Attic ventilation: net free area ≥ 1/150 of vented space (1/300 if Class I/II vapor retarder + 40-50% high/low vent balance)
2025 RCNYS §R806.2 Attic ventilation: 1/150 minimum (1/300 with vapor retarder + balanced vents)Description
Enclosed attics + enclosed rafter spaces must be ventilated. Minimum net free ventilating area: 1/150 of the vented space area. This drops to 1/300 when: (a) a Class I or II vapor retarder is on the warm-in-winter side of the ceiling, AND (b) at least 40% and not more than 50% of the vent area is in the upper portion of the space (ridge / gable / high soffit), with the rest within 3 ft of the eave. Eave/cornice vents must not be blocked by blocking, bridging, or insulation.
Why this exists
Attic ventilation prevents moisture buildup + ice dams. The 1/150 default works without a vapor retarder; the 1/300 reduction (with retarder + balanced vents) is the modern norm. Architects designing cathedral ceilings or shed roofs need the same air flow at every rafter bay — typically via continuous soffit + ridge vents and unobstructed channels (use cardboard 'air chutes').
Measurements
| Property | Operator | Value | Unit | Note |
|---|---|---|---|---|
defaultRatio | max | 150 | sqft attic per sqft vent | Vented attic ratio (no vapor retarder) |
reducedRatio | max | 300 | sqft attic per sqft vent | Vented attic ratio (with vapor retarder + balanced high/low vents) |
highVentMin | min | 40 | % of vent area high | Min share of vent area in upper portion |
highVentMax | max | 50 | % of vent area high | Max share of vent area in upper portion |
Categories
Applies to
- Jurisdiction: New York State
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
- Habitable space minimum ceiling height · IRC R305.1
- Carbon monoxide alarms in sleeping areas · IRC R315
- Wind design · IRC R301.2.1
- Seismic provisions · IRC R301.2.2
Last reviewed 2026-05-15.