Basement wall insulation: continuous rigid foam interior face, no fiberglass batts against concrete
BASC Guide — guides/basement-wall-insulation-interior-rigid-foamDescription
Basement walls should be insulated on the INTERIOR with continuous rigid foam (XPS or polyiso, R-10 to R-20 depending on climate zone). NEVER install fiberglass batts directly against concrete or against poly that's against concrete — concrete walls are a moisture source, and any insulation against them will eventually wet, lose R-value, and grow mold. Best practice: 2 in rigid foam on the wall + 2×4 studwall with cavity insulation + drywall. Foam continuous across the rim joist too.
Why this exists
The most common basement-insulation failure mode is fiberglass + poly against concrete — produced as a standard detail in many older builder manuals. Modern foam-on-the-wall is robust against the concrete's moisture and provides a thermal break. Architects should call out the specific basement insulation detail in the wall section, not leave it generic.
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.