Vapor barrier under slab

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Yvette Louka

July 17, 2023 15:59

Trying to understand vapor barrier, insulation and thermal bridging with this detail from one of the reference books from NCARB. Can someone please point out where is the slab (which I’m assuming is the top layer from the materiel indication similar to the concrete? So what’s underneath between this and the vapor retarder ? Thank You

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  Shernise Richardson July 20, 2023 14:45

  What book is this? AGS? Im thinking this is a compacted soil layer for the concrete slab to bare on...but not sure. I can look on the page and see if I see the hatch in other sections.

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  Yvette Louka July 20, 2023 15:25

  Thanks Shernise, yes it is AGS, under basement construction in cold climate.

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  Yvette Louka July 20, 2023 15:33

  Shernise, I took your idea and searched for compacted soil, but found a different hatch.

  But got another graphic showing that same hatch under "basement waterproofing underslab" and it is marked as "mud slab 2" min"

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  Shernise Richardson July 21, 2023 16:33

  Great. Glad you found the answer. Ill have to check out these graphics and review for my studies too :)

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  Christine Williamson Cronin July 21, 2023 21:09

  Pretty sure it’s sand. See comment thread here:

  https://are5community.ncarb.org/hc/en-us/community/posts/10379356402199

  If it were a mud slab, it would be under the poly. We use mud slabs (also called rat slabs) to get a more even substrate for waterproofing membranes on sites that are very large.

  This is not a terribly common way of designing slabs, and it’s not a very good one. The poly should nearly always be in direct contact with the concrete. It’s too bad this is in AGS.

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  Shernise Richardson July 21, 2023 22:14

  Thanks for the clarification, Christine and for including the other post. Your explanations make a lot of sense.

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  Ben Thouthip August 17, 2023 04:34

  It’s a slab that contact with ground level. They usually required 6 mil vapor barrier. Reference IBC chapter 19.

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