Interior moisture: bath + kitchen exhaust to outdoors, no bath fans into attics, vapor retarder per climate zone

Description

All bathroom + kitchen exhaust fans must vent DIRECTLY to the outdoors (not into attics, soffits, or floor systems). The vapor retarder strategy must match the climate zone — Class I (foil-faced batts, polyethylene) on the warm-in-winter side in zones 5-8; smart vapor retarders (Class II) work across more zones. Crawlspaces must be sealed + conditioned (or fully vented per local practice). Foundation walls must have an interior vapor retarder in basement living spaces.

Why this exists

Wrong-side vapor retarders trap moisture in walls. The climate zone determines which side is the warm side and what permeance is permissible. The airPLUS checklist is the easiest reference — RESNET 301 also catalogs the same rules.

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