Crawl space moisture control
Moisture gets under a house in three ways: rain, the ground and the air. Good moisture control deals with each one in order.
A dehumidifier alone won't keep up if storm water is running under the house, and a sump won't help much if the real problem is August air pouring through open vents. Sorting out the source first saves money and avoids a fix that doesn't work.
First, rain and surface water
Florence gets more than 48 inches of rain in a typical year, and tropical systems can drop much more. The cheapest moisture fix is often outside the house:
- Gutters that are clean and sized for heavy downpours
- Downspouts that empty several feet from the foundation instead of at the wall
- Soil sloped away from the house, not toward it
- Flower beds and mulch kept below the top of the foundation
Next, groundwater
Where the water table rises in winter and spring, as it does in poorly drained soils like the Rains series found around Timmonsville, water can seep up into a low crawl space. Options include an interior drain along the footing that leads to a sump pump, a perimeter drain outside, or regrading the crawl space floor. Under the IRC, if groundwater can rise to within 6 inches of the floor of a crawl space, the floor grade has to come up to the outside grade unless an approved drainage system is provided.
Finally, the air
In a sealed crawl space, the IRC allows a dehumidifier sized to remove 70 pints of water a day per 1,000 square feet of floor as one way to keep the air dry. Crawl space dehumidifiers are built to run in tight, low spaces and usually drain to a pump or to the outside.
What moisture level is too high?
South Carolina's wood infestation regulation, 27-1085, treats wood moisture readings of 20% or more, or standing water in the crawl space or around the foundation, as excessive moisture, and says decay fungi become active at 28% and above. Those are the readings a CL-100 inspector records. The regulation also says a sump pump can't be installed without a drain or trench system that carries water to it.
Questions to ask about a sump or dehumidifier
- Where will it drain, and is that discharge kept away from the foundation?
- How will I know if it stops working?
- What maintenance does it need, and how often?
- Is it on its own circuit, and is the outlet protected?
Every crawl space is different, so the company prices the job after someone has been under your house and measured it. We don't quote prices, and a number given over the phone before an inspection is only a guess.
Where we connect you with help
Common questions
Can I just put a regular dehumidifier in the crawl space?
A household unit isn't built for a dirt-floored, unsealed space and usually can't keep up. Crawl space models are designed for the job, but they work best once the space is sealed.
Should I close my crawl space vents in summer?
Closing vents without sealing the ground and drying the air can make things worse. Talk to a crawl space company about sealing the space properly instead.
Related services
More background: Florence crawl space guide.
Damp insulation, a musty hallway or a floor that dips by the tub?
Call and we'll put you in touch with a crawl space company that works around Florence.