Crawl Space Vapor Barrier Installation in Tacoma, WA
Polyethylene over the crawl space soil is the least Washington code allows and the thing most often missing, or in shreds, under a pre-1980 Tacoma house.
What a Vapor Barrier Does and What Code Requires
A crawl space vapor barrier is polyethylene sheeting laid over the soil to keep ground moisture from evaporating into the house. Washington's residential code requires at least 6-mil black polyethylene overlapped six inches at the joints. Replacing one under a Tacoma house runs about $1,500 to $4,000 with cleanup.
Soil under a Tacoma house is wet for most of the year even when no water is visible. The Alderwood till that underlies most of the city holds a perched water table one to three feet down from December into April, and soil that wet gives off water vapor continuously. Without a barrier that vapor rises into the floor insulation, condenses on the joists, and drifts up into the house. The Tacoma-Pierce County Health Department's mold guidance for homeowners is to cover the dirt in the crawl space with heavy plastic, and the state residential code makes it a requirement.
6-mil and heavier
WAC 51-51-0408 requires a ground cover of 6-mil black polyethylene, or an approved equal, overlapped six inches at the joints and extended to the foundation wall. With a Class I vapor retarder on the ground the required vent area drops from one square foot per 300 square feet of crawl space to one per 1,500, which is why the barrier and the vents get sized together. Six mil is thin. It tears when a furnace technician crawls across it, and it is what most Tacoma houses have in pieces today. A 10- to 12-mil reinforced sheet costs more per roll and holds up; a 20-mil liner is encapsulation territory and belongs with sealed seams and closed vents rather than under an open-vented crawl space.
Overlap, wall lap, and posts
A barrier laid edge to edge with gaps around every post does about half its job. On a good installation the seams overlap at least six inches and are taped, the sheet runs several inches up the stem wall, and each post and pier is cut around and sealed. On a sloped crawl space floor the sheet is pinned so it cannot slide downhill into a heap, which is how many Tacoma barriers end up.
When a Vapor Barrier Is Enough
A vapor barrier on its own is the right scope when the crawl space is dry, the vents are open and clear, the floor insulation is in reasonable shape, and the only problem is a missing or torn ground cover. That describes a fair number of houses on the outwash gravel south of the city and in Lakewood, where the ground drains and the crawl space stays dry through the winter.
It is the wrong scope when there is standing water, when the vents are admitting wet air that condenses on the framing all winter, or when the insulation and joists are already wet. A barrier laid on a wet crawl space floats, and a barrier under wet insulation keeps the wood damp from above. Those crawl spaces need drainage first and often encapsulation after.
What Replacement Involves
- Remove the old sheet along with the debris, fallen insulation, and rodent material lying on it.
- Clear and level the soil, pulling sharp rock and construction scraps that would puncture the new sheet.
- Lay the new sheet, overlapped and taped, run up the wall, and cut around and sealed at every post.
- Check the vents are open, unblocked, and not covered by the sheet or by the floor insulation.
- Photograph the finished floor so the homeowner can see it without crawling.
A straightforward replacement on a 1,200 square foot crawl space is a one-day job for a two-person crew. Rodent cleanup, which is common in Tacoma's older neighborhoods, can double both the time and the price.