Builder Basics: Conditioned Crawl Space (Part 2) Once the vapor barrier is sealed tight — covering every pier and wrapping up the foundation walls — it’s time to seal and insulate the crawl itself. Closed-cell spray foam is applied to the interior block walls, and insulation is added to the rim joists. This locks in temperature control, blocks moisture, and turns the crawl space into part of the home’s conditioned air system. The result? A clean, dry, energy-efficient space that protects your in