Basement & Crawl Space Insulation in Washington, DC

Basement and crawl space insulation in the DC area deals with two problems at once. In winter, cold floors and drafts come up from uninsulated foundations and rim joists. In summer, humid outdoor air gets into basements and vented crawl spaces and meets cooler surfaces, which leads to condensation, musty smells and mold. Sealing and insulating the space below the house addresses both.

Rim joists

The rim joist sits above the foundation wall and leaks air at every seam. Closed-cell foam seals and insulates it in one step. HomeGuide puts rim joist foam at about $2,000 to $2,500.

Basement walls

Rowhouse basements often have old brick foundation walls; suburban houses have block or poured concrete. Closed-cell foam on the inside face insulates and slows vapor, and goes on before framing if you are finishing the space. DC's 2017 IECC-based code lists R-15 continuous, or cavity plus continuous combinations, for basement walls in new work.

Crawl spaces

Vented crawl spaces under additions, porches and suburban homes pull in humid summer air. Encapsulation, meaning a ground vapor barrier, sealed vents and insulated walls, turns the crawl space into a drier, semi-conditioned space. HomeGuide lists encapsulation at $3 to $10 per square foot, or $3,000 to $15,000.

Water first

Foam is not waterproofing. If a basement leaks, grading, gutters or drainage need to be fixed first. We check before quoting.

Finishing a rowhouse basement

Many DC owners are turning basements into living space or rental units. Foam goes on the rim and walls before framing, and we explain what covering the foam needs. Sealing the basement also cuts musty air that otherwise moves up into the house.

Under rear additions

Rear additions and kitchen bump-outs are often built over shallow crawl spaces or open piers. Closed-cell foam on the underside of the floor seals and insulates in one step. We look at clearance and access first.