Crawl-Space Moisture: Drainage, Vapor Barriers, and When to Get Help
Water in a crawl space is usually a report on the outside of the house. Roof runoff, a grade falling toward the wall, or ground water arrives first; the damp soil, the rusted hanger and the smell follow. Work in that order — the water’s route at the exterior, the ground cover under the floor, then the vents — and treat the space as something to look into from the hatch, not somewhere to spend an afternoon.
The Environmental Protection Agency calls a crawl space a “hole in the ground” with more contact with soil than a slab-on-grade foundation — which is why it is often where a drainage fault shows first. The Department of Energy’s Building America Solution Center states that the ground cover exists to keep ground moisture and soil gases out of the space “where they can be pulled into the home through cracks in or around the subfloor.”
What you can see from the hatch
The federal guide lists common causes as a water leak, a sewer leak, water entry from rain or melting ice and snow, a dehumidifier that has stopped working, and a vapor barrier that is torn, badly installed or missing. It also gives two figures worth carrying: wood moisture above 18 percent is a problem, and air above 70 percent relative humidity is a problem.
| What you see | What it points to | The check that separates it |
|---|---|---|
| Standing water after rain | Bulk water arriving from outside | Time it against the weather |
| Damp soil, no puddle | Ground moisture, or a slow leak | Whether it is wet in a dry week |
| White powder on the wall | Water passing through concrete and evaporating | Where it enters, and at what height |
| Rust and condensation on ducts | Humid air rather than liquid water | Whether the ground cover is whole |
| Musty smell upstairs, no visible water | Vapor moving up through the floor | The ground cover and the vents |
The safety line comes first
Look at the crawl space; do not work in it. The federal guide’s own method assumes as much: it asks for a respirator, eye protection, a coverall with hood, gloves and two lights, and states that the job “could take over an hour and is a physically demanding task” — “hire a contractor if you don’t feel up to it.” Where there is significant standing water, the instruction is not to enter that area but to document it and have a professional remove the water; exposed wiring is documented from a distance and not touched.
What follows is limited to what can be seen from the hatch, measured from outside, or written into a scope. No method for entering or working in a crawl space is published here.
Check one: the water arriving from outside
The EPA’s design guidance asks for a 5 percent slope — 6 inches per 10 feet — to the finish grade away from the building over the 6 to 10 feet nearest it, and for at least 8 inches of foundation left exposed after final grading. Both are taken with a tape and a level, in the rain; the checks at the wall cover how.
Roof water is the fastest route and the easiest to redirect. An outlet discharging a few feet from the wall puts a concentrated stream where the grade is trying to take water away; the outlet decision is its own subject in this cluster. Spill over the gutter’s front edge while the outlet runs dry is a gutter diagnosis, and an outlet that has stopped passing water is checked at the downspout.
The Solution Center’s retrofit guidance sets the order: re-grade away from the structure, make sure patios, sidewalks and driveways slope away too, then attend to gutters and downspouts. The interior floor is also meant to sit above the local water table, which in an existing house is a fact to establish; the wet-season observation that does it, part of the foundation checks, cannot be made in August.
Check two: rain, plumbing, or the ground?
Three of the federal guide’s five causes are plumbing: a water leak, a sewer leak, a dehumidifier that has stopped working. Separating them is a matter of record. A supply leak runs whether or not it rains and whether or not anyone is home. A drain leak tracks use: it appears after a shower or a load of laundry, not otherwise. Water present in a dry week with nothing running is a plumber’s question, and a sewer smell is plumbing rather than moisture.
Check three: the ground cover
Washington’s residential code, in its under-floor section, requires a ground cover of six-mil black polyethylene or an approved equal, overlapped at least 6 inches at the joints and extended to the foundation wall; it may be omitted only where a concrete slab at least 2 inches thick forms the floor. The section is WAC 51-51-0408.
The federal guidance is more demanding where it matters: six mil or thicker, seams lapped 6 to 12 inches and sealed, the sheeting sealed around pipes, edges lapped up walls and piers at least 6 inches and fastened. The thickness agrees; the lap does not.
