How do I encapsulate a crawl space?

The short answer

Encapsulating a crawl space turns a damp, vented space under the house into a sealed, dry one. Water is kept out, the dirt floor and walls are covered with a sealed plastic liner, air leaks are closed, and something runs to dry the air. The liner and the sealing are a reasonable homeowner project. Closing the vents and choosing the drying system are where it goes wrong, so those decisions come last and often involve a contractor.

It is worth doing carefully. Research for the Department of Energy found that "vented crawlspaces located in mixed or hot-humid climates tend to increase the moisture level within the space instead of keeping it drier".^[1] ENERGY STAR calls sealing and insulating a crawl space a moderate to difficult do-it-yourself project,^[3] mostly because of the crawling: plan on several long sessions on your back in a tight space.

What has to be fixed before the liner goes down

The Department of Energy guide starts every conversion with a full evaluation of the outside and inside of the space, and says "This inspection must be based on federal, state, and local regulations and carried out by a licensed or certified contractor."^[1] Your own look with a flashlight is a good way to know what to ask about, but it does not replace that evaluation.

These conditions stop the work until they are solved: mold on the framing, exposed wiring or questionable electrical work, atmospherically ("B") vented appliances, signs of termites, standing water, and a high radon reading.^[1] Sealing a crawl space over any of these traps the problem inside it.

Outside water. Start at the gutters. Ideally the ground slopes at least 5 percent away from the foundation walls for the first 10 feet, and downspouts discharge into drains or at least 10 feet from the foundation.^[1] A liner cannot fix water that pours in after every storm; it only hides it.

Appliances. A furnace or water heater in the crawl space may draw its combustion air through the foundation vents. In the Department of Energy's words, "simply sealing the vents could make the situation worse, increasing the levels of moisture and potentially creating a deadly situation if naturally ventilated types of combustion equipment are located in the crawlspace".^[1] That equipment has to be assessed by a mechanical contractor first.

Radon. An effective way to reduce radon in crawl space homes is covering the earth floor with a high-density plastic sheet and drawing radon from under it with a vent pipe and fan. When properly applied, the EPA calls this the most effective way to reduce radon levels in crawl space homes.^[5] That is the same sheet you are about to lay, so test first. ENERGY STAR's advice: "If you are in a high radon area, consider contacting a contractor to conduct a radon test and install radon mitigation systems before sealing your home."^[3] Place the test kit in the lowest lived-in level of the home.^[4]

Permits. Virginia's Uniform Statewide Building Code requires a permit before alterations involving electric wiring, mechanical systems or fuel supply systems.^[6] A liner and caulk are not on that list, but a new outlet for a dehumidifier, a supply duct from your air handler, or a sump pump can be. Ask your local building department before any work that touches them.

Materials that make the job last

• Liner. Use "a minimum 10-mil or cross-laminated puncture-resistant 6-mil poly".^[1] Protect unreinforced 6-mil sheeting where people will crawl to service equipment; thicker reinforced material is more durable.^[2]
• Seam sealing. Advanced Energy recommends "duct mastic with embedded fiberglass mesh tape" for seams, or a tape the liner's manufacturer accepts for long-term use.^[2]
• Air sealing. Use caulk for gaps or cracks of 1/4 inch or less and spray foam for anything larger.^[3]
• Fasteners. Something mechanical to hold the liner on the walls, plus ground pins for the floor.

Laying the liner so it stays sealed

Work from the far end toward the access door so you are not crawling over finished sheet. Lap sheets at least 12 inches at the joints and tape them,^[1] and pin the field down wherever it lifts.

On the walls, seal the liner and also fasten it mechanically. Adhesives, sealants or tapes alone may be insufficient to support the material,^[2] and a liner that sags off the wall stops working. Leave a termite inspection gap of at least 3 inches between the top of the wall liner and the top of the masonry wall.^[2] Run it at least 4 inches up the piers,^[2] and seal it around any sump lid or drain.

Drying the air once it is sealed

"Crawlspaces must always have a drying mechanism."^[1] There are two usual ways. One is a small supply of conditioned air from the house's heating and cooling system, which is ductwork and belongs to an HVAC contractor. The other is a permanent dehumidifier draining to a sealed drain point, aiming for 30 to 50 percent humidity in the space.^[1] Frost can form on a dehumidifier's coils below 65 degrees,^[9] so pick a model rated for cool spaces.

Close the foundation vents only after the drying system is running. Then watch the numbers. Good installers aim to hold relative humidity typically at 70 percent or lower over time.^[2] A water leak alarm at a low spot or near the drain warns you of pipe leaks that could otherwise go unnoticed for a long time.^[1]

Insulating the walls is a separate decision. Rigid foam is the usual material, and most codes require a thermal barrier or an ignition barrier, such as gypsum board, over foam insulation unless the product is tested and documented for use without one,^[1] so it is worth having a contractor price that part. Rapid drying can cause cosmetic damage to wood floors and trim upstairs,^[1] so a slow, steady start is better than a big dehumidifier on high.

For what the full job costs from a contractor, see how much crawl space encapsulation costs in Virginia, and for keeping it dry afterwards, how to keep a crawl space dry. Pipes down there need their own attention in winter: protecting pipes in an unheated crawl space.

Stop and call a pro if

• There is standing water, or water comes back after rain: that is a drainage job before it is a liner job.
• Mold is growing on the joists, subfloor or any other framing. The Department of Energy guide says a mold remediation professional will need to be hired^[1] before the space is sealed. Soft or sagging wood also needs a pro.
• You smell gas: leave the house immediately, do not switch on anything electrical, and call your gas supplier or 911 from a neighbor's house.^[7]
• A carbon monoxide alarm sounds, or you get a headache, dizziness or nausea while working near a fuel-burning appliance: get fresh air immediately and leave the home.^[8] Have the appliance checked before anyone goes back down.
• You see exposed, damaged or spliced wiring, or you need a new outlet for a dehumidifier or sump pump.
• A furnace, water heater or other gas or oil appliance sits in the crawl space, or a gas line regulator is down there.
• Ducts need sealing, moving or a new supply run.
• You find termite tubes, a dead animal, or old wrap on pipes or ducts that you cannot identify.
• Your radon test comes back at 4.0 pCi/L or above, the level at which the EPA recommends action.^[4]

Licensed crawl space and restoration businesses are listed in the Blacksburg restoration directory, and the rest of the topic is under restoration answers and crawl space services. To get the job quoted, describe your crawl space on Loclis.