Crawl Space Encapsulation in Farmington Hills

The vapor barrier is the base layer and the cheapest part. Under the residential code, all bare soil gets one unbroken Class I vapor retarder. Seams overlap by 6 inches and are taped or sealed, and the sheet climbs 6 inches or more up the wall, where it's fastened and sealed. It doesn't name a thickness. HomeGuide prices material from 6 mil to 20 mil at $0.30 to $0.70 per square foot, and $2 to $4 per square foot installed. We'd lean toward a heavier, reinforced sheet where a furnace, ducts or plumbing mean someone will crawl over it every year.

Encapsulation carries the liner past that 6-inch minimum, up the walls and around each pier, and seals it to the wall or the wall insulation. Every seam is taped and every penetration sealed. Building Science Corporation's guidance is to treat the crawl space as a mini-basement: unvented, ground sealed, walls insulated and the air conditioned, rather than vented to the outdoors. Under the code, a crawl space with its vents closed also needs a way to condition the air, covered next.

Sealing the vents keeps outdoor air and its moisture out, but soil, concrete and leaks still add water. EPA wants indoor relative humidity under 60%, and between 30 and 50% if you can hold it. A crawl space dehumidifier hangs from the joists or sits on a stand, with its condensate piped to a drain or the sump. HomeGuide puts one at $1,000 to $3,000 installed. One note for Michigan: the 2015 code text that our residential code follows lists exhaust ventilation and conditioned supply air as the ways to condition an unvented crawl space, and doesn't list a dehumidifier. The contractor should confirm what the Building Division will accept.

Once the crawl space is sealed, its vents come out of service. DOE's Building America guide cuts rigid foam to fit each vent and seals it in with spray foam or caulk, fastens foam board to the walls with masonry fasteners, gaskets the access door and foams each rim joist bay. DOE notes that in cold climates this protects the floor above and the pipes from the cold. With the walls insulated, the floor between the joists doesn't have to be. HomeGuide puts sealing vents at $150 to $450 and insulation at $500 to $3,700.

If water stands on the floor after rain, a liner will only float on it. A shallow perimeter drain in gravel carries water to a small sump, and the pump sends it out. HomeGuide puts an interior crawl space drain with a pump at $800 to $3,000. The code is strict here: where groundwater can rise to within 6 inches of the crawl space floor, or surface water doesn't drain from the lot, the inside grade has to be as high as the outside grade unless an approved drainage system is installed. The sump pump installation rules for pit size and discharge apply to this pit too.