Ask Capt. Crawl Space – Crawl Space Vents: Open or Closed?

Crawl Space Vents: Open or Closed?
Blocking Crawl Space Vents

Dear Captain Crawl Space, 

I have crawl space vents that open and close, and I keep them open in summer and closed in winter – am I doing the right thing?

James B., New Kent, VA.

Hello James, and thank you for your question. As a matter of fact, I get asked this question a lot. I certainly understand your reasoning: in winter, close vents to keep cold air out; and in summer, open vents to let humidity/ moisture out.

First, I would recommend encapsulating and conditioning your crawl space. This will make your crawl space healthier and energy efficient. But, to do that you would have to block your crawl space vents.

If you don’t want to encapsulate, I would recommend keeping your vents closed year round. Your house is subject to a phenomenon called “The Stack Effect”. Warm air rises through your house and escapes through the Gable Vents in your attic. Your house always has the same amount of air, so every molecule of air that escapes is replaced by another molecule of air.

Moisture in Crawl Space

The most likely place for this new air to be drawn into your house is through open crawl space vents. In the summer, warm humid air is drawn in. This air is humid and full of moisture- when it enters the cooler crawl space, condensation happens and the air releases its moisture and it gets all over everything in your crawl space.

When this moisture gets on your floor joists, it can promote mold, mildew, and can lead to wood rot and serious foundation issues. When it gets on your HVAC it can ruin your unit and certainly make it less efficient.

I hope I answered your question fully. If anyone has a question about your crawl space, please send it to


Nov 22, 2017 | Posted by in Capt.'s Blog, Crawl Space | 0 comments

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