Guide · Crawl spaces
Crawl space water and mold: causes and fixes
A damp crawl space can rot floor framing, grow mold, and send musty air into the rooms above. The fix depends on where the water is coming from.
Find the source first
| Source | Clues | Usual fix |
|---|---|---|
| Rain and surface water | Water after storms, worst near downspouts or low corners | Extend downspouts, clean gutters, regrade soil away from the house |
| Groundwater | Water seeping through walls or up through the floor in wet seasons; high water table | Perimeter drain and sump pump, then a vapor barrier |
| Plumbing leaks | Wet spot under a bathroom, kitchen or water heater; water year-round | Repair the pipe, dry the area, replace wet insulation |
| Ductwork | Wet insulation and condensation on ducts in summer; disconnected duct | Seal and insulate ducts, reconnect loose runs |
| Summer condensation | Damp joists and dripping pipes in humid weather with no rain | Ground vapor barrier, air sealing, and dehumidification or conditioning |
Vapor barrier, encapsulation and drainage
- Vapor barrier: polyethylene sheeting laid over the dirt floor, with seams overlapped and taped. It's the basic step that slows moisture rising from the soil.
- Encapsulation: a heavier liner over the floor and up the walls, sealed at seams and piers, with vents and rim joists sealed, and a dehumidifier or a small supply of conditioned air to keep humidity in check.
- Drainage: an interior perimeter drain that carries water to a sump pump. It's needed when liquid water collects; a liner alone just hides the water underneath.
- Insulation: in a sealed crawl space, insulation usually moves from between the floor joists to the walls. Fiberglass batts that are wet or falling down should be removed.
Questions to ask a contractor
- Where is the water coming from, and how will you fix the source, not just cover it?
- What liner thickness are you using, and how are seams, piers and walls sealed?
- How will the space be dried afterward: dehumidifier, supply air from the HVAC system, or both? Who maintains it?
- Do you need a permit, and does the design meet local code for crawl space ventilation and combustion appliances?
- Is mold on the framing being cleaned, treated or removed, and is that in the written scope?
Test for radon before sealing
Sealing a crawl space changes how air moves between the soil and the house. Where soil gas contains radon, the U.S. Department of Energy notes that air-sealing can raise indoor radon levels, so test first and plan for mitigation if levels are high.
Signs it's working
A crawl space that's under control stays below about 60% relative humidity, has no standing water after storms, and has no musty smell drifting into the house. A cheap hygrometer left in the crawl space and checked through a summer will tell you more than any sales pitch.
Frequently asked questions
Is some moisture in a crawl space normal?
Crawl spaces are rarely bone dry, but standing water, wet insulation falling from the joists, visible mold, condensation dripping from ducts or pipes, and wood moisture readings consistently above about 19% to 20% are signs of a problem.
Should crawl space vents be open or closed?
Vents were meant to dry crawl spaces, but in humid summers they can let in warm, moist air that condenses on cool joists and ducts. Many building scientists now recommend sealed, conditioned crawl spaces in humid climates. Don't simply close vents without adding a ground vapor barrier and a way to dry the air; local building codes set the requirements.
Is crawl space mold dangerous?
Air from the crawl space moves up into the house, so mold and musty odors there can affect indoor air. People with asthma or allergies are most likely to notice. Fix the moisture, clean or remove the moldy material, and seal gaps between the crawl space and the living space.
Does insurance cover crawl space water?
Water from a sudden plumbing failure usually is covered. Groundwater, seepage, flooding and long-term humidity or condensation usually aren't, because insurers treat them as maintenance or as excluded flood and surface water.