Crawl Space Encapsulation in Tacoma, WA
A sealed liner on the ground and walls, closed vents, and controlled humidity, built for a crawl space that sits in glacial hardpan water from December to April.
What a Sealed Crawl Space Includes
Crawl space encapsulation seals a crawl space from the ground: a reinforced 12- to 20-mil polyethylene liner over the soil and up the stem walls, seams taped, foundation vents closed, and humidity held down by a dehumidifier or conditioned air. A typical Tacoma job runs about $6,000 to $14,000.
In Tacoma the word is sold at every level from a heavier vapor barrier to a fully conditioned space. A crawl space counts as encapsulated when the ground and stem walls are both covered, the seams are sealed rather than overlapped, outdoor air is kept out, and something controls the humidity. Skip any of the four and the crawl space is still connected to the wet ground it sits in.
The liner
Washington's code minimum for a crawl space ground cover is 6-mil black polyethylene. An encapsulation liner is a different product: 12 to 20 mil, string-reinforced to survive twenty years of plumbers and inspectors crawling over it, run up the stem wall and fastened there, cut around each post and pier and sealed, and taped at every seam. The residential code's unvented crawl space section asks for joints overlapped six inches and sealed or taped, with edges carried at least six inches up the wall and sealed to it; a contractor building to that line is building to Section R408.3.
The vents and the air
Foundation vents in a Tacoma crawl space admit air that is near saturation for half the year. In winter it condenses on framing chilled by wet ground, which is why joists in a vented crawl space can drip in January with no groundwater in sight. Encapsulation seals the vents with foam or rigid blocks and replaces ventilation with a crawl space dehumidifier draining to a sump or outdoors, or a small supply of conditioned air from the house. Closing the vents with neither is the version that grows mold, and it is why the code attaches conditions to an unvented crawl space.
The radon provision
Because a sealed crawl space stops soil gas leaving through the vents, WAC 51-51-0408 requires a radon system built to Appendix AF for any unvented crawl space, and WAC 51-51-60101 applies that appendix to unvented crawl spaces in every Washington county. Pierce County sits in the EPA's lowest radon zone, and the EPA advises testing every home regardless of zone. The system is a perforated pipe under the liner run to a vertical vent stack, passive unless a test shows a fan is needed. It is cheap to build while the liner is going in and expensive to add afterward.
Where Tacoma Encapsulations Go Wrong
- Liner laid over standing water or damp soil with no drain or sump. On till ground the water returns every December and floats the liner.
- A 6-mil ground cover sold as encapsulation. It is a vapor barrier, and a legitimate one, at a fraction of the price.
- Vents closed with no dehumidifier or conditioned air. Humidity climbs, the joists grow mold, and the encapsulation gets the blame.
- Wet fiberglass left on the joists under a new liner. The insulation keeps the wood damp long after the ground is sealed.
- No radon provision. The code requires one for an unvented crawl space, and a later home inspection may ask about it.
An inspection that measures wood moisture, looks for water lines on the stem wall, and checks the downspouts outside before quoting the liner is the one worth waiting for.
The Work, Step by Step
- Inspect and measure. Square footage, clearance, standing water, wood moisture readings, rodent activity, posts and sills, and where the downspouts discharge.
- Fix the water. Interior drain and sump on hardpan lots; downspout extensions and grading on outwash lots; an exterior footing drain where the stem wall itself is weeping. Details on the moisture page.
- Clean out. Old barrier, wet insulation, debris, and rodent contamination removed and disposed of; framing cleaned where mold is present.
- Repair. Sistered joists, replaced sills, new posts on new footings, done while access is open. See structural repair.
- Seal. Liner on the soil and walls, taped seams, post wraps, vents closed, radon pipe under the liner if the crawl space will be unvented.
- Condition. Dehumidifier set and drained, or a conditioned-air supply, and new floor insulation at R-30 where the floor is being insulated.
A plain liner over a dry, clean crawl space is a one- or two-day job. A Tacoma crawl space with water, rodents, and rot is a week.