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Crawlspace question climate zone 6

Question about sealing up a crawlspace with foam.

Our house at some point had an addition on the front put on. It is more like a bump out that is 6' x 16'. They did dig for a foundation and built a cinder block wall to hold this up. There is a dirt floor in the crawlspace that is above the level of the dirt outside the block wall. This crawl space is only 18" deep stays dry and only accessible through an old basement window well window that they built this in front of. Needless to say I never went in there.

We are renovating our house and the subfloor over this crawlspace has been removed so I now have easy access to this area. I am planning on putting at least 2" of foam (polyiso) on the crawlspace walls and was planning on doing the same on the floor. I will be using 1" sheets and will stagger the seams. I will tape all of the seams. I will air seal the rim joist area with the polyiso and can foam.

My question is about the floor. If I put down the 2" of polyiso over the dirt do I need a poly vapor barrier over the foam? Should I do a 6 mil vapor barrier over the dirt and then put down the foam? Or should I skip the poly all together? I will never access this area again so I do not need any sort of rat slab to protect the foam.

I will say that this area gets ventilation from the basement through the old basement window that was removed and I may take out another cinderblock to promote more free airflow with the basement.

So to reiterate. Poly vapor barrier over dirt floor yes or no? If so where? Above or below the foam?

Is the amount of foam I'm talking about enough for climate zone 6?

I hope this all makes sense, I try to be detailed and sometimes I just make it confusing with too much information.

Thank you.

Asked by William Heiden
Posted Fri, 08/22/2014 - 12:32
Edited Fri, 08/22/2014 - 12:35

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2 Answers

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William,
Crawl space walls should be insulated to at least the same R-value as the minimum required R-value for basement walls. In Climate Zone 6, the 2009 International Residential Code calls for basement walls to be insulated to at least R-10. So 2 inches of polyiso (about R-13) is adequate.

You should install a layer of polyethylene above the dirt floor, and then install the rigid foam on top of the polyethylene. I suggest that you use a few old bricks or rocks to keep the rigid foam in place, in case the racoons decide to have a party.

For more information on this work, see Building an Unvented Crawl Space.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 08/22/2014 - 13:26

2.
Helpful? 0

Thank you Martin.

Answered by William Heiden
Posted Fri, 08/22/2014 - 22:00

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