Window U-factor: 0.27 (CZ 6+); 0.30 (CZ 4-5); 0.40 (CZ 1-3). SHGC ≤ 0.40 in cooling-dominant CZ 1-3 + 4-mixed
IECC R-value (DOE climate-zone) — Fenestration — Window U-Factor and SHGC by climate zoneDescription
IECC 2021/2024 prescriptive window U-factor by climate zone: CZ 1-2 (subtropical) U ≤ 0.40 + SHGC ≤ 0.25; CZ 3 U ≤ 0.30 + SHGC ≤ 0.25; CZ 4 (mixed) U ≤ 0.30 + SHGC ≤ 0.40; CZ 5 U ≤ 0.30 + no SHGC max; CZ 6 U ≤ 0.27 + no SHGC max; CZ 7-8 U ≤ 0.27 + no SHGC max. Skylights stricter. Specify by zone-appropriate U-factor + SHGC on the window schedule.
Why this exists
Wrong-zone windows can cost the project ENERGY STAR / IECC compliance. Architects designing residential should pull the window schedule with U-factor + SHGC by orientation from the zone — many manufacturers offer dual-line products that meet several zones with different IGU specs.
Measurements
| Property | Operator | Value | Unit | Note |
|---|---|---|---|---|
cz6Plus | max | 0.27 | U-factor | Window U-factor CZ 6+ |
cz4_5 | max | 0.3 | U-factor | Window U-factor CZ 4-5 |
cz1_3 | max | 0.4 | U-factor | Window U-factor CZ 1-3 |
coolingSHGC | max | 0.25 | SHGC | SHGC max in cooling-dominant CZ 1-3 |
Categories
Source
Solver enforcement
Browsable only — the solver does not currently enforce this directive (no spec-level data to check against). This entry exists so the architect personas can cite it in conversation and the user can read what the rule says.
Related directives
- Natural light in habitable rooms · IRC R303.1
- Natural ventilation in habitable rooms · IRC R303.1 (vent)
- Whole-dwelling mechanical ventilation rate · IRC R303.4
- Required heating · IRC R303.8
- Habitable rooms: glazing ≥ 8% of floor area, facing street/yard/court (or 6 fc artificial light) · 2025 RCNYS §R325.1.1 Natural light
Last reviewed 2026-05-15.