Slab-on-grade vapor barrier + capillary break required
HUD RSDG §4.6Description
Slab-on-grade foundations should be placed on 2-3 inches of washed gravel or sand + a 6-mil polyethylene vapor barrier. The sand / gravel layer is a capillary break; the polyethylene blocks moisture vapor diffusion. Tie the gravel into the foundation drain system where one is present.
Why this exists
Without the assembly, soil moisture wicks through the concrete (concrete is 12-15% porous) and emerges as floor-finish failure, mold, or visible damp. The 2000s rebuild of slab-on-grade detailing made this the residential default.
Measurements
| Property | Operator | Value | Unit | Note |
|---|---|---|---|---|
gravelDepth | min | 2 | in | Gravel / sand depth |
vaporBarrierThickness | min | 6 | mil | Polyethylene vapor barrier thickness |
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 2026-05-14.