Piecing Together a Closed Crawl Space

Q: I’d like to close my crawl space, but I’m not sure where to begin. A company that does pest control and crawl space work gave me a quote that includes closing the vents, laying plastic over the soil and installing a dehumidifier. Is this all I need?

A: It can be daunting to hire a professional to do work when you are not sure what needs to be done. For starters, a closed crawl space is more than just closed vents, plastic and a dehumidifier. Let’s break down the required parts into four manageable categories.

Safety First!

Gas furnaces or gas water heaters located in the closed crawl space must be direct-vent or power-vented. This ensures there is no opportunity for harmful gases to leak and linger in the space. Another safety consideration is radon testing, which is recommended for all homes by the U.S. surgeon general and U.S. Environmental Protection Agency (EPA). The EPA recommends that all homes with radon concentration of 4 picocuries per liter and greater inside the living space be mitigated.

The Box

Think of the crawl space as GladWare® — you know, the plastic containers that store leftover soup or peach cobbler. Like GladWare, a closed crawl space needs to be sealed tight so no outside air gets in. To build this air-tight box, the contractor will install plastic sheeting over the soil plus the masonry walls and piers, overlapping and sealing the seams. They will also seal all holes and gaps linking the crawl space to the great outdoors and the interior of your home with caulk or foam. This includes installing weatherstripping on the crawl space doors and covering crawl space vents with rigid foam or wood. Once all six sides of this crawl space “box” are sealed, insulation should be installed either on the perimeter walls or touching the wood subfloor.

Pest Control

A 3-inch-tall portion of the foundation wall should remain visible immediately below the wood floor framing to allow for termite inspections. Be sure to check with your pest management professional and contract to avoid any problems with this change.


Managing liquid water and water vapor are very important. Water should already be directed away from the exterior of the foundation with gutters, downspouts, sloped soil and foundation drains. Closed crawl spaces additionally need a floor drain or sump pump so the crawl space doesn’t turn into a swimming pool if there is condensation, wicking, a plumbing leak or other unexpected water event.

Water vapor is managed with a dehumidifier, supply air from the HVAC system, house air or an exhaust fan. Advanced Energy’s research found that dehumidifiers and supply air are the most effective methods for controlling water vapor in a closed crawl space. If ductwork is already located in the crawl space, a supply duct is a terrific and inexpensive option. Be aware of the long-term electrical and maintenance costs of running a dehumidifier or exhaust fan.

For more information about crawl spaces, Advanced Energy has research findings and other resources available at www.crawlspaces.org.

As always when hiring a contractor, get at least three estimates, check references and have a detailed scope of work to make sure that you and the contractor are on the same page. Prices and descriptions may vary, but all estimates should include work that addresses the above four research-based categories. It’s also reasonable to ask for before and after pictures. Go for it and trust your gut!

This article was originally published in Carolina Country magazine.