Window-to-floor area: 12-20% for daylight quality; 8% is code minimum
ROT/EXT-WINDOW-AREA-TO-FLOORDescription
While code requires only 8% glazing-to-floor area for natural light (IRC R325.1.1), architects designing for daylight quality target 12-20% of floor area. Beyond 25% the room often becomes uncomfortable (glare, heat loss / gain). Distribute glazing on TWO or more orientations whenever possible — single-side-lit rooms have a 'dim back' even at high glazing percentages.
Why this exists
8% is functional code compliance. 15% is comfortable. 25%+ is dramatic but starts to fight HVAC. The dual-orientation rule is the daylight-quality lever that 8% glazing on one wall can't achieve.
Measurements
| Property | Operator | Value | Unit | Note |
|---|---|---|---|---|
targetMin | min | 12 | % of floor area | Daylight-quality target |
targetMax | max | 25 | % of floor area | Beyond this, comfort drops |
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
- Living spaces south-facing in cold climates · LIVING-SOUTH-FACING
- Kitchen east-facing for breakfast · KITCHEN-EAST-FACING
- Studio / office north-facing for consistent light · STUDIO-NORTH-FACING
- Mechanical room central, not corner · MECHANICAL-CENTRAL
- Water heater near fixtures · WATER-HEATER-NEAR-FIXTURES
Last reviewed 2026-05-15.