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Crawl space vents: open, closed or sealed?
Crawl space vents let outside air move through a vented crawl space.
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Written by HyreFoundation Research Desk Primary-source research and fact checking
The short answer
Crawl space vents let outside air move through a vented crawl space.
The IRC asks for 1 square foot of net vent area for every 150 square feet of under-floor area, or 1 per 1,500 where the ground is covered with a Class I vapor retarder.
Vents can be closed only when the crawl space becomes a sealed, unvented design with a continuous vapor retarder and a conditioning method.
Key takeaways
How to decide
Standing water, wet insulation or rotted wood come first. The crawl moisture check helps.
DOE Building America research supported adding sealed, conditioned crawl spaces to the code. Humid climates are where the choice matters most.
If vented, keep vents clear and the ground covered. If sealed, close vents, run a continuous retarder and add a conditioning method.
A hygrometer shows whether humidity stays down. EPA advises below 60%, ideally 30 to 50%, indoors.
Two designs that work, and one that does not
| Option | What it means for you |
|---|---|
| Vented | Open vents at the code ratio, a ground cover, dry grading outside. Works where outside air is dry enough to carry moisture away. |
| Sealed (unvented) | Vents closed, continuous Class I vapor retarder lapped 6 in and sealed 6 in up the walls, plus a conditioning method such as 70 pints/day per 1,000 sq ft of dehumidification or 1 cfm per 50 sq ft of exhaust. |
| Half of each | Vents blocked with no conditioning, or a sealed space with gaps. This is the version that grows mold and rots wood. |
What to ask before you sign
The full checklist is on choosing a foundation contractor.
Where this goes wrong
Moisture that used to leave has nowhere to go.
In humid climates warm air can condense on cool crawl space surfaces.
A sealed crawl space changes airflow. EPA sets an action level of 4 pCi/L; test after major changes.
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Questions
Should crawl space vents be open or closed?
How many crawl space vents do I need?
Should I close crawl space vents in winter?
Do crawl space vent covers work?
Is a sealed crawl space better?
Written and audited by
HyreFoundation Research Desk
Primary-source research and fact checking
We read the model building code, the federal soil and moisture guidance and the insurer position ourselves, and we publish each figure with the document it came from and the date we retrieved it.
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