Crawl space: ≥ 18 in clear, ventilation 1 sqft per 150 sqft floor (or unvented per R408.3); vapor retarder when air-permeable insulation
2025 RCNYS §R408 Under-floor (crawl) space — access, ventilation, vapor retarderDescription
Crawl spaces must have at least 18 in clear height (12 in below joists where stretched by mechanical penetrations). Vented crawl spaces require a minimum 1 sqft of net free vent opening per 150 sqft of under-floor area, distributed with at least one vent within 3 ft of each corner. Unvented crawl spaces (R408.3) require continuous ground vapor retarder + mechanical conditioning. Vapor retarder on air-permeable insulation between floor joists is required in Climate Zones 1A/2A/3A (max permeance 1.5 perms per ASTM E96 procedure B).
Why this exists
Crawl space moisture problems are the #1 cause of mold + structural rot in pier-foundation residential. Modern best practice is the SEALED (unvented) crawl space with a continuous ground vapor barrier and dehumidification — far better moisture control than open vents. Architects working in mixed-humid climates should default to the unvented detail.
Measurements
| Property | Operator | Value | Unit | Note |
|---|---|---|---|---|
minHeight | min | 18 | in clear | Minimum crawl space height |
ventArea | max | 150 | sqft floor per sqft vent | Ventilation area ratio (vented crawl) |
vaporPermeance | max | 1.5 | perms (ASTM E96 B) | Vapor retarder permeance |
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.