If your crawl space is damp but there are no plumbing, HVAC or gutter leaks, the water is most likely coming from the outside air. Humid air enters through the foundation vents and condenses on cooler surfaces like ductwork, brick, and the floor joists cooled by your air conditioning. The fix is to control the humidity and temperature under the house so condensation can't form.
What's going on under here?
We get this call often. A homeowner looks under the house and finds surfaces that are damp to the touch, or a contractor tells them the crawl space is wet. So they:
- Call a plumber, who finds no leaks
- Call their HVAC technician, who confirms the air handler and unit aren't leaking
- Check the gutters and the flashing around decks, and can't find where the water is coming from
If that sounds like your crawl space, the answer may simply be that the water is coming from the outside air.
How can air make everything wet?
The average humidity in the Upstate of South Carolina is around 69%. If your home has foundation vents, that warm, humid air fills the crawl space and moves up into the house through the stack effect.
Warm weather also makes water in the crawl space soil evaporate. A moisture barrier slows that down, and a sealed crawl space slows it even more. But with open vents, you're depending on the outside air to regulate your crawl space. Lower crawl spaces, or those with fewer vents, tend to have problems sooner because ground moisture gets trapped under the home.
When that warm, humid air meets a colder surface, condensation forms. That includes:
- Metal flashing
- Cooler brick or concrete
- Floor joists and subfloor cooled by the air conditioning above
That condensation is what makes the joists feel wet to the touch.
How water builds up in wood
This is usually a slow process. A new board from a building supply store might have a wood moisture content (WMC) of 4–5%. Your foundation framing is treated lumber, with preservatives pressed deep into the wood under pressure to deter rot and pests. Over time, high humidity (which is just water in the air) works into the wood and slowly breaks that protection down. If you have regular pest treatments, your technician likely checks these readings for you.
| Wood moisture content | What it means |
|---|---|
| 4–5% | New lumber |
| 6–7% | Typical after a few years |
| 11–13% | A few years after that |
| 20% | Excessive moisture. Dehumidification is needed.1 |
| 28%+ | Active wood rot |
You may live in your home 20 years or more before the wood gets wet enough to cause problems. By the time WMC reaches 20%, a new moisture barrier on its own won't change much. Better ground coverage can keep things from getting worse, but it won't pull moisture back out of the wood.
In most homes we see, the highest readings are in tight corners or on the low side of the crawl space. Homeowners often say the floor feels soft or a little bouncy in those spots. The key to keeping repair costs down is catching the problem before it reaches wood rot.
So what's the fix?
To solve the problem, we have to look at why condensation forms. Condensation happens when water vapor reaches its dew point. In a South Carolina crawl space in July, the dew point may be 70°F or higher. So when that moist air touches any surface that's 70°F or cooler, water forms on it. That's why ductwork carrying cool air, and framing cooled by the AC, get wet. If you've ever slid under a duct line in a crawl space in summer, you've felt it dampen your hair or the back of your shirt.
There are only two ways to stop it: change the temperature, or change the humidity.
- Bring the crawl space temperature close to the house temperature. Then the humidity would have to be much higher to cause condensation.
- Lower the relative humidity. Then the gap between the air and a cooler surface would have to be much bigger to cause condensation.
Our goal is to do both. By sealing and insulating the crawl space and adding a dehumidifier, we control both the temperature and the humidity, so condensation can't form at all. The tighter the seal and the better the insulation, the more efficient the system, and the less your dehumidifier and HVAC have to run.
That's also why encapsulation is a good long-term investment. In U.S. Department of Energy-sponsored research, homes with closed crawl spaces used about 15% or more less energy for heating and cooling on average.2 To find out whether encapsulation is the right fix for your crawl space, book a free inspection.