Should Crawl Space Vents Be Open or Closed?

The short answer

In Washington, leave the vents open unless you are prepared to do all of what the code asks of a sealed crawl space. The state's residential code makes a vented crawl space the default. It allows an unvented one only where two conditions are both met: a continuous Class I vapor retarder sealed to the stem walls, with a radon system under it, and an exhaust fan that runs continuously. Shutting the vents without those removes the only moisture control the space had and puts nothing in its place.

Two things follow. Vent covers bought in October and forgotten are not a plan. And a contractor who seals the vents and leaves out the radon vent or the exhaust fan has not built what Washington's code describes, whatever the invoice calls it. The rest of this page is what the code requires, why ventilation behaves the way it does, and how to tell which case your house is in.

A vented crawl space is open to outdoor air through foundation vents. The theory is that airflow carries moisture out. In cold, dry weather it does.

A closed (sealed, unvented, encapsulated) crawl space has the vents shut, the ground and the lower walls covered with a sealed liner, and the space brought inside the building's moisture-controlled envelope. It has to get its air managed some other way, because you have removed the mechanism that was doing it. Washington's answer to "some other way" is specific: continuous exhaust.

Neither is universally correct. A vented crawl space with its soil covered is often dry enough. A sealed one, built as the code describes, gives more control over the space and brings the ducts and pipes inside the envelope, at a higher price and with a fan that has to keep running.

Why vents were required in the first place

Foundation vents solve a real problem. A crawl space over bare soil continuously receives moisture evaporating out of the ground. Without a way for that moisture to leave, it accumulates, condenses on framing and insulation, and creates conditions for mold and eventually decay.

Air exchange is one way to move it out, and in cold weather it works well. Cold outdoor air holds very little moisture in absolute terms, so bringing it into the crawl space and letting it leave carries water vapor away with it. Around Seattle the wet season is also the cold one: at the Sea-Tac and Sand Point weather stations, 73 to 75 percent of the year's rain falls between October and March. Washington's code still pairs vents with a ground cover — a 6-mil black polyethylene sheet over the soil — because the ground is the larger moisture source, and ventilation alone over bare soil often loses.

Why vents can work against you

The mechanism that dries a crawl space in winter can wet it in warm weather, and this is the part that surprises people.

Warm air holds substantially more water vapor than cold air. When warm outdoor air enters a crawl space that has stayed cool — as crawl spaces do, being shaded, below grade, and in contact with earth that warms slowly — that air is cooled. If it cools to its dew point, the moisture it carries can condense on the coolest surfaces available: ducts, pipes, the underside of the subfloor, the joists.

How often that happens under a given house depends on summer humidity and on how cold the crawl space runs, and it varies from house to house. It is the core building-science argument for sealed crawl spaces, and the reason "just add more vents" is often the wrong response to a damp crawl space.

Two other realities in older housing stock compound it. Vents get blocked by landscaping, stored material, soil or added insulation, so the ventilation that was designed often is not the ventilation that exists. And a vented crawl space under a heated house loses heat through the floor all winter, which is why cold floors and a vented crawl space so often appear together. Sealing is not the only answer to that, though, and often not the first one — what is actually making your floors cold separates the insulation, duct, hatch and air-leakage causes, which have very different prices.

What Washington code actually requires

Washington's residential code covers this in Section R408, as amended in WAC 51-51-0408. It is worth reading the requirements rather than a contractor's summary of them, because the specifics decide what a legitimate bid contains, and Washington's version differs from the national model code that most product literature describes.

The edition in force is the 2021 International Residential Code with Washington amendments, effective statewide since March 15, 2024. A 2024 edition is proposed and not adopted; the council's planned effective date for it is May 3, 2027. Editorial only. The proposal reprints both conditions and changes the appendix reference from AF to BE. Its own summary classes the change as alignment with the model code’s reorganization. The City of Seattle's own copy of the code reads the same as the state text for these sections. The Washington crawl space code page quotes each section with the document behind it and tracks where the 2024 edition stands, and the Washington requirements section collects the registration and rebate rules that sit alongside the code.

If the crawl space stays vented (R408.1 and R408.2)

• Ground cover (R408.1). A ground cover of 6-mil black polyethylene, or an approved equal, must be laid over the ground in the crawl space, overlapped at least 6 inches at the joints and extended to the foundation wall. It may be left out where the crawl space has a concrete slab floor at least 2 inches thick.
• How much ventilation (R408.2). The under-floor space must have ventilation openings through the foundation or exterior walls, with a net area of at least 1 square foot for each 300 square feet of under-floor area. Openings are placed to give cross ventilation, and one side of the building may have none.
• What covers the openings (R408.2). Vent openings must be covered for their full height and width with one of six listed materials, such as hardware cloth, cast-iron grating or corrosion-resistant wire mesh, and the least dimension of the covering’s openings may not exceed 1/4 inch. Operational louvers are permitted. The 1/4-inch limit is the figure a vent screen is checked against, and a torn screen is an open door for rodents.
• Reduced venting (R408.2, exception). Vent area may be reduced to 1/1,500 of the under-floor area where the ground is covered with an approved Class I vapor retarder and the openings still give cross ventilation. If the installed ventilation is less than 1/300, or operable louvers are installed, a radon vent must be installed from a point between the ground cover and the soil, built to the radon appendix.

Read those together and the pattern is clear: the basic rule is modest, a 6-mil sheet lapped and run to the wall, with no sealing or taping. Covering the soil with a Class I vapor retarder is what earns the far smaller vent area, and the moment the vent area drops below 1/300 or louvers go in, a radon vent comes with it.

