Windows and Fenestration Requirements

Windows are one of the first things your plan reviewer will check. The WSEC-R sets prescriptive U-factor limits for glazing assemblies, and understanding how area-weighted averaging works can save you thousands of dollars in upgrade costs.

The U-0.30 Requirement

Under the prescriptive path, both Climate Zone 4 and Climate Zone 5 require a maximum area-weighted average U-factor of U-0.30 for fenestration. This applies to all glazed openings in the building envelope — windows, sliding glass doors, and any other glazed assemblies.

Area-Weighted Averaging

You do not need every single window to hit U-0.30. The code allows area-weighted averaging across all fenestration. This means a high-performance window in one location can offset a weaker one elsewhere.

Example: Suppose you have 200 sq ft of windows at U-0.25 and 100 sq ft of windows at U-0.35. The area-weighted average is (200 × 0.25 + 100 × 0.35) / 300 = U-0.283. That passes the U-0.30 requirement, even though some individual windows exceed the limit.

The NFRC Label

Every new window sold in the United States carries an NFRC (National Fenestration Rating Council) label. The U-factor printed on this label represents the performance of the entire assembly — glass, frame, spacer, and all. This is the number you use for code compliance, not the center-of-glass value sometimes quoted in marketing.

Skylights

Skylights have a separate, more lenient requirement: U-0.50. They are averaged separately from vertical fenestration. If your skylight U-factor exceeds 0.50, it does not drag down your window average — it simply fails its own check.

Total Fenestration Area Matters

Under the R406 credit path, your building size tier depends partly on total fenestration area. If your total glazing is 300 sq ft or more, your home cannot qualify as a“Small” dwelling (which only needs 5.0 credits). This can bump you into the Medium tier requiring 8.0 credits — a significant jump. Count every window and glazed door carefully.

Exempt Ornamental Windows

The prescriptive path allows up to 15 sq ft of ornamental or decorative glazing to be exempt from the U-factor calculation. This is useful for a small decorative sidelight or transom that would otherwise be difficult to source at U-0.30.

Frequently Asked Questions

Does every window need to be below U-0.30?

No. The requirement is an area-weighted average across all fenestration. Individual windows can exceed U-0.30 as long as the overall average comes in at or below U-0.30.

What if I don’t know my U-factor yet?

Check the NFRC label on the window or look up the product on the manufacturer’s website. For standard double-pane vinyl windows available in 2026, U-factors typically range from U-0.28 to U-0.30. Your window supplier can provide a schedule of U-factors for your specific selections.

For even better performance, triple-pane windows typically achieve U-0.22 or lower. Triple-pane adds cost but gives you significant headroom on the area-weighted average and can help earn additional envelope credits under Option 1. If your project is close to the credit threshold, upgrading a few key windows to triple-pane may be cheaper than adding insulation elsewhere.

Do regular doors count as fenestration?

No. Opaque doors (solid wood, insulated steel, fiberglass) are NOT fenestration— they’re calculated separately with their own U-factor requirements. However, glazed doors— sliding glass doors, French doors with glass panels, any door with more than a small decorative lite — ARE fenestration and count toward your window area and U-factor average. If it’s mostly glass, it’s fenestration.

Do sliding glass doors count?

Yes. Any glazed opening in the building envelope counts as fenestration —sliding glass doors, French doors with glass panels, sidelights, transoms, and skylights all factor into the calculation.