GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Polyethylene as WRB and air barrier – comments?

Ben47297 | Posted in Green Building Techniques on

I’m considering the following wall assembly, from interior to exterior:
vapor permeable interior finishing, gypsum board, wood frame wall with unfaced fiberglass batt insulation (diagonal braces, no wood-panel sheathing), 2″ polyiso with no seam detailing, polyethylene (shingle lapped with sealed seams) transitioning to fully adhered membrane around all envