Crawl space vents: open or closed, and when

The short answer, by climate

Humid climates (the Southeast, Gulf Coast, Mid-Atlantic, and most of the Midwest and Northeast in summer): close the vents year-round, cover the ground with a sealed vapor retarder, and give the space a way to dry: a dehumidifier or conditioned air from the house. Open vents in July add water to the crawl space; they do not remove it.

Dry climates (the interior West and Southwest): open vents work most of the time, because the outside air is drier than the crawl space for most of the year. Cover the ground anyway, and close the vents for hard freezes if there is plumbing down there.

Marine Northwest: vented crawl spaces can stay dry here. Washington State University's monitoring found vented crawl spaces in Vancouver and Moses Lake rarely, if ever, reached dew point, with no moisture buildup in the framing. If yours is dry with the vents open and the ground covered, leave it; if it is not, close it and dry it like a humid-climate space.

Cold climates with pipes or ducts in the crawl space: close them for the winter at minimum, and consider a sealed, wall-insulated crawl space so the plumbing lives inside the heated envelope.

The vents were never the moisture control; the ground cover and the drying mechanism are. The rest of this guide is the reasoning and the work.

What vents were supposed to do, and why dew point defeats them

Foundation vents date from the idea that outside air blowing through the crawl space carries moisture out. That works only when the outside air is drier than the crawl space, and a crawl space in summer is a cool place: the soil keeps it in the 60s, and air-conditioning ducts run colder than that. Air does not care about relative humidity; it condenses on any surface colder than its dew point. The table shows what happens to ordinary summer air when it meets a 60 to 65°F joist or duct.

The field data match the table. Advanced Energy's study of twelve identical houses in the humid Southeast, part of the Building America research program, left four crawl spaces wall-vented with a ground cover and closed the rest. Over the summer of 2003 the wall-vented crawl spaces were above 70 percent relative humidity 98 percent of the time and above 80 percent for 86 percent of the time, and repeatedly reached condensing conditions; the closed crawl spaces were above 70 percent only 5 percent of the time and never reached condensation. Over the following eleven months the closed-crawl-space houses used 15 to 16 percent less electricity for heating and cooling, and the Department of Energy's Building America summary credits the approach with 15 to 18 percent savings and a humidity reduction of more than 20 percent. That research put the unvented crawl space into the 2009 and 2012 editions of the International Residential Code as Section R408.3.

It matters for the wood because framing in a dry crawl space reads about 8 to 14 percent moisture content on a pin meter, and the USDA Forest Products Laboratory's rule of thumb is that wood above about 19 percent can support mold growth. A crawl space that spends the summer above 80 percent relative humidity pushes the joists past that line by August.

What the data does not say is that closing vents alone fixes anything. The closed spaces in the study had a sealed liner and a measured supply of conditioned air; a closed crawl space over bare dirt with no drying mechanism is a terrarium. The same WSU monitoring that found vented crawl spaces dry in the marine Northwest measured radon in closed crawl spaces without an exhaust fan at roughly ten times the vented level, and found that about 40 percent of the air in those houses had come up from the crawl space. Closing the vents makes the crawl space part of the house, and it has to be treated like one.

Measure before you decide

Three readings settle the question for your house better than any map.

Crawl space humidity. A hygrometer that logs or holds a maximum, in the middle of the space, off the ground and away from a vent, for a week in summer. The EPA's guidance for indoor spaces is to keep relative humidity below 60 percent, ideally 30 to 50; a crawl space running in the 70s and 80s with the vents open is being wetted by them.

Duct and joist temperature against the outdoor dew point. An infrared thermometer on the supply ducts and the underside of the subfloor at the perimeter, compared with the dew point in the forecast. If the duct reads 58°F and the dew point is 70°F, every cubic foot of outside air that touches it leaves water behind.

Framing moisture content. A pin meter in a floor joist and the subfloor at three or four points, especially under bathrooms and at the perimeter. Dry framing reads 8 to 14 percent; above 16 percent the space has a moisture problem whatever the vents are doing; at 20 percent or more, or with visible growth, you are in remediation territory, and the crawl space humidity guide covers that boundary.

