How to fix crawl space humidity

Measure before you spend anything

Two numbers tell you whether you have a problem and, later, whether you fixed it.

Relative humidity in the crawl space air. Put a hygrometer that logs or at least holds a maximum reading somewhere in the middle of the space, off the ground and away from a vent, and leave it a week. The EPA's guidance for indoor air is to keep relative humidity below 60 percent, ideally between 30 and 50; the target crews worked to under a house was 50 to 55 percent, because a crawl space is cooler than the rooms above it and the same air reads higher there. Readings that spend the summer in the 70s and 80s mean condensation on anything cooler than the air: ducts, water lines, the underside of the subfloor at the perimeter.

Moisture content of the wood. A pin moisture meter pushed into a floor joist and the subfloor at three or four places, especially near the perimeter and under bathrooms, tells you how much of that humidity the structure has absorbed. Framing in a dry crawl space reads about 8 to 14 percent. Above 16 percent it is wet for our purposes, and the USDA Forest Products Laboratory's rule of thumb is that wood above about 19 percent moisture content can support mold growth. Framing at 20 percent or more, or visible growth on the joists, moves this from a humidity job to a remediation job; see the end of this guide.

Take a flashlight down with the meter and look for the physical evidence while you are there: a water line on the piers or block, a vapor barrier that is torn, buried or missing, wet soil, efflorescence (white mineral crust) on the block, rust on duct straps and nail plates, sagging or fallen insulation, and the staining pattern under any bathroom or kitchen.

Where the moisture is coming from

A crawl space has five moisture sources, and most wet ones have more than one.

1. The ground. Bare soil under a house evaporates water continuously, even soil that looks dry; a crawl space of 1,500 sq ft can put gallons a day into the air this way. This is the source a vapor retarder stops, and it is the one almost every damp crawl space has.
2. Outside air through the vents. In a humid climate, summer air at 85°F and 70 percent relative humidity has a dew point around 74°F. The crawl space, cooled by the ground, sits in the 60s. Every cubic foot of that air that comes through a vent gives up water on the first surface it touches that is below 74°F: the ducts, the pipes, the framing. The vents that were supposed to dry the space are wetting it, and no dehumidifier can keep up with an open vent in July.
3. Plumbing and appliance leaks. A dripping drain line under a tub, a sweating cold water line, a washing machine standpipe that overflows, a dryer vent that disconnected and blows into the crawl space, an air conditioner condensate line that drains onto the soil. Each is small, continuous and easy to miss, and the staining pattern under the fixture gives it away.
4. HVAC. Leaky supply ducts push conditioned air out and pull crawl space air into the return side; sweating ducts drip; an air handler in the crawl space with a blocked condensate drain overflows its pan.
5. Bulk water. Rain that runs toward the foundation because the grading is flat, downspouts that discharge at the wall, a window well or vent well that fills, a high water table in spring. Standing water or a wet perimeter after rain is this, and it is fixed outside first.

The order of operations

This is the sequence, and the reason it is a sequence is that each step makes the next one possible. Skipping to the dehumidifier is the mistake we were called back for most.

1. Stop bulk water. Extend downspouts 6 ft or more from the foundation, regrade so soil falls away from the house, cover window and vent wells, fix the leaks you found with the flashlight. If water still pools after rain, the fix is a perimeter drain to a sump pit with a pump, and our sump pump roundup covers the pump. Nothing below works on a crawl space that fills.
2. Cover the ground. A continuous vapor retarder over every square foot of soil, seams overlapped and taped, run up the walls and sealed to them, and around every pier. This is what cuts off source one, and it is the largest single improvement on most crawl spaces.
3. Close and seal the vents. Once the ground is covered, the vents are only bringing in water. Block them with rigid foam cut to fit and sealed with foam, or with the manufacturer's vent covers, from the inside.
4. Air-seal and insulate the rim. The band joist between the sill and the subfloor leaks air at every joint; foam the gaps, and insulate the rim and, if you are converting to a sealed crawl space, the foundation walls.
5. Dehumidify. Now a dehumidifier is working against a closed space with one remaining source, the air that leaks in, and it can win. Size it to the square footage and the climate, drain it by gravity or pump, and set it to 50 to 55 percent.
6. Monitor. The hygrometer stays; a water alarm goes in the dehumidifier's pan; the framing gets metered again in a month.

The drawing shows what steps 2 through 5 look like together.

Vents: what the code actually says

The idea behind foundation vents is that outside air dries the space. In dry climates it can. In the humid half of the country it does the opposite in summer for the dew-point reason above, and in winter open vents freeze pipes. The model residential code recognizes both paths. For a vented crawl space, the International Residential Code (Section R408.1 and R408.2) requires 1 sq ft of net vent opening for every 150 sq ft of under-floor area, which can be reduced to 1 for every 1,500 sq ft when the ground is covered with a Class I vapor retarder. For an unvented crawl space (R408.3), the vents can be omitted entirely when exposed earth is covered with a continuous Class I vapor retarder whose joints overlap by 6 in and are sealed or taped, whose edges extend at least 6 in up the stem wall and are attached and sealed to it, and the space is given one of four things: continuously operated exhaust at 1 cfm per 50 sq ft of floor area, a conditioned air supply at the same rate, use as a plenum, or dehumidification sized to provide 70 pints of moisture removal per day for every 1,000 sq ft of crawl space floor area.

