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Multiple layers of 6 mil poly sheet in crawlspace

1869farmhouse | Posted in General Questions on

I think I’ve figures out the best method of dealing with my crawlspace.  Namely spraying the perimeter walls with closed cell foam, lining the floor with poly sheeting, air sealing the floor joists (due to contaminents), and installing an exhaust fan with supply air grates in living space.

My question is, since I only have access to 6 mil sheet, can I use multiple layers?  Or will layers of vapor barrier cause problems?

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  1. Expert Member


    You can use multiple layers, but unless you are wandering around there all the time one is fine. The odd hole makes no difference to it as a vapour barrier.

    1. 1869farmhouse | | #4

      Makes since. I don’t plan on bein in there a lot, but I’m sure things will come up. I actually considered doing 6 mil now and then something much thicker down the road.

  2. Expert Member
    BILL WICHERS | | #2

    It’ll probably be extra slid-y to work on two sheets. Why not just order thicker material? Americover has many options, and the right tape to seal edges. I’m sure there are other suppliers with heavier sheet too.


    1. 1869farmhouse | | #3

      My living situation has changed due to the coronavirus and I have to be moved out of my current home and into the home I’ve been remodeling on Monday. So I’m limited to what I can find locally and don’t want my newborn to spend a single night there without the contaminants covered.

      1. nshirai | | #5

        If you're concerned about contaminants, delete the closed cell spray foam and do this instead:
        - Air seal subfloor and band joist
        - If you have ducts in the crawlspace, seal the duct system extremely well, especially the return side
        - Seal any foundation vents (except the one you are using to depressurize the crawlspace)
        - Attach your vapor barrier to vertical surfaces with some form of sealant (adhesive, tape, whatever) and tape joints and penetrations in the field
        - Put in a dehumidifier with permanent drain to outdoors
        - If you really need to insulate, use something less volatile than CCSPF such as rigid mineral wool board or a borate treated EPS

        1. GBA Editor
          Brian Pontolilo | | #7

          Hi Nathan.

          Excellent point about seling the ducts. I was part of a conversation on indoor air quality recently and the question of duct cleaning came up. An engineer who was also part of the panel reminded us that poor efficiency isn't the only result of leaky ducts, but spreading dust and potentially unhealthy contaminants is also an issue with them.

        2. Andrew_C | | #9

          @Nathan -
          Your suggestions make sense to me, except for the option of using rigid wool on crawlspace walls. I thought basement insulation was supposed to keep moist air from hitting cold surfaces, so air permeable insulation was not recommended. The rigid foam seems like a better idea.

      2. Expert Member
        BILL WICHERS | | #8

        Understood, you’re in a time and materials availability crunch, so this needs to be done in “get’er done!” Mode.

        You can just use one layer of 6 mil to accomplish your goal, then replace it down the road when you can. I don’t think you really gain anything with the second layer. The extra thickness of the recommended 10+ mil is more for physical durability than anything else.

        Don’t use spray foam if you have to live in the house immediately. You really need at least a few days of air out time after applying spray foam before you start living in the house. Unless you have a stone foundation where you need spray foam to deal with the irregular surface, use reclaimed rigid foam. Reclaimed polyiso is great for these kinds of projects and it’ll save you some money.

        You can use urethane caulk to hold the poly to the walls. Nathan’s idea to seal any ductwork is a good one too. Get a roll of fiberglass mesh tape and a tub or two (it’s amazing how fast you use this stuff up) of duct mastic and seal those ducts. I actually noticed an improvement in my own home after sealing the ducts in my crawl space. I had a day of “I smell duct mastic” after I did the work, but then I noticed an improvement. I was surprised I was able to notice any immediate improvement but I did.

        If you can, add some permanent light fixtures in the crawl space controlled by a switch near the entrance. The lights will make all the rest of the work much easier.

        BTW, if you’ll be working in old insulation or doing any cleaning in your crawlspace, I recommend wearing a tyvek suit while you’re working. I did some work in my crawlspace last week and got some spider bites which are annoying and still a little itchy. Next time I’ll suit up for some protection.


        1. 1869farmhouse | | #10

          Thanks a ton. It’s an ancient farmhouse built in 1869, so definitely rubble stone and irregular surface. I may have to make another post, my friend that’s done all my spray foam elsewear is saying that spraying the “rim joist” area that is old balloon framing (and already sealed and isolated at the top) will cause the wood to rot out... now I’m not sure what to do.

          But I have no ductwork at all. I’m 100% mini splits.

  3. Eric__S | | #6

    For what it's worth, I bought my crawl space poly from Crawlspace Repair. Their guy Larry sketched out the layout based on my crawl's dimensions and pier locations, and then did a material quote to be sure I didn't over or under-run.

    If your time and the workspace permit, I would suggest using rigid polyiso foam boards on the walls and rim joist, rather than spray. You can check Craigslist for reclaimed roofing polyiso in your area to reduce cost.

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