Missing Crawl Space Vapor Barrier: What It Means on Your Home Inspection Report

Your inspector found bare soil in the crawl space where there should be a sheet of heavy plastic, or a sheet that's torn, gapped, or pulled aside. Reports call it a vapor barrier, vapor retarder, moisture barrier, or ground cover; they mean the same layer.

The Department of Energy's Building America program says all crawl spaces should have a ground covering of polyethylene sheeting to keep ground moisture and soil gases out, whether the crawl space is vented or not. Oregon State University Extension's advice to homeowners is just as direct: is there a layer of plastic on the soil floor of your crawl space? There should be.

Why It Matters

Building America explains that moisture can move up through the soil and enter the crawl space as liquid water or water vapor, which can then condense on the floor joists. Moisture in crawl spaces, it says, can harm the home by promoting mold and rot along floor joists and rim joists. The EPA's 2013 moisture-control guidance listed exposed earth in crawl spaces among the sources of indoor humidity and said that in low-rise buildings, damp basements or crawl spaces may add as much water vapor to the air in a day as all the other internal sources combined.

Building America adds that the plastic helps keep soil gases from being pulled into the home through cracks in or around the floor above. If you haven't tested the house for radon, now is a good time; see our radon test kit guide.

A ground cover isn't the whole story, though. In vented crawl spaces, Building America says the dominant source of moisture is bulk water: improper grading, a lack of perimeter drainage, or improper irrigation. In humid climates, it adds, moist outdoor air coming in through the vents can condense on cooler framing too. So check the outside as well; see our pages on negative grading and downspouts draining near the foundation.

Urgent (beyond repair-soon): standing water, mold on the joists, soft or rotted wood, or floors that sag or feel springy above the crawl space. Those mean the moisture has already done damage; get the water source and the framing evaluated first.

What to Do Before Closing

• Ask the inspector for details. Is there no cover at all, or a torn or partial one? Was the soil damp, was there standing water, and did they see mold, wood rot, or wood in contact with soil? Under the InterNACHI standards of practice, one of the standards home inspectors work under, inspectors aren't required to enter a crawl space that isn't readily accessible or to move a vapor retarder, so ask whether they went in.
• Get a contractor to evaluate it. Choose one who does crawl space moisture work, and have them check drainage and framing as well as the cover. Rot in the joists, sill, or beams calls for a licensed structural engineer or qualified contractor.
• Talk to your agent. Your real estate agent can advise whether to ask the seller to repair it or offer a credit within your inspection contingency.

For other items on your report, see more findings in our inspection report decoder.

What to Do After You Move In

Water first. Building America says that if any standing water, mold, rotten wood, or hazardous materials are present, those conditions should be addressed before the vapor barrier is replaced. Oregon State Extension makes the same point about rot: eliminate the source of the water before replacing rotten wood.

Then the cover. Building America's specification:

• polyethylene sheeting at least 6 mil thick covering the entire soil floor;
• seams lapped 6 to 12 inches and sealed with acoustical sealant, butyl caulk, or tape made for polyethylene;
• edges lapped at least 6 inches up the walls and piers and attached with mechanical fasteners or pressure-treated furring strips, or else staked to the ground around the perimeter.

If there's already a cover, Building America says a 6-mil sheet that is torn or unsealed may be repairable by taping the tears, overlapping and taping the seams, and adding pieces to extend it up the walls. A more durable option is a new layer of 6-mil or thicker polyethylene over the old one, sealed at every seam and fastened to the walls and piers.

Can You Fix It Yourself?

Taping a small tear, or laying and sealing plastic in a dry crawl space you can move around in, is work some homeowners take on. Oregon State Extension's crawl space kit is coveralls, gloves, a cap, and a headlamp or flashlight. Work only where you can move freely and get out easily.

Leave it to a pro if there's standing water, mold on the framing, rotted wood, or materials you can't identify; Building America says those come first. Drainage, structural repairs, and closing the crawl space are contractor jobs, and damaged wiring belongs to a licensed electrician.

Frequently Asked Questions

Is a crawl space vapor barrier required?

Building America's guidance is that every crawl space should have one, vented or not. Whether it was required when your house was built depends on your jurisdiction and the age of the home; your local building department can tell you what applies now.

Is a torn vapor barrier a big deal?

Tears in an existing 6-mil cover may be repairable with tape, Building America says. Large gaps, missing sections, or plastic that has been pulled back leave soil exposed, so treat them like a missing cover.

Should I encapsulate the crawl space instead?

Maybe, especially in a humid climate, where Building America says building scientists recommend insulated, unvented crawl spaces. But a ground cover is part of either approach, and Building America says drainage and moisture problems must be dealt with before a crawl space is sealed. Closing one changes how it's dried and conditioned, so it's a design job for an experienced contractor, and local codes vary.

The Bottom Line

A crawl space without a sealed ground cover lets soil moisture rise toward the floor framing, where it can promote mold and rot. Ask the inspector what they saw, get a crawl space contractor's evaluation, and talk to your agent before your contingency ends. After you move in, fix any water problems first, then install or repair a 6-mil or thicker polyethylene cover with sealed seams, lapped up the walls and piers.

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