Crawl Space Venting

Should crawl spaces have open vents in the warmer months? I live in NC. I am hearing that I should not vent my crawl space in the summer anymore.

NC is in the zone of “good arguments on both sides”.

It really depends on the amount of moisture in the ground under the particular house and the quality of the vapor barrier installation. Other important factors include, surface water control, height of framing above the ground, amount of moisture generated inside the living space, size of living space and amount of sunshine/shade on the house.

Its a complicated subject that really just depends on the exact conditions present at your site. Sealed/conditioned crawls are the best and almost all others will have some negative issues at some point during the life of the structure.

Vent it in all but the extreme winter months. Our experience shows that wood moisture and humidity are more of a concern in the summer. Even with all the vents open, stagnant, humid air can contribute to excessive moisture in the wood structures that can lead to expensive repairs. We therefore always recommend visual monitoring of the crawl space at least annually, and especially during the humid summer months. A licensed pest control company can be retained to test the moisture content of the wood using specialized instruments. It should be < 20%. If it’s higher, put down a 100% vapor barrier (plastic) on the ground. Test again. You should be aware that wood will decay at moisture levels > 28% and will support fungi growth at levels in the low-20s.

I would not close the vents unless the results I outlined above are poor. If so, you can “condition” the crawl space. That means sealing the floor and walls and blowing conditioned air into the crawl. That can be and expensive undertaking. You may not need to do it. Vent it first and test the wood this summer and see what you get.

Mine is vented and it’s been OK for over 100 years.

I inspected one house a few years ago. It was about 12 years old and the vents had been closed for quite some time. A large portion of the floor framing was rotted.

Most of those 100 years it did not have floor insulation to trap moisture.
Or put another way, the house kept the crawlspace drier due to easy airflow between the two areas.

A vapor barrier is needed way before the wood reaches 20% !

Anything above around 15% indicates elevated and prolonged moisture/humidity in the crawlspace.

That’s true. And, my house sits on top of a hill and water flows away from the house…moisture intrusion is not a problem. I don’t even have a vapor barrier…it’s dry as Egypt under my house.

Thanks guys!

My lot is very steep, and shady. I think I will continue to close vents in the winter, and open them in the warmer months.