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Crawl spaces

Crawl space vents: open or closed?

By Robert Stulpt Last reviewed

Short answer

Whether your vents should be open or closed depends on how your crawl space is designed to work, not on the season. If you have a vented crawl space, keep the vents open year round and use a vapor barrier on the ground. If you want to close them, do it as part of a closed crawl space: a sealed liner and a way to keep the air dry. Simply blocking your vents will usually cause more problems.

On this page
  1. Why crawl spaces have vents
  2. Why vents can backfire
  3. What the code says
  4. If your crawl space stays vented
  5. Automatic vents
  6. What happens when you just block the vents
  7. Replacing or sealing vents

Why crawl spaces have vents

The idea behind crawl space ventilation is to let outside air flow through and carry moisture away. Codes have required them for many years, and in drier conditions they can work.

Why vents can backfire

The trouble comes in the summer, when it’s hot and humid outside and you’re letting that air into your crawl space. If the crawl space is cooler than outside, condensation will form on ducts, cold water lines and even the framing when they’re below the air’s dew point. The more air you let in, the more condensation you can get. Building scientist Joseph Lstiburek argues in BSI-115 that a crawl space should be either clearly inside the house or clearly outside it, and that vented crawl spaces under air-conditioned homes tend to get wet.

Vented crawl space Schematic section of a vented crawl space. Outdoor air enters through vents in the foundation walls (1) and leaves through vents on the other side. In humid weather, water condenses on cool ducts and pipes (2). Insulation sits between the floor joists (3). Moisture rises from soil that is bare or only partly covered (4). Schematic, not to scale. 1234
  1. Vents in the foundation walls
  2. Cool ducts and pipes, where condensation forms
  3. Insulation between the floor joists
  4. Bare or partly covered soil
Schematic, not to scale.

What the code says

If your crawl space is vented, the code sets how much vent area it needs. In the 2021 Virginia Residential Code (R408.2), for example:

  • At least 1 square foot of net vent opening for every 150 square feet of crawl space
  • One vent within 3 feet of each outside corner
  • Vents covered with mesh, grates or similar, with openings no larger than 1/4 inch
  • If the ground is covered with a Class I vapor retarder, the vent area can drop to 1 square foot per 1,500 square feet, and the corner rule doesn’t apply as long as the vents provide cross ventilation

For a 1,500-square-foot crawl space, that’s 10 square feet of net vent area, or 1 square foot with the ground covered. Your state may use a different edition, so check with your local building department.

If your crawl space stays vented

If a crawl space is going to stay vented, I keep the vents working:

  • Nothing growing in front of them
  • No insulation blocking them
  • Screens in good shape
  • Reasonable cross-ventilation
  • No cardboard stuffed in them for the winter

I still like to have a vapor barrier on the ground. Having vents doesn’t mean your dirt floor should be exposed. See crawl space vapor barriers.

Automatic vents

I’m not a fan of automatic vents as a way to control humidity. They open and close based on temperature, and humidity doesn’t follow temperature. It can be hot outside with a high dew point, so the vent opens and lets that moisture into your crawl space. They’re not a replacement for a proper design.

What happens when you just block the vents

I’ve seen plenty of crawl spaces with the vents blocked off. Homeowners block them to keep the cold out, but they still have a dirt floor and no dehumidifier. Moisture keeps coming up from the ground, and there’s very little air movement. The result is odors, condensation, high humidity and sometimes wood above 20% moisture. Blocking off your vents doesn’t make it an encapsulated crawl space. If you want a closed crawl space, see crawl space encapsulation.

Replacing or sealing vents

These are the mistakes I see most:

  • Foam sprayed in a hurry, leaving gaps
  • Material that comes loose
  • Untreated wood touching damp masonry
  • Closing some vents and leaving others open
  • Never deciding whether the crawl space is vented or closed
  • Blocking flood vents, which must stay open in flood hazard areas
  • Blocking the access door
  • Sealing up before checking gas or oil appliances in the crawl space

Sources