How the two designs differ
A vented crawl space keeps insulation under the floor and treats the space as outdoors. A closed crawl space seals the vents, covers the ground with a sealed vapor barrier and insulates the foundation walls, so the space becomes part of the house.
A closed crawl space still needs a way to dry. The Building America guide to unvented crawl spaces notes that the 2009 International Residential Code (section R408.3) calls for a drying mechanism such as a continuously running exhaust fan or a conditioned-air supply. The Department of Energy’s conversion checklist sets that supply at 1 CFM per 50 square feet of crawl space floor, so a 1,000-square-foot crawl space would get about 20 CFM.
Find the moisture source before blaming the vents
The pattern tells you a lot. Water that shows up after rain points to gutters, downspouts, grading or foundation leaks. A wet spot under a pipe points to a plumbing leak. Damp, cool surfaces in humid weather with no puddles point to condensation, which is where open vents can be part of the problem. Building America also lists bare soil, dryer vents exhausting into the space, and AC or furnace condensate lines.
Sealing comes last. DOE’s checklist has the contractor verify foundation drainage and damp-proofing before starting, and Building America says standing water, mold, rotten wood or hazardous materials get dealt with before any air sealing or insulation. The same thinking applies to documenting basement water before agreeing to a repair scope.
What you can check without crawling in
From outside, look for downspouts dumping at the foundation, soil sloping toward the house, and vents sitting at or below the soil line. Building America recommends ground that slopes away from the house at a 2 percent grade. From the access opening only, photograph puddles, bare soil, torn plastic and wet or sagging insulation. Leave the space to professionals if you see standing water near wiring, damaged electrical parts, suspected mold or rotted framing.
EPA’s mold guidance says to keep indoor relative humidity below 60 percent, ideally 30 to 50 percent, and Building America says supplemental dehumidification may be needed if humidity above 70 percent persists after a crawl space is enclosed. Air humidity isn’t the same as wet wood, so ask whoever inspects to measure the moisture content of the joists too.
Three things to check before any vent gets closed
Combustion appliances. If a furnace, boiler or water heater in the crawl space is a natural-draft type that pulls air from the space, Building America says to replace it with sealed-combustion, direct-vent equipment or have a qualified contractor check for gas leaks, carbon monoxide spillage and makeup air, with depressurization testing before and after the sealing work.
Flood openings. Flood vents let floodwater pass in and out to even out pressure on the foundation walls. Building America’s flood-resistant crawl space guide says many ordinary air vents are not suitable flood vents and that closed, conditioned crawl spaces should not be used in flood-prone areas. In Philadelphia, check the address on the City’s floodplain map; work in the Special Flood Hazard Area needs flood protection documents with the permit application, and projects costing $50,000 or more require a scoping meeting first.
Radon. The Pennsylvania DEP says about 40 percent of homes tested in the state have radon above EPA’s 4 pCi/L action level. The CDC says to test before and after renovations because major work can change radon levels, and Building America calls for a radon venting system under the vapor barrier where radon is a risk.
What a closed-crawl-space bid should spell out
Each water-entry fix, listed separately.
Ground cover: DOE’s vented-to-unvented checklist calls for at least 6-mil polyethylene over the entire ground, lapped 6 to 12 inches, with seams and tears sealed.
How the vents get sealed, what insulation goes on the walls, and an airtight, insulated access door of at least 16 by 24 inches.
The drying method: conditioned supply air, an exhaust fan or a dehumidifier. If you’re weighing a dehumidifier, see sump pump vs. dehumidifier.
Code items: Building America notes most codes require foam insulation to be covered by a thermal barrier, typically ½-inch gypsum board, or an ignition barrier, and a termite inspection strip of bare wall where termites are a risk.
A combustion safety test after completion if any natural-draft equipment remains.
Ask for monitoring, too. Building America recommends tracking crawl space temperature, humidity and joist moisture after the work, preferably for a year. Get in writing who comes back if dampness returns.
Who to call
Start with a home-performance or building-science contractor who measures first. Ask what measurements support the plan and what would make them advise keeping the vents. Send plumbing leaks to a plumber, grading and drainage problems to a drainage or waterproofing contractor (see interior drainage vs. exterior waterproofing), suspected mold to a remediation company, and rotted framing to a structural professional. Have the contractor confirm the design with the local building department.