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Exterior Polyiso Insulation and Air/Vapor Barrier

denis_cosby | Posted in Energy Efficiency and Durability on

Hello, I am in the planning stages of a new home build. I want to install an exterior, continuous  insulation and air barrier. I am on a very tight budget for this build.

climate zone 4 marine, no huge temp swings here in northern Calif, approx 2 miles from the ocean. 2000 sq.ft single story.

So here is my plan,
2×4 walls with  r-13 cavity insulation,  1/2″ plywood or osb, not taped, 1″ poly iso foil faced, taped and sealed, 1×4 bats then hardiplank siding. Do I need to have a house wrap over the polyiso or is tape enough for a wrb? Do I need a house wrap over the plywood?

Roof is 1/12 pitch shed roof, plywood sheeting, 2/2″ layer polyiso taped and sealed, another layer of 1/2″ plywood or osb,  tigerpaw polyproplene roof underlayment then standing seam metal roofing.
Do I need a wrb on the first layer of plywood?

I’m concerned about too many vapor/air barriers in the assembly.
Is the polyiso considered and vapor or air barrier?
Will taping just the foam provide the need air sealing?

Thanks for any input

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  1. Expert Member
    PETER Engle | | #1

    If cost is a big driver, polyiso is one of the more expensive options. In answer to one of your questions, the taped foil-faced polyiso can act as your WRB, vapor and air barrier. A somewhat more standard approach would be to either use taped ZIP sheathing as air, water and vapor layer with EPS or XPS on the outside of that. Or, tape the seams in OSB or plywood sheathing for air control and use a standard housewrap WRB. Again, EPS or XPS over that. EPS is generally "greener." Recycled foam of any flavor is even better, and cheaper.

  2. Expert Member
    BILL WICHERS | | #2

    You'd be fine using the foil faced polyiso as your air barrier, and also your WRB -- just be careful not to damage the exterior side foil facer. I would use foil tape to tape the seams since it sticks well, and it gives a clean finished appearance (a bit OCD on my part, since this will all get covered up).

    Try to find reclaimed polyiso to save some money. Note that you sometimes see "roofing polyiso" with a fiberglass mat facer which is more difficult to tape. Reclaimed polyiso has been in relatively short supply lately though.


  3. denis_cosby | | #3

    Peter & Zephyr7 Thanks for the reply
    In my area poly iso is cheaper on a cost per R value, that was my deciding factor.
    As far as taping the sheeting I looked at that, but the cost for the tape on each layer of sheeting and foam was approaching 3K. It seemed redundant to tape 3 layers on the roof and 2 layers on the walls. My logic was telling me that if just the foam was taped carefully, would work.
    As for recycled foam I live in a rural area and that option is not available here.

    What about the multiple layers of house wrap and foam? is there any chance of trapping moisture between the layers?

    Thanks, Denis

    1. Expert Member
      BILL WICHERS | | #4

      You can get by with only taping the outer layer of polyiso for your air barrier. Taping extra layers just gives you extra insurance. Don’t forget to caulk the perimeter of the polyiso as you put it up to seal the edges to the framing. I would use polyurethane sealant for that. If you use multiple layers of polyiso, just stagger the seams so that you don’t have any seams lining up all the way through the stackup. If you stagger the seams like that, you don’t need to worry about taping the inner layers.


      1. denis_cosby | | #6

        When you say caulk the perimeter, do you mean the perimeter of each sheet or just the perimeter of the wall?


        1. Expert Member
          BILL WICHERS | | #8

          Just the perimeter of the wall. The goal is to make sure air can’t find a sneaky way to leak through. If you tape the seams between sheets, and caulk the sheet/framing interface around the perimeter of the wall, you’ve made a pretty airtight assembly. Note that I’d install the interior side drywall airtight too, as both additional insurance and also a bit more robustness of the wall as a whole.


  4. Expert Member
    Akos | | #5

    I prefer to tape the sheathing, easy to do when the walls are flat before standing them up. It also puts the tape behind housewrap and rigid insulation for extra protection. With thin exterior rigid there is no need to also tape the foam although it doesn't hurt.

    The best WRB is a lapped sheet good, by that I mean standard house wrap. Trades know how to work with, it relies on gravity for drainage instead of tapes and even badly done it still works well enough.

    I would budget a quality tape for the sheathing and rough openings and something like standard housewrap tape if you also want to tape the foam. The total tape budget should be well under $3k.

    2x4+R6 is around an R17 assembly. 2x6 walls with high density batts (mineral wool or HD fiberglass) is about the assembly R value for much less cost. In your mild climate, going with exterior insulation doesn't have much benefit.

    1. denis_cosby | | #7

      So your recommended wall assembly is plywood sheeting with taped joints, WRB, Foam, then batts and siding. no need for a WRB over the foam?

      About the roof, if I used a WRB over the first layer of plywood, two layers of foam and another WRB over the second layer of plywood, would that cause any problems. With a 1/12 pitch I want to make sure no water penetrates the layers.


      1. Expert Member
        Akos | | #9

        Most foam products have no issues with water, so no need for an extra WRB.

        Sometimes it makes sense to have the WRB over the foam instead of over the sheathing if your windows are outies. This keeps your water management in a single plane which makes detailing flashing much easier.

        All WRB are fairly vapor open, so there is generally no issues with having multiple layers in an assembly. Mostly just extra material and install cost.

        Roofs need to have one water proof layer, trying to including proper multiple backup layers underneath is very costly to detail right. Once your main roof fails and starts leaking, you want to know about it right away before it saturates the roofing polyiso. Once polyiso is saturated, it has to be replaced. I know it feels like the extra layers make the roof better, but it really is not adding anything in this case.

        The best way to improve the robustness of your roof is to increase the pitch.

        1. denis_cosby | | #10

          Thanks for the info

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