The Crawlspace Vapor Barrier:
Displaced, Missing, Stained or Brand New
Bare soil under a house gives off water all year. A sheet of plastic is what keeps most of it out of the joists, the insulation and the air you breathe upstairs.
The plastic on the crawlspace floor came up in 80 of Russ’s 269 recent reports. Forty of those were simply displaced: pulled back, bunched up or shoved aside, leaving bare soil. Others were missing in sections, stained with water, laid over the foundation vents, or brand new and hiding whatever was there before.
It’s one of the cheapest parts of the house to fix. It also does more than it looks like.
What the plastic is for
Soil is damp, even in summer. In a vented crawlspace, that moisture evaporates into the air under the house, where it can raise the moisture content of the joists and subfloor, feed mold-like growth and drift upstairs. The vapor barrier is a sheet of polyethylene laid over the ground so that moisture stays in the soil.
Russ’s report wording: a missing or displaced vapor barrier allows ground moisture to accumulate, leading to elevated moisture levels in the crawlspace. The residential code calls for at least 6-mil plastic, lapped at the seams. The crawlspace water intrusion post covers what happens when the moisture comes from outside instead.
Real Inspection Photos
Vapor Barrier Findings From Russ's Inspections
Displaced, incomplete, stained and new crawlspace vapor barriers from Russ's recent reports.
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Vapor barrier displaced, leaving soil exposed. -
Plastic that didn't cover all of the exposed soil. -
Plastic pushed back in one area, leaving bare dirt. -
Stains on a new house's vapor barrier, likely from rain during construction. -
A newly installed vapor barrier, which also covers whatever was on the ground before.
Displaced and missing plastic
Displaced plastic was the most common finding by far. Every trade that goes into a crawlspace drags the plastic: plumbers, electricians, pest crews, cable installers, insulation crews. Nobody puts it back. Rodents tear and tunnel under it.
In Happy Valley and Ridgefield, gaps were left where sheets weren’t overlapped. In Battle Ground and Washougal, the plastic was pulled back far enough that live plants were growing in the crawlspace. In a Gresham crawlspace, it was displaced in several areas and covered in mud. Portland and Sandy houses had sections missing, and one Portland crawlspace had no vapor barrier at all. A Clackamas manufactured home had bare dirt under it.
Stains, water and new construction
Water stains on top of the plastic show that water was there at some point. Russ found heavy staining in a Beaverton crawlspace that appeared to be from prior moisture intrusion.
On new houses, it’s often rain. In Tigard and Ridgefield new builds, the staining on top of the plastic most likely came from rain before the house was closed in. Russ notes it either way. A new Beaverton house had minor staining on a freshly installed barrier, which still means water got there after it went down.
New plastic and what it covers
Sellers often lay new plastic before listing. Russ found new vapor barriers in Portland and Lake Oswego crawlspaces, one fastened to the foundation and taped at the seams. That’s an improvement. It also covers the ground, so evidence of old water, rodent activity or debris underneath isn’t visible. His reports say so.
A new barrier is a good reason to ask the seller why it was replaced. The rodent activity post covers one common answer.
Vents and radon pipes
Plastic can cause problems where it’s installed carelessly. In Clackamas, the vapor barrier was run up over foundation vents, blocking airflow; the report recommended cutting it back at the vents. The foundation vents post covers why those openings matter.
In houses with a radon system on a crawlspace, the plastic is part of the system. The fan pulls soil gas from under the sheet, which only works if the sheet is sealed, including around the pipe. Russ found radon pipes not sealed to the barrier in two Vancouver houses. The post on buying a house with a radon mitigation system covers the rest of that system.
What Russ checks
He enters the crawlspace when it’s accessible and safe, looks at the vapor barrier’s coverage, overlap and condition, notes standing water, staining and debris, and takes moisture readings on the framing. Areas he can’t reach or see, and what’s under new plastic, get noted as limitations. Repairs usually go to a crawlspace or insulation contractor.
- Ask whether anyone has been in the crawlspace recently and what they did
- From the hatch, look for bare soil showing between sheets
- Look for water or mud on top of the plastic
- If the plastic is new, ask when and why it was replaced
- With a radon system, check the plastic is sealed around the pipe
A vapor barrier only works where it covers the soil. Displaced plastic was the most common crawlspace finding in Russ’s reports, usually left behind by whoever was last under the house. Stains on top of it mean water got there at some point.
For the rest of the crawlspace, the guide to what to look at in the crawlspace before you make an offer covers framing, insulation and drainage. To set up an inspection, you can book online.
Vapor Barrier FAQ
