Sealed + conditioned crawl space: continuous ground vapor barrier, sealed walls, dehumidification or HVAC supply
BASC Guide — guides/conditioned-crawlspaceDescription
A sealed + conditioned crawl space outperforms a vented crawl in moisture control and energy efficiency. Steps: continuous 6-10 mil polyethylene vapor barrier on the ground lapped 12 in at joints + sealed; barrier extended up the foundation walls + sealed at the band joist; foundation walls insulated (R-10 to R-15 rigid foam interior); a small HVAC supply OR a dehumidifier (typical) maintaining the space at ≤ 60% RH. Vents to the outdoors are CLOSED.
Why this exists
Sealed crawls eliminate the moisture-mold-rot cycle that plagues vented crawls. The cost premium is small (~$1500-3000 for a typical home); the comfort + IAQ + structural-longevity benefit is substantial. Architects should default to sealed crawl in mixed-humid climates (most of the US south of CZ 6).
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
- Continuous load path from roof to foundation · HUD RSDG §2.4
- Residential structural reliability targets 1-in-100 to 1-in-1000 annual probability of failure · HUD RSDG §2.5
- Residential floor live load: 40 psf minimum (30 psf sleeping rooms) · HUD RSDG §3.4
- Wind load design uses ASCE 7 basic wind speed for the locality · HUD RSDG §3.6
- Ground snow load for Virginia: 25 psf eastern, up to 40 psf western mountains · HUD RSDG §3.7
Last reviewed 2026-05-15.