If the crawl space is closed (R408.3)

Ventilation openings are not required where both of the following are provided:

1. A sealed liner with a radon system under it. Exposed earth is covered with a continuous Class I vapor retarder; joints overlap 6 inches and are sealed or taped; the edges run at least 6 inches up the stem wall and are attached and sealed to it; and a radon system meeting the code's radon appendix is installed.
2. Continuous mechanical exhaust at 1 cubic foot per minute for each 50 square feet of crawl space floor, terminating outside. For a 1,200 to 1,500 square feet crawl space that is about 24 to 30 cfm, running all the time.

A sealed crawl space in Washington is a liner, a radon system and a continuously running exhaust fan, together. The rule prints the two conditions as a numbered list with no “or” between them, and the City of Seattle’s marked-up copy of the same text shows why: the model code offered a menu of ways to condition the space, and Washington struck the menu and kept exhaust. A dehumidifier is not in the section, and neither is conditioned supply air. Either can be added, but neither takes the place of the exhaust fan or the radon system as the rule is written.

This is the single most useful thing on this page for evaluating a quote. "Encapsulation" that seals the vents and leaves out the radon vent or the exhaust fan is not the system Washington's code describes. If a bid closes the vents, look for three line items: the sealed liner, the radon pipe from under the liner, and the exhaust fan with its rate.

Why a sealed crawl space in King County still needs a radon vent

The radon appendix applies in the seven counties the code lists as high radon potential (Clark, Ferry, Okanogan, Pend Oreille, Skamania, Spokane and Stevens), and to every building built with an unvented crawl space under R408.3, in any county. King and Snohomish counties are not in the table. In a listed county an unvented crawl space is not permitted at all.

That surprises people who know the local numbers, which are low. King and Snohomish counties are in Zone 3, EPA's lowest radon category. Of the laboratory radon tests from King County submitted to the Washington State Department of Health in 2025, 5% (63 of 1,177) were at or above 4 pCi/L, against 14% statewide. Those are tests people chose to send in, not a sample of homes, and the state publishes no city or ZIP code figure.

The code's reasoning does not depend on the county average. Sealing a crawl space changes how soil gas moves into a house, and radon is a soil gas: the outdoor air that was incidentally diluting it is no longer passing through. So the rule follows the construction, not the map. A vented crawl space at the full vent area, without operable louvers, carries no radon requirement in King or Snohomish County; the same house picks one up the moment its vent area is cut below 1/300, louvers go in, or it is sealed.

For a crawl space the appendix describes a passive system: a continuous soil-gas retarder of polyethylene at least 6 mil thick, lapped at least 12 inches and extended to the foundation walls, and a 3- or 4-inch vent pipe from beneath the sheeting up through the building to at least 12 inches above the roof. It has an exception for buildings with an approved mechanical crawl space ventilation system. A sealed crawl space scope that follows the appendix includes a pipe that starts under the liner and ends above the roof, which is worth looking for in a bid. The appendix asks for 12-inch laps where R408.3 asks for 6-inch sealed laps; which applies to a sealed crawl space, and whether the appendix’s exception for mechanical ventilation covers the R408.3 exhaust fan, are questions for the building department.

Test anyway. EPA says every home should be tested whatever its zone. The Department of Health notes that a test taken in the crawl space says little about the air the household breathes, and it lists a crawl space vapor barrier and unobstructed foundation vents among the simple steps that reduce radon. Test in the living space before a crawl space is sealed and again afterward. If a result comes back at or above 4 pCi/L, mitigation is specialist work to design alongside the crawl space, not after it.

Which is right for your house?

If you close it, what has to be true

A closed crawl space done properly is a durable improvement. Done partially, it is worse than the vented space it replaced, because you have removed the drying mechanism without installing its replacement. The non-negotiables:

• No active water intrusion. Drainage before liner, always.
• Continuous Class I vapor retarder over all exposed earth, lapped and sealed at the seams and run at least 6 inches up the stem walls, sealed there (R408.3).
• A radon system built to the code's radon appendix: a vent pipe from beneath the liner up through the house to above the roof.
• Continuous exhaust at 1 cfm per 50 square feet, vented outside. A dehumidifier can be added; it does not replace the fan.
• Insulation where the code puts it. Insulating the crawl space walls in place of the floor is not a prescriptive option in the Washington text this site has read. Do not accept it as code-compliant on a permitted job without the building department’s confirmation.
• Combustion safety. If a gas appliance draws combustion air from the crawl space, sealing it changes that. This is a safety item, not a detail.
• Radon tested before and after, in the living space.
• An access hatch you can still get through, and a plan to check the space periodically. Sealed does not mean forgotten.

In the Seattle metro this site models a full closed-crawl-space system at $10,650 to $13,000, reaching $15,675 to $21,500 with a dehumidifier and premium scope, before sales tax (10.55% in Seattle). The radon vent and exhaust fan are outside those ranges, so ask for them as line items. See encapsulation cost for the breakdown, and how long encapsulation lasts if you are weighing that against its service life.

Seattle Crawlspace Pros does not issue code interpretations. Confirm requirements for your specific project with your city or county building department, and check that any contractor is registered, bonded and insured with the Washington State Department of Labor and Industries through L&I Verify (how to do that, and what it does and does not prove). A free in-home inspection is how to find out which configuration your crawl space is actually a candidate for, and the written, no-obligation quote that follows should address each item in the list above — combustion air and radon included.