Crawl space moisture
Three sources, three different fixes. Most wrong repairs come from treating the wrong one.
Crawl space moisture almost always comes from one of three places: water vapor rising from the soil, humid outdoor air. Or liquid water that gets in. The fix for each is different, which is why guessing is expensive. A short check with a flashlight and a humidity meter usually tells you which one you are dealing with.
This page gives no prices. Costs vary by region and year. It gives you the order of checks instead.
Source one: vapor from the ground
Soil holds water, and it gives off vapor all year, even when it looks dry. In a crawl space with bare dirt, that vapor rises straight into the air and the wood framing. This is the most common source and the one people miss, because nothing looks wet.
A clue is a dry-looking crawl space with damp, dark wood, rusty fasteners or condensation on cold pipes and ducts. A sealed ground barrier is the usual answer, covered in the encapsulation guide.
Source two: humid air from outside
Warm, humid air that enters through vents meets cooler surfaces under the house and drops its moisture there. It is worst in humid summers, when the soil and the floor above are cooler than the air. This is why the old advice to keep vents open has been questioned. The full reasoning is on the page about crawl space vents.
Do not close vents on a hunch, though. Whether to seal them depends on your climate, on the ground cover, on any fuel-burning appliance nearby and on what local code requires. Your building department can tell you the local rule.
Source three: liquid water
Standing water, wet soil after rain, a leaking pipe or a downspout that dumps beside the foundation all add liquid water. This one needs fixing first, because no liner or humidity control works well over a flooded floor.
Look for tide marks on the foundation, mud lines on piers, and soft, stained soil. Then walk the outside. The exterior drainage section covers grading, gutters and downspouts. Use the water diagnosis tool if you are not sure where it enters.
A quick check you can do
- Look at the soil: dry and dusty, damp, or wet with standing water.
- Look at the wood: pale and dry, or dark, swollen and stained.
- Check insulation in the floor above: is it sagging or damp?
- Put a small humidity meter in the space for a few days and note the readings.
Many building guides aim to keep the air in an enclosed space under about 60 percent relative humidity. Treat that as a rough target, not a rule for your house, since local practice varies.
What to do in what order
Fix liquid water first, then cover bare soil, then decide about the vents, then control humidity if the space is sealed. Skipping the order is how people end up with a sealed crawl space that is still damp.
Call the damp wood what it is: evidence of persistent moisture, not a verdict on what is growing on it. If you want to know what a surface growth is, a qualified third party has to sample it. The page on growth on floor joists shows what to look for and how to dry the source.
When damp wood becomes a structural matter
Long-term moisture can soften joists and sill plates. If floors sag, bounce or feel soft, read the floor structure section. Repairs to framing that carries load belong to a licensed professional. And an independent structural engineer can tell you how serious it is without a repair to sell you.
Take a position: a dehumidifier on its own is not a plan for a crawl space with open dirt and a downspout draining beside it. Deal with the source, then the air.
Frequently asked questions
What causes moisture in a crawl space?
Usually soil vapor, humid outdoor air entering through vents, or liquid water from rain, leaks or poor drainage.
How do I tell if my crawl space moisture is from the ground?
A dry-looking space with damp, dark wood or condensation on cold pipes often points to soil vapor. A ground vapor barrier addresses it.
Will a dehumidifier fix a damp crawl space?
Not alone. If bare soil or standing water keeps adding moisture, the dehumidifier just runs without catching up.
Should I close the vents to stop moisture?
It depends on your climate, ground cover, nearby appliances and local code. Check with your building department before sealing them.
When is crawl space moisture a structural problem?
When floors sag or feel soft, the wood framing may be damaged. Have a licensed structural engineer assess it.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.