That last option is the one most homeowners end up with, and it gives you a sizing floor straight from the code: a 1,500 sq ft crawl space needs at least 105 pints a day of rated capacity. Your local amendments may differ, and a sealed crawl space that houses a gas furnace or water heater needs its combustion air accounted for, so check with your building department before you close the last vent.

Tools and materials

• A hygrometer that logs or holds a maximum, and a pin moisture meter for the joists; the General Tools MMD4E is enough for this
• Vapor retarder: 6-mil polyethylene is the code minimum in most places and tears the first time you crawl on it; 10- to 20-mil reinforced crawl space liner is what lasts. Buy 15 to 20 percent more than the floor area for overlaps and wall runs
• Seam tape made for the liner (not duct tape), butyl or polyurethane sealant for the wall termination, and mechanical fasteners (masonry nails with washers or a termination bar) for the wall
• Rigid foam board and spray foam for vents, rim joist gaps and penetrations; foam-compatible sealant
• A dehumidifier rated for the square footage at a sealed or moderately sealed condition, with a condensate pump if there is no drain below the unit; 3/4 in drain hose; hose hangers
• A water alarm or leak sensor for the dehumidifier pan; a GFCI-protected outlet for the unit (crawl space receptacles must be GFCI-protected under the National Electrical Code)
• Knee pads, a creeper or a sheet of plywood to slide on, a headlamp, nitrile gloves, an N95 respirator for the dust, contractor bags for the old liner and fallen insulation
• Utility knife with spare blades, tape measure, a staple gun for temporary holds

Sizing and choosing the dehumidifier

Three things decide which unit: capacity for the square footage and how well sealed the space is, performance in cold air, and whether it physically fits through the hatch and under the joists.

Capacity. Crawl space and whole-home units are rated in pints per day at the AHAM condition of 80°F and 60 percent humidity, not the cooler DOE condition retail portables use, so compare like with like. Manufacturers publish coverage by how tight the space is: Santa Fe rates the Compact70 for up to 2,600 sq ft tightly sealed, 2,200 moderately sealed and 1,800 loosely sealed, which is a useful reminder that sealing is worth more square footage than the next model up. The code's 70 pints per 1,000 sq ft is the floor for an unvented space; the calculator below adds headroom for a cool space and air leakage.

Cold performance. Crawl spaces sit in the 40s and 50s for much of the year across most of the country, and a refrigerant dehumidifier's output falls as the air cools until it reaches a floor where the coil ices and the unit cycles off. Santa Fe and AprilAire publish minimums of 49 and 50°F; AlorAir's Sentinel line publishes 33.8°F with automatic defrost. In a sealed crawl space under a heated house the temperature usually holds above 50°F; in a partially sealed one in a cold climate it does not, and the AlorAir floor is the difference between a unit that works in March and one that waits for May.

Fit. Measure the hatch and the lowest clearance on the way to where the unit will sit, and subtract 3 in for the liner, a stand and the drain fitting. The Compact70 is 12 in tall; the Sentinel HD55 is 13.3; the AprilAire E080 is 15; the Sentinel HDi90 is 17.7. The crawl space roundup compares all six we have documented, and the Santa Fe versus AprilAire comparison covers the two ducted favorites.

Doing it yourself versus hiring an encapsulation contractor

Everything above is physically possible for a homeowner with a weekend, a creeper and a tolerance for crawling. What a contractor adds is a heavier liner installed faster, the drainage work (trenching a perimeter drain under a house is miserable), wall insulation done to code, and a warranty on the result. What they sometimes add that you do not need is a sales pitch: an encapsulation quote that leads with mold, radon or structural fear deserves a second quote. The parts of the job most worth hiring out are the sump and drain, any insulation of foundation walls in a cold climate, and anything involving the gas appliances. The parts most worth doing yourself are the leak repairs, the cleanout, the liner on a simple rectangular crawl space, and the dehumidifier.

Three cautions apply either way. A sealed crawl space with a gas furnace or water heater in it changes the combustion air supply; have a heating contractor confirm the appliance is direct-vented or has a dedicated air supply. Sealing can change how radon moves into the house; test before and after. And in termite regions, pest control companies and some jurisdictions want an inspection gap of bare wall left between the top of the liner and the sill, so do not run the liner to the wood without asking.

When it is mold remediation, not moisture control

If the joists and subfloor show growth, the question changes. The EPA's mold guidance is that a homeowner can clean an area under about 10 square feet with detergent and water, an N95 respirator, gloves and goggles; a crawl space with growth on the framing is almost always larger than that, is overhead, and is in a confined space, which is professional work under the IICRC S520 mold remediation standard: containment, HEPA filtration, removal of the growth from the wood, and verification afterward. The humidity fix is still required, because remediation without moisture control grows back, but it comes after or alongside the remediation, not instead of it.

When to call a pro

Call a waterproofing or crawl space contractor if the space takes standing water after rain, if a perimeter drain or sump is needed, if the foundation walls need insulating to code, or if the crawl space is too low to work in. Call a heating contractor before sealing a crawl space with a gas appliance in it. Call a mold remediation contractor if the framing shows growth over more than a small patch, or reads over 20 percent moisture content with staining. Call a plumber for a drain leak you cannot reach. The form below reaches vetted local contractors through HelpMeNow.com.