Vapor Barrier in Crawlspace and Vent to the outside
michaeldrehl
| Posted in General Questions on
Hello!
I have a crawlspace that has about 1/4 of an inch layer of concrete. It is in Northern NJ and was probably built in 1900s. It has a small access from the basement and from the outside. Currently, it is vented to the outside. The floor joist already have white mildew on them.
I wanted to lay down a 6 mil poly vapor barrier and insulate and air seal the exterior vent.
I have heard that laying down vapor barrier will help stop the vapor migration from floor to the ceiling but not completely stop it.
Should I not close of the vent to the outside?
Thank you!
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Replies
Michael,
There are two types of crawlspaces: Vented and unvented. If you seal the vents you need to do some other things to make it work. This article explains what's necessary: https://www.greenbuildingadvisor.com/article/building-an-unvented-crawl-space
Hi, I read the article and on building science about ventilation and unventilated crawlspace. What they say about unventilated is that there should be forced conditioned air in the confined area through a duct or a dehumidifier.
In my situation, the crawlspace has two entrance (please see the two photos attached). One from the basement in the house and one from the outside. The outside access serves as ventilation and so does the access from the basement.
My plan was to close off and air seal the outside ventilation and put down a vapor barrier and leave the inside access from the basement as it is.
I recommend this article by Joe Lstiburek:
https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces
In your climate the moisture you're seeing is likely coming from humid summer air and not the ground.
Thanks for sending that article.
It does get humid down there.
My question is can I put a vapor barrier down and close off the outside ventilation if I have an access from the basement to the crawl space?
It should either be inside the heated and cooled part, or outside. A lot of older houses have attics, basements and crawlspaces that are sort-of inside the house, sort-of outside. The modern building science concept is that you should be able to take a plan of the house and draw the boundary between the interior and the exterior without ever lifting your pencil off the paper. And that boundary should be air-tight, vapor-tight and insulated.
So yes, you can bring the crawl space into the interior of the house. You need to seal and insulate the exterior of it, which includes the bottom.
The same rules apply to the basement. It should either be sealed and insulated against the outside, or sealed and insulated against the interior of the house. The latter is rarely done because it complicated the mechanicals.