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Vapor barriers inside AND outside of unvented crawl space?

I have been trying to figure something out for days. Maybe you can provide some clarity.

If I am building an unvented crawl space, and have an interior vapor barrier on the ground and sealed to the stem wall, with rigid insulation over it, do I need to have a vapor barrier on the EXTERIOR of the foundation wall, as well?

Do I even need to brush asphalt on the exterior CMU or do I want it to breathe?
Thanks so much!

Asked by Beyhan Trock
Posted Feb 7, 2013 5:43 PM ET


2 Answers

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Yes, you need to install asphalt dampproofing (or a waterproofing system) on the exterior of your CMU walls. This asphalt dampproofing is required by most building codes, and it's always a good idea.

It's normal for below-grade CMU walls to be somewhat damp. Concrete blocks and mortar aren't damaged by moisture, so you don't have to worry too much about the drying rate of the CMU wall. The asphalt dampproofing is a vapor retarder, not a vapor barrier. In most such walls, there is some drying through the exterior of the above-grade portion of the wall. In any case, don't worry about it.

Answered by Martin Holladay, GBA Advisor
Posted Feb 8, 2013 8:19 AM ET


Thanks. BTW - Does exposed insulation in an unvented crawlspace have to be covered with something like cement board, so that it won't burn and give off toxic fumes in a fire?

Answered by Beyhan Trock
Posted Feb 8, 2013 10:33 AM ET

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