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Vapor Barrier on top of SIP floor?

Kevin Camfield | Posted in Energy Efficiency and Durability on

We are working out the details for the construction of our new house in the Pacific Northwest. Our architect and structural engineer have specified a 9 1/4″ SIP floor for the 1st floor. Since we will be building in a flood zone, our foundation will be an open design with concrete columns extending from a mat foundation. Horizontal beams will sit on top of the columns and the SIP panels will sit on top of the beams. The structural engineer has specified a factory treatment of “bluewood” preservative to the bottom face of the SIP panels. SIP panels make some sense in this situation because they are easier to install than joists and insulating from below. I understand they are often used for floors over garages and other open spaces.

I talked to the largest SIP panel manufacturer who distributes in the area. When I asked the representative how they recommend detailing the air barrier around the SIP panel, he informed me that they recommend placing an vapor barrier (< 1 perm) between the top of the SIP panel and the t&g plywood subfloor. He recommended attaching the subfloor to the SIP panel with screws. This doesn’t make sense to me. Wouldn’t this be trapping moisture in the top layer of the OSB? And all the screws used to attach the subfloor would penetrate the vapor barrier which would allow moisture to enter the OSB from above either via moist air or liquid from say a kitchen or bathroom floor. As always, I found some very helpful information on this forum. In the article “How to Protect Structural Insulated Panels from Decay” they were talking about a similar situation for SIP roofs. There is a lot of information in the article, but it seems to recommend using an air permeable moisture barrier and good attention to sealing the seams with foam, gaskets and tape. Would it work better in my situation to foam and tape (or use sealant on) both sides of the joints and leave out the vapor or moisture barrier? I can’t figure out how we might use a barrier on either side of the panel without penetrating it with a lot of screw or nail holes. Will the OSB without a moisture barrier then dry to the interior on the top and to the exterior on the bottom? Would the bottom need any additional protective layer? Finally would others recommend sealing the 2×10 rim in the floor perimeter under the exterior sheathing?

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  1. Brendan Albano | | #1

    For the air barrier, can you just seal the seams of the SIPs (with tape or foam or whatever is most appropriate)?

    For the vapor barrier, my understanding is that poking holes in it with screws typically isn't a big deal, as the functionality of vapor barriers is directly proportional to their area, so small holes aren't a big deal. This is not necessarily the case for air barriers or for a "spill barrier" like you are describing, so it still might be an issue, but the vapor barrier performance should be fine with a few holes.

    As to whether or not this is a good place for a < 1 perm vapor barrier, hopefully someone else will chime in!

    In walls, the "when in doubt" vapor barrier of choice seems to be a 2-mil nylon like Certainteed Membain, as it allows a certain degree of drying in the summer, while still performing its job as a vapor retarder in the winter. Not sure if the same logic applies to floors.

  2. Kevin Camfield | | #2

    Brendan, thanks for the reply. It makes sense that the holes don't make a lot of difference when the vapor barrier is in the vertical position. Most homes have that, although it is one reason why rain screens are good. In the horizontal position it seems like it is a way in for water from above and then not a way out for the moisture vapor. I'll take a look at the Certainteed Membrain to see if that would help in this situation although it seems like an air barrier is all I need since the SIP panels act as a vapor barrier already.

  3. GBA Editor
    Martin Holladay | | #3

    Your SIP distributor doesn't know what he is talking about. SIPs don't need an interior vapor barrier. There will be almost no vapor diffusion through a SIP floor, wall, or roof.

    What you need is a good air barrier, not a vapor barrier. The usual approach is to install SIP tape at the seams on the interior. If you don't want to use SIP tape, you could use a layer of plywood or OSB subflooring if you want -- with seams staggered so that the subflooring seams don't line up with the SIP seams. If you go this route, it still wouldn't hurt to tape the seams in the subflooring (unless the tape interferes with the finish flooring installation).

  4. Kevin Camfield | | #4

    Thanks for the reply. It's good to hear you confirm that they are giving out bad advice. SIP tape seems like a prudent thing to do and simple enough. We are evaluating the heating system now and one of the considerations is hydronic radiant floor heat on the first floor with tubes set in 1 1/2" of grout. If we put that directly on top of the taped OSB, I'm assuming that would actually help to keep things dry as the floor would be hotter and tend to extract any moisture from the OSB. Is that a fair assumption?

  5. GBA Editor
    Martin Holladay | | #5

    The interior facing of your SIPs are not at risk of getting wet (unless, of course, you have a plumbing leak). Keeping the OSB facing dry is not a problem.

    What you are worried about is air leakage. A layer of gypcrete (gypsum concrete) will act as an air barrier, so that will work -- as long as the hydronic heating contractor knows what he or she is doing. You'll probably need some type of membrane between the SIPs and the gypcrete.

  6. Kevin Camfield | | #6

    Perfect. Thanks for the counsel.

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