For homeowners

Crawl space encapsulation material: which one fits your crawl space?

Choose Polyethylene ground cover (sheet) for a ground cover in any crawl space, vented or unvented; Thin concrete slab over polyethylene for a floor that can ease access to HVAC equipment in the crawl space; Rigid foam under a thin slab for a crawl space that will be accessed frequently for storage; Closed-cell spray foam on the floor for insulating the crawl space floor directly over a polyethylene vapor barrier; Rigid foam board on the walls for an insulated, air-sealed space for ducts and HVAC equipment.

The right choice depends on how often people go into the crawl space, whether the floor or the walls are being covered, whether ducts or HVAC equipment sit in the crawl space, and your climate zone and termite risk.

How we know: By InstaBids editorial team with AI research agents. · Sources read: 22 October 2026 · Updated 27 October 2026

BASC (DOE, 2015): when a crawl space is included within the home's thermal enclosure, it should be insulated and air sealed on its perimeter and should have a continuous sealed ground cover.
Type	Best for	Main advantages	Main drawbacks
Polyethylene ground cover (sheet)	a ground cover in any crawl space, vented or unvented	BASC (DOE, 2017): 6-mil or thicker polyethylene spread across the entire ground surface; BASC (DOE, 2017): also helps keep soil gases from entering the home	BASC (DOE, 2017) suggests a vinyl runner or extra plastic over any areas that will get traffic, such as a service path to the furnace
Thin concrete slab over polyethylene	a floor that can ease access to HVAC equipment in the crawl space	BASC (DOE, 2017): poured over the polyethylene, it holds the sheeting in place and adds durability; BASC (DOE, 2017): typically 2 to 4 inches thick, sometimes called a rat slab for its resistance to penetration from burrowing rodents	BASC (DOE, 2017): a concrete slab alone will not prevent moisture from coming up from the surrounding ground
Rigid foam under a thin slab	a crawl space that will be accessed frequently for storage	BASC (DOE, 2015): floor insulation is also recommended in cold climates or very low-energy-consuming buildings; BASC insulation table (PNNL, 2021): extruded polystyrene (XPS) rigid foam, R-value per inch 5, an air barrier, typically Class II vapor retarder (1.2 perm/inch)	no official source names one
Closed-cell spray foam on the floor	insulating the crawl space floor directly over a polyethylene vapor barrier	BASC insulation table (PNNL, 2021): high-density polyurethane spray foam, R-value per inch approx. 6, an air barrier, typically Class II vapor retarder (1 perm/inch)	BASC (DOE, 2015): not appropriate in a crawl space that needs to be accessed frequently; BASC (DOE, 2015): should not be used under a future concrete slab; BASC (DOE, 2015): a spray-on thermal/ignition barrier is applied over the foam as required by code
Rigid foam board on the walls	an insulated, air-sealed space for ducts and HVAC equipment	BASC (DOE, 2021): polyisocyanurate or XPS board is recommended for its low permeance to water and high insulation value per inch	BASC (DOE, 2015): the likelihood of termites in climate zones 1 and 2 discourages below-grade foam insulation in unvented crawl spaces; BASC (DOE, 2021): a pest control inspection strip is left where local codes need one

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Sources for the facts on this page

Pacific Northwest National Laboratory, Building America Solution Center (U.S. DOE), Insulating Crawl Space Floors in Existing Homes guide (last updated 2015-12-15), national laboratory, read 22 October 2026.
Pacific Northwest National Laboratory, Building America Solution Center (U.S. DOE), capillary break guide for crawlspace floors (polyethylene lapped up walls and piers or secured in the ground) (last updated 2017-07-25), national laboratory, read 22 October 2026.
Pacific Northwest National Laboratory, Building America Solution Center (U.S. DOE), capillary break guide for crawlspace floors (polyethylene sheeting under a concrete slab) (last updated 2017-03-14), national laboratory, read 22 October 2026.
Pacific Northwest National Laboratory, Building America Solution Center (U.S. DOE), Typical R-values and Vapor Retarder Classifications for Common Insulation Materials table (last updated 2021-12-23), national laboratory, read 22 October 2026.
Pacific Northwest National Laboratory, Building America Solution Center (U.S. DOE), Unvented, Insulated Crawlspaces guide (last updated 2021-09-23), national laboratory, read 22 October 2026.
Pacific Northwest National Laboratory, Building America Solution Center (U.S. DOE), Water Management of Existing Crawlspace Floor guide (last updated 2015-02-09), national laboratory, read 22 October 2026.