2021 IECC Section R402.3
Windows, skylights, and glazed doors are the one part of the thermal envelope that the code rates two ways at once: how fast heat conducts through the unit, and how much solar heat comes through the glass. On the Gulf Coast, where the cooling season runs most of the year, the second number matters more than most builders expect. This lesson covers how those two ratings are established and documented, how a schedule with a mix of window performance can still comply on average, what small amounts of glazing and doors are exempt, why skylights get their own limits, and what changes when a window is a replacement rather than new construction.
R402.3 regulates fenestration on two properties. U-factor describes how fast heat conducts through the whole window assembly, glass, frame, and spacer together, and a lower U-factor means less heat transfer in either direction. Solar heat gain coefficient, SHGC, describes the fraction of incident solar radiation that ends up as heat inside the building, on a scale of zero to one, and a lower SHGC means less solar heat gain. A cold-climate code leans hard on U-factor because conduction losses run all winter. Climate Zone 2 flips that emphasis: the heating season is short, so conduction through glass is a minor part of the annual energy picture, while the cooling load from solar gain runs for months. That is why Table R402.1.3 sets a comparatively relaxed maximum U-factor of 0.40 for Zone 2 vertical fenestration, alongside a tight maximum SHGC of 0.25. Climate Zone 3 tightens the U-factor requirement to 0.30 while holding SHGC at 0.25, the same solar limit, because solar gain remains the dominant issue even as winters get a little more demanding.
The code does not let anyone estimate these numbers. Under R303.1.3, U-factor is determined under NFRC 100 and SHGC under NFRC 200, by an accredited independent laboratory, and the manufacturer labels and certifies the result. Those National Fenestration Rating Council procedures test the whole assembly, not just the center of the glass. Every rated window, skylight, and glazed door ships with an NFRC label that a plans examiner can check against the schedule. Center-of-glass numbers, sometimes printed on the spacer bar or in marketing material, are always more flattering than the whole-unit rating because they ignore the frame and edge-of-glass losses, so they are not an acceptable substitute for the certified label.
Older, site-built, or otherwise unrated fenestration cannot always carry an NFRC label. For those products the code supplies default U-factor and SHGC tables (Table R303.1.3), deliberately conservative numbers that assume the worst reasonably likely performance for that frame and glazing type. A double-pane window in a nonmetal or metal-clad frame defaults to U-0.55, and clear double glazing defaults to SHGC 0.7, so an unlabeled window cannot meet Zone 2's 0.40 and 0.25. A default value almost always makes compliance harder than a certified rating would, which gives a real incentive to get products rated and labeled rather than to rely on the default. Once labels are removed after installation and no schedule or certified documentation survives, the burden falls on the builder to document what was actually installed before an inspector can accept it.
A house rarely has identical windows throughout, and the code does not require every single unit to individually clear the U-factor and SHGC limits. Under the area-weighted average method, each unit's U-factor is multiplied by its area, the products are summed across the whole building, and the total is divided by the total fenestration area; the resulting weighted average has to meet the zone's maximum. SHGC can be averaged too (R402.3.2), but only across products that are more than 50 percent glazed, and separately, because the two properties are averaged within their own category rather than combined into one number. U-factor averaging is covered by R402.3.1. A better U-factor on one window cannot offset a poor SHGC on another. Averaging only helps when at least some units are better than the limit; if every unit in the schedule already sits at the maximum, or above it, the weighted average cannot come out any better than that value.
In the field
GCHS reviewers see two recurring window-schedule problems on Gulf Coast plans. The first is a schedule where every unit lists SHGC 0.30, a value that passes in some cold-climate jurisdictions but fails outright in Zone 2, and averaging cannot rescue it because nothing in the schedule is better than the limit. The second is a schedule with correct U-factors but no SHGC column at all, because the supplier only tracks the conduction number. Both are worth flagging before a plan review comment, since either one is a straightforward, inexpensive fix if it is caught while the windows are still on order rather than after they are installed.
