Crawl Space Moisture and Mold Cleanup in Tacoma, WA
Where the water under a Tacoma house comes from, how it is stopped, and what cleaning the mold it left behind involves.
Where the Water Comes From
Water in a Tacoma crawl space is nearly always groundwater perched on glacial hardpan from December to April, or roof water from downspouts discharging at the foundation. Mold on the joists follows. Cleanup runs about $2,000 to $7,000, and the water must be stopped first or the mold returns.
Perched groundwater on the till
The Alderwood soil series that covers most of Tacoma's uplands is described by the USDA as moderately well drained, with a dense layer 20 to 40 inches down and a perched seasonal water table 12 to 36 inches below the surface at times from December through April. A crawl space floor dug a foot or two below grade sits in that water. It comes past the footing and up through the soil, and it does so on schedule every winter, which is why a crawl space that was dry in August is a pond in January.
Roof water on the outwash
South Tacoma, Parkland, Spanaway, and most of Lakewood sit on glacial outwash gravel, the Spanaway series, which the USDA calls somewhat excessively drained. Groundwater rarely reaches a crawl space here. When one is wet the cause is usually a downspout discharging beside the foundation, a grade that slopes toward the house, or a leaking irrigation line, and the fix is a downspout extension and some regrading rather than a drain.
The valley floor
In Fife, Puyallup, and Sumner the ground is river alluvium, and on some valley soils the seasonal water table sits within a foot of the surface unless the land is drained. Crawl spaces here are rarely dry by gravity. A sump pump with a battery backup is the working solution, and it works year round rather than only in winter.
Condensation from the vents
A vented crawl space takes in outdoor air that is close to saturated for most of the Tacoma wet season. Against framing chilled by wet ground it condenses, so joists can be wet and moldy in a crawl space that never had liquid water on the floor. Closing the vents and drying the air is the fix, which is encapsulation.
Stopping It
- Interior perimeter drain to a sump: a trench along the inside of the stem wall with perforated pipe and rock, collecting groundwater and pumping it out. The standard Tacoma fix on till ground, about $1,500 to $5,000 depending on the perimeter.
- Sump pump and basin: about $1,200 to $3,500 installed. A battery backup adds about $600 to $1,500 and matters during a November windstorm outage, which tends to arrive with the heaviest rain.
- Downspout extensions and regrading: the cheapest fix and the first one to check, often a few hundred dollars.
- Exterior footing drain: dug outside the stem wall when the wall itself is weeping and the yard is accessible. More expensive, and a drainage contractor's job rather than a crawl space contractor's.
- Dehumidifier: about $1,200 to $2,500 installed. Required when the vents are closed, and useful anywhere the air stays damp after the water is gone.
The contractor should say which of these the crawl space needs and why, and should name where the pumped water goes. Tacoma's municipal code prohibits routing groundwater into the sanitary sewer, so a sump discharge line to the yard or to a permitted storm connection is part of the design.
Mold Cleanup
Mold on crawl space framing in Tacoma is a symptom of the moisture described above, and it is treated after the water is handled. The Tacoma-Pierce County Health Department's guidance is that an area larger than about ten square feet, roughly a full sheet of newspaper, is a job for a professional; that testing is not usually needed unless documentation is required for medical or legal reasons; and that indoor humidity belongs between 40 and 50 percent. The Department lists eye, nose, and sinus irritation, skin rash, and problems with asthma among the reactions people can have to mold.
Cleanup in a crawl space means removing the contaminated insulation and debris, HEPA-vacuuming and cleaning the framing with an antimicrobial, sometimes sanding or media-blasting heavily stained wood, and applying a sealer where the contractor uses one. It runs about $2,000 to $7,000 for a typical crawl space. Surface mold on sound wood cleans off; wood that has gone soft is a repair job.