The Department of Health’s radon program gives a second Washington reason to keep it whole. It asks homeowners to “inspect and fix, or install, a good crawl space vapor barrier,” with any seams, holes or tears well taped, because the goal is a gas-tight barrier between the house and the ground beneath it. Radon is a radioactive gas from the decay of elements in rock and soil; the department states that exposure over time increases the likelihood of lung cancer, that testing is the only way to know the level, and that a crawl space is not where to test.
A ground cover does not drain anything. The EPA asks for the earth to be sloped so water is directed to a disposal location before the barrier goes over it. Where water collects under the sheeting, the sheeting is hiding a drainage question rather than answering it.
Check four: the vents
A vented crawl space is a code-defined assembly, not a set of holes someone made. The residential code sets the minimum net area of ventilation openings at 1 square foot for each 300 square feet of under-floor area, requires them evenly placed for cross ventilation — one side may have none — and specifies the covering, down to corrosion-resistant wire mesh with a least dimension of 1/8 inch.
The same section allows that area to fall to 1/1,500 of the floor area where the ground carries an approved Class I vapor retarder and the openings give cross ventilation. A good ground cover does part of the work the vents were doing.
Two things block vents in practice: debris, and foam blocks put in to “winterize” the space and left there all year. The Department of Health adds the same instruction from the radon side — keep the foundation vent-blocks unobstructed.
Vented or sealed is a decision, not a repair
Sealing a vented crawl space is not a homeowner repair. The EPA states that crawl space foundations may be vented to the outdoors or air sealed, and sets out what an unvented space requires: a continuous Class I vapor retarder with joints overlapping 6 inches and sealed or taped, edges extending at least 6 inches up the stem wall and sealed to it, and a radon system built to the code’s radon appendix.
The retrofit sequence is equally plain: before any sealing and insulating of the crawl-space walls, the drainage and moisture issues in and around the space have to be dealt with, so that it will be dry and stay dry once enclosed. Sealing a wet crawl space encloses the water.
So the decision is not “seal or vent.” Fix the water first, then decide with someone who knows the code and the building whether the space should be vented or sealed, and what that does to its radon provisions.
Mold follows the water
The EPA’s homeowner guidance is direct about order: “The key to mold control is moisture control,” water-damaged areas should be dried within 24 to 48 hours, and “if you clean up the mold, but don’t fix the water problem, then, most likely, the mold problem will come back.” Its threshold for a household job is a patch of about 10 square feet; beyond that it directs the reader to its remediation guidance and a contractor experienced in mold cleanup.
Nothing here diagnoses mold from a color, a photograph or a smell, and the whole-house question of where indoor moisture comes from is its own subject.
What to ask before anyone works in there
When you hire, you are buying a diagnosis and a scope, not a chemical. These questions separate one written proposal from another:
- Which source did you identify, and what did you observe that identifies it?
- Will the earth be re-sloped toward a disposal point, or only covered?
- What is the ground cover specification — thickness, seam lap, sealing at pipes, walls and piers?
- Do the vents stay open, and if not, what is the code path for sealing them and what happens to the radon provisions?
- How will standing water be removed, and where does it go?
- What does the price exclude, and how will the before-and-after condition be documented?
Where this stops
Entry into a crawl space, removal of standing water, electrical work, gas work, structural repair and mold remediation above a small patch are not household tasks. Three conditions end the household’s judgement entirely. Structural evidence — sagging joists, a post no longer bearing on its pier, a large crack in a wood member — is a question for a qualified professional. Water appearing inside the living space is a diagnosis before it is a repair. Radon mitigation is its own trade, and the Department of Health directs homeowners with elevated results to a certified professional.
Next step
Look in from the hatch after the next significant rain — the Solution Center names spring or fall, after a significant rain event, as the useful window — and write four lines: the date, where the dampness is, what the weather had done, and what had been running in the house. Then walk the outside in the same rain: the grade, the outlets, the paving. Those two records decide whether the next conversation is with a drainage contractor, a plumber, or nobody at all.