of cooling loads in commercial buildings. Limiting the window-to-wall ratio (WWR) caps the thermal penalty of fenestration. The 30% limit represents a balance between energy performance and the architectural need for daylight and views. The 40% exception with daylighting controls acknowledges that additional fenestration can reduce electric lighting energy if daylighting is actively managed, offsetting some of the thermal penalty.
What plan reviewers look for
Plan reviewers calculate the gross above-grade wall area from the building elevations (height from grade to roof parapet, times the perimeter length). They calculate the total vertical fenestration area from the window schedule or curtain wall elevations. They divide fenestration area by wall area to determine the WWR percentage. If the WWR exceeds 30%, they check for daylighting controls per Section C405.2.3 (which allows 40%) or verify the building uses an alternative compliance path. They verify the wall insulation and fenestration thermal values meet the prescriptive requirements independently of the area limit.