Look while you are down there: droplets or rust streaks on the ducts, batts sagging out of the joist bays from the weight of absorbed water, growth on the joists, a water line on the piers, and the smell.

Winter: close them everywhere

Open vents in winter are a wind tunnel across the water lines. The Insurance Institute for Business & Home Safety puts the point where unprotected pipes in unheated spaces begin to freeze at about 20°F outdoors, and a crawl space with open vents on a windy night gets there. Close and cover the vents before the first hard freeze, sleeve the pipes and air-seal the rim; the routine is in our pipe winterizing checklist. Winter air is dry, so closed vents cost nothing in moisture, and the floors above feel warmer.

Summer: humid climates close, dry climates can vent

In a humid climate the summer decision is the one the table makes: close the vents and dry the space with a dehumidifier or conditioned air. In a dry climate, where summer dew points sit in the 40s and 50s, open vents move drier air through and the ground cover does the rest; close them during monsoon weeks if your region has them. In the marine Northwest, test rather than assume: a week of readings under 60 percent with the vents open means the vents are not hurting you.

Automatic vents: what they fix and what they do not

Temperature-actuated foundation vents use a bimetal coil, no electricity, to open as the air warms and close as it cools; the common Temp-Vent design is fully open at about 70°F and fully closed at about 40°F. That solves the freezing half of the problem without anyone crawling around the house in November. It does nothing for the moisture half, because the vent is open exactly when the outside air is warmest and wettest and the ducts are coldest. In a humid climate an automatic vent is a convenience for a crawl space you have decided to keep vented, not a moisture strategy.

Tools and materials for closing the vents

• Rigid foam board (2 in. XPS or polyiso) cut to each vent opening, or purpose-made vent covers; low-expansion spray foam and exterior sealant
• A Class I vapor retarder for the ground: 6-mil polyethylene is the code minimum, a reinforced 10 to 20 mil liner survives being crawled on; seam tape rated for it, fasteners or termination bar for the wall
• Foam or caulk for the rim joist and penetrations; weatherstripping for the access door
• A dehumidifier sized to the space with a condensate drain line or pump, or a supply register tapped off the house ductwork
• A hygrometer, a pin moisture meter and a radon test kit
• Knee pads, headlamp, coveralls, an N95 respirator and gloves

Closed vents with a dehumidifier, or full encapsulation?

The steps above produce a closed, dried crawl space. Full encapsulation adds the liner run all the way up the walls, insulation on the foundation walls instead of (or as well as) the floor, a sealed sump if ground water is present, and often a drainage mat under the liner. It is the right next step in a cold climate where the pipes and ducts need to be inside the insulated envelope, in a house whose crawl space floods seasonally, and where the owners want the space clean enough to store things. Most humid-climate crawl spaces do not need it to stop the summer condensation; the sealed ground cover, sealed vents and a correctly sized dehumidifier do that, and the crawl space humidity guide walks the whole sequence including the drainage work.

Sizing is the decision that matters either way. The code floor is 70 pints per day per 1,000 sq ft, cold air cuts a refrigerant dehumidifier's output, and most crawl spaces sit in the 40s and 50s for months: Santa Fe and AprilAire publish minimum operating temperatures of 49 and 50°F, AlorAir's Sentinel line 33.8°F with automatic defrost. Height matters too, since many crawl spaces have 18 in. of clearance or less. The two below cover the common cases; our crawl space dehumidifier roundup ranks the field, the Santa Fe vs AprilAire comparison covers the two ductable brands, and the calculator sizes the unit from the floor area and how well the space is sealed.

When to call a pro

Call a crawl space or encapsulation contractor when there is standing water or a seasonal stream through the space (a perimeter drain and sump come before the liner), when the clearance is too low to work in, when the joists show growth or read 20 percent or more on a pin meter (remediation first, following EPA and IICRC guidance for the extent of the growth), or when you want the wall insulation, drainage mat and warranty that come with full encapsulation. Call a heating contractor before closing vents on a crawl space with a gas furnace or water heater in it, and a radon mitigator if the post-closure test is above 4 pCi/L. A frozen or burst line in front of an open vent is a plumber's job and is covered in our winterizing guide. The form below reaches vetted local crawl space, waterproofing and remediation contractors through HelpMeNow.com.