Plastic on the Dirt or a Sealed Crawl Space: Choosing a Fix for Crawl Space Mold
A vapor barrier is a plastic ground cover over the dirt floor of a crawl space. It stops moisture evaporating up from the soil, which is the biggest single source of crawl space dampness. Encapsulation goes further: heavier plastic sealed at the seams and run up the walls, vents closed off, and the space kept dry with a dehumidifier or conditioned air. A good ground cover plus fixed drainage is enough for many crawl spaces. Encapsulation makes sense when the space stays damp anyway.
“Our crawlspace smells like a wet basement.” When someone tells me that, there’s often bare dirt down there. The ground under a house is always giving off moisture, and in a crawl space it has nowhere to go but into the wood above it. A vapor barrier stops most of that. Encapsulation seals the whole space and keeps it dry on purpose. Which one you need depends on whether the barrier alone gets the wood dry.
Why crawl spaces get moldy
Three sources, usually in this order:
- The ground. Bare soil evaporates water constantly, even when it looks dry. That moisture rises straight into the floor joists, subfloor and insulation.
- Summer air. Warm, humid outdoor air comes in through the vents, meets the cool ground and the cool framing, and drops its water right there. In humid weather, vents can let in more moisture than they let out.
- Water that gets in. Gutters dumping at the foundation, soil sloping toward the house, a plumbing leak above.
What I usually find: fiberglass insulation sagging or lying on the dirt, white or gray fuzzy growth along the joists, dark staining on the subfloor, and a sill plate that’s softer than it should be.
A vapor barrier: plastic on the dirt
A vapor barrier is a sheet of polyethylene laid over the whole dirt floor, overlapped at the seams and run a little way up the walls. It’s the cheapest, most effective single step for a damp crawl space, because it cuts off the biggest source of moisture.
When it’s enough: the space is mostly dry, the outside drainage is good, and the wood reads dry a few weeks after the barrier goes in.
What makes one work: full coverage with no bare dirt showing, seams overlapped well, and sturdy enough plastic that it doesn’t tear the first time someone crawls across it. Thin plastic shredded by years of plumbers and cable installers is one of the most common things I see under houses.
Encapsulation: sealing the space
Encapsulation treats the crawl space like part of the house instead of part of the outdoors:
- Heavier, reinforced plastic over the floor, with seams taped or sealed
- The liner carried up the foundation walls and sealed there
- Vents and other openings to the outside closed off
- If there’s a gas or oil furnace or water heater down there, an HVAC contractor confirming it still gets combustion air before anything is sealed
- The space kept dry with a dehumidifier or a small supply of conditioned air from the house
- Insulation moved from between the joists to the walls, in many designs
The model residential code allows sealed crawl spaces, as long as the ground cover is sealed and the space is dehumidified or gets a little conditioned air. Done right, it holds the space at a steady, low humidity year-round.
When it’s worth it: the crawl space stays damp even with a good ground cover, there’s ductwork or a furnace down there, or the house has had repeat mold problems in the floor framing.
The catch: it’s more work and more money, and it has to be done completely. A crawl space with the vents closed but bare dirt still showing is worse than one that’s left alone.
What has to happen first, either way
- Fix the water outside. Gutters, downspouts, grading.
- Clean what’s already growing. Encapsulation keeps a space dry. It doesn’t remove mold. Active growth on joists and subfloor gets cleaned under containment, wet insulation comes out, and the wood should read dry before anything gets sealed. The readings are explained in moisture meter readings explained.
- Check the wood. A screwdriver or pocket knife into the sill plate and a few joist ends. Soft wood is a carpenter’s job before it’s a sealing job.
If you’re adding a dehumidifier to a sealed crawl space, the same logic from sizing a dehumidifier for an Adams County basement applies.
Older houses and additions
Some older homes have a basement under the original house and a crawl space under a later addition. If yours does, that crawl space is often the dampest, least-visited part of the house. Check it even if the main basement is fine. The basement side is covered in basement mold in Gettysburg homes.
Start with the cheap step
Get the outside water handled, clean what’s grown, put down a proper ground cover, and check the readings again in a few weeks. If the wood is dry, you may be done. If it isn’t, encapsulation is the next step, and you’ll know it’s needed instead of guessing. That’s how my crawl space mold work is approached, and it’s part of how I do mold remediation in Gettysburg, PA.
Questions about crawl spaces
Should crawl space vents be open or closed?
It depends on how the crawl space is built. A traditionally vented crawl space relies on those vents, though in humid summer weather they can let in more moisture than they let out, because warm wet air hits the cool ground and framing. A sealed, encapsulated crawl space has the vents closed on purpose and is kept dry another way. Closing vents on an unsealed crawl space with bare dirt usually makes it worse, not better.
Does the mold on the joists have to be cleaned before encapsulation?
Yes. Encapsulation keeps a crawl space dry going forward. It doesn't remove what's already growing. Active growth on joists, subfloor and sill plates should be cleaned under containment first, wet or fallen insulation should come out, and the wood should read dry before the space is sealed. Sealing over active mold just traps it in a smaller, darker box.
What happens to the fiberglass insulation between the joists?
In a damp crawl space it often sags, gets wet, falls to the floor and stays there. Wet fiberglass doesn't insulate and holds moisture against the wood. It usually comes out during cleanup. Whether new insulation goes back between the joists or on the walls depends on whether the crawl space is staying vented or being sealed, so decide that first.
John will tell you which fix it needs
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