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Community and Q&A

xps insulation under crawl space vapor barrier

woodguy00 | Posted in General Questions on

I’m building a home in NW AR on a foundation part basement and part  conditioned crawl space. In the crawl I plan to use 20 mil plastic as a vapor barrier. I have access to 1/2” 4×8 xps insulation at no cost. My thinking is to place an imperfect layer or two on the ground before laying and sealing the vapor barrier.  The insulation would mostly be there to protect the vapor barrier from small rocks but would also add some amount of insulation value.  Does anyone see any problem with doing
this? To clarify I will be insulating the crawl space walls and cobbling foam for the rim joist. Foam is going under the concrete slab in the basement area . The main priorities with what I’m suggesting is to protect the vapor barrier in the crawl space sections from holes and try to avoid any mustiness. Are there any pitfalls to this idea?

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  1. tnergyservices | | #1

    It seems more appropriate to consider using the rigid foam for the foundation perimeter walling and maybe cut and cobble along the rim/sill (but, after air sealing the rim/sill). As the vapor retarder will be sealed to barrier status, the intent of said barrier is moisture/vapor control. Insulting under it shouldn't hurt anything, but I suspect isn't as necessary based on your climate zone vs. other locales. It's more important to get a good seal, insulate the foundation walling, and air seal and insulate the rim/sill. And then manage the air space of course...

    1. woodguy00 | | #6

      Yep, I plan to do all of these. Agree that insulation under the vapor barrier may not be necessary in this climate, but with no cost other than my own labor, I'm thinking to put layers down first to protect and cushion the vapor barrier from the somewhat rocky grade below.

  2. Expert Member
    BILL WICHERS | | #2

    I see no problem here, but I'd expect you'll see some cracking of the XPS while you're working on it. I would use at least two staggered layers here to help keep things intact as much as possible, but the more the marrier.

    I would absolutely follow the advice in comment #1 too, and be sure to insulate the perimeter walls up to at least code minimum levels. You could use the same 1/2" XPS in layers to build up to the R value you need, just be sure to stagger the seams.

    If the insulation is available for free, there is really no downside to using it, as long as you're not concerned with extra labor costs to build up lots of layers of relatively thin XPS sheets to get to more useful R values.


    1. woodguy00 | | #7

      Very helpful. Thanks. My labor cost is very low as I'm retired. I like the idea of two staggered layers

  3. Expert Member
    Michael Maines | | #3

    It's best to install the vapor retarder over the foam and directly below the concrete slab, so there is no problem with your order of materials. 20-mil is very thick for a vapor retarder; even with 10-mil I don't worry about damage during concrete placement. Concrete is very heavy, especially when wet; it will flatten out any foam under the slab so I also wouldn't worry about cobbling it together.

    1. woodguy00 | | #5

      Thanks Michael. To clarify, I plan to use foam under the concrete slab in the basement area below the vapor barrier. What I am asking here is for the areas of the crawl space where there is no concrete, is it ok to put free foam under the plastic vapor barrier to cushion the vapor barrier and prevent damage to it

      1. Expert Member
        Michael Maines | | #9

        Ah, thanks for clarifying. Your building code may not allow the foam to be on the floor without an approved covering; I have avoided this type of assembly as a result, using rigid mineral wool instead since it doesn't need fire protection. But I've also had builders substitute foam for the mineral wool and the building inspectors haven't seemed to notice or care.

        I just noticed your screen name--I used to use Woodguy as a screen name, good choice!

        1. woodguy00 | | #10

          After checking the flame spread and smoke numbers on this product, I'm rethinking if I want it inside my house. I've already used a layer to protect the external foundation waterproofing from the backfill and will use it under the concrete but may pass in the the crawlspace.

          I've been woodguy00 online for many years. Lots of time spent in the timber industry and woodworking hobbies.

          1. Expert Member
            BILL WICHERS | | #11

            If you put the XPS under a thin concrete "rat slab", or even a layer of sand, it will still provide insulation but will be protected from fire so those fire ratings won't matter. I think most PE vapor barriers probably have similar fire spread issues. I know the polyethylene (PE) jacketed fiber optic cables are required to transition to different materials within 50 feet of entering a building, or be completely enclosed in conduit, because they will spread fire (PE doesn't self extinguish).

            I do wonder how much difference flame spread makes with these materials when used in open areas of wood frame structures though, since the wood itself will certainly burn and spread fire too. If you're really worried about this, a rat slab over the insulation AND the vapor barrier is the best way to go. Concrete is pretty much fireproof :-)


  4. Expert Member


    Last year I laid poly on the levelled earth floor of a crawlspace, taped the joints and ran it up the low stem-walls and fastened it to the sill-plates.

    Lat week I was back there doing more work on the crawlspace, and quickly found that while moving around didn't damage the 6 mil vb, the poly moved under me, and was soon pulling away from the walls creating voids and folds. Exposed poly seems fine if left largely undisturbed, but if you are going to use the area for storage, or plan future work there, I would consider some sheet good as a protective cover.

    1. woodguy00 | | #8

      That's a wrinkle I hadn't thought of. I don't expect to go into the crawl space often but may toss an additional foam layer on top the vapor barrier (along likely path) to prevent movement

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