Crawl Space/Slab insulation assembly questions

A homeowner building a new house in southeast British Columbia switched from a slab-on-grade foundation to a crawl space after digging revealed conditions the foundation contractor and engineer didn't expect. The site sits near a river with flood risk, so groundwater is a real concern, and the homeowner turned to an online forum for advice on insulating the new foundation.

A building science expert weighed in with specific guidance. For under-slab insulation, he recommended R-10, typically achieved with 2 inches of XPS (extruded polystyrene) foam board, noting that going much thicker offers little added benefit since ground temperature swings less the deeper you go. He also said XPS holds up better than EPS (expanded polystyrene) when buried in soil, citing an Alaska Department of Transportation study, though newer XPS formulas have addressed the environmental concerns that once set the two materials apart.

On the walls, the home uses 11-inch ICF (insulated concrete form) blocks with a 6-inch foam core, already providing better insulation than many building codes require even in cold climates. The expert suggested skipping additional batt insulation inside the ICF wall and instead attaching rigid polyiso foam directly to the interior surface for a bigger efficiency boost in less space. Local flood-mitigation rules bar a full basement here, since mechanical equipment must sit above a set elevation, which shaped the decision to build a crawl space instead.