Two narrow exemptions keep small, incidental openings from forcing a custom-rated unit. Up to 15 square feet of glazed fenestration per dwelling unit is exempt from the U-factor and SHGC requirements of the prescriptive table, which is enough for a decorative sidelight or a small stained-glass panel. Separately, one side-hinged opaque door assembly up to 24 square feet is exempt from the U-factor requirement; that is a single-door allowance, not a per-door credit, and it does not touch a door's glazed sidelights, which are counted against the 15-square-foot glazed allowance instead. Neither exemption may be used under the total UA alternative. A house with a 20-square-foot opaque entry door and two 6-square-foot sidelights can exempt the door outright, but the sidelights use 12 of the 15 allowed square feet, leaving only 3 for anything else.
Skylights carry their own, more permissive U-factor limit in every climate zone that sets a fenestration U-factor at all, 0.65 in Zone 2 against 0.40 for vertical windows, because of how skylight curbs and frames are built and installed. That relief does not make skylights energy-neutral on the Gulf Coast. A horizontal or low-slope glazing plane faces almost straight up into the highest summer sun for most of the day, so a skylight typically delivers far more solar heat gain per square foot than a vertical window of the same rating, which is why unshaded skylights are a common source of summer overheating complaints in rehab and new construction alike. The Zone 2 SHGC limit of 0.25 applies to skylights too, with one relief: in Zones 0 through 3 a skylight may be excluded from the glazed SHGC requirement if its own SHGC does not exceed 0.30.
SHGC is the number a GCHS plans examiner should scrutinize hardest on a Zone 2 window schedule, more than frame color, grille pattern, or glass thickness, because it feeds directly into the Manual J cooling load calculation and the equipment sizing that follows it. Two window packages with identical U-factors but different SHGC values will not perform the same in July: the lower-SHGC package blocks more solar heat, which can support smaller, less expensive cooling equipment and a more comfortable house in the hottest months. Exterior shading, deep overhangs, and orientation are real physics that reduce solar gain in practice, but the prescriptive path gives them no credit; a designer who wants credit for shading has to move to the simulated performance path, where a compliance model can account for it directly.
When a window, skylight, or glazed door is replaced in an existing house rather than installed new, R503.1.1.1 requires the replacement unit to meet the U-factor and SHGC values in Table R402.1.3 for that climate zone, not the performance of the unit it replaces; where several units are replaced, an area-weighted average of all the replacement units can be used to show compliance. A GCHS rehab project that swaps a leaky, single-pane 1980s window for a new unit rated U-0.35 and SHGC 0.22 is compliant in Zone 2 because both values beat the 0.40 and 0.25 maximums; a replacement that only matches the old window's performance would not be.
A worked example on area-weighted averaging: a Zone 2 house has 200 square feet of windows rated U-0.44 and 100 square feet rated U-0.32. The weighted average is (200 times 0.44, plus 100 times 0.32) divided by 300 total square feet, which is (88 plus 32) divided by 300, or 120 divided by 300, equal to 0.40, exactly at the Zone 2 limit. If the same schedule's SHGC values were 0.28 on the larger group and 0.18 on the smaller one, the weighted SHGC would come out to (200 times 0.28, plus 100 times 0.18) divided by 300, or (56 plus 18) divided by 300, equal to 0.2467, which clears the 0.25 limit with room to spare even though one group alone would not have.
1. U-factor and SHGC describe:
2. The 2021 IECC prescriptive maximum fenestration U-factor for Climate Zone 2 is:
3. The 2021 IECC prescriptive maximum glazed fenestration SHGC in Climate Zone 2 is:
4. Compared with a vertical window in the same Zone 2 house, a skylight's maximum U-factor is:
5. Fenestration U-factor and SHGC are established for code compliance by:
6. Compared with a certified NFRC rating, a code default value for an unrated product is generally:
7. A Zone 2 house has 200 sq ft of window at U-0.44 and 100 sq ft at U-0.32. Does the area-weighted average comply with the U-0.40 limit?
8. When averaging a Zone 2 window schedule, U-factor and SHGC are:
9. How much glazed fenestration per dwelling unit is exempt from the U-factor and SHGC requirements?
10. The opaque door U-factor exemption covers:
11. Under R402.4.3, how does the manufactured air leakage limit for swinging doors compare with the limit for windows and sliding glass doors?
12. Why does a skylight typically contribute more heat gain per square foot than a vertical window in Climate Zone 2?
13. A designer wants credit for exterior awnings over west-facing glass instead of specifying lower-SHGC glazing. On the prescriptive path this is:
14. A replacement window is installed in an existing Zone 2 house at U-0.35 and SHGC 0.22. Under R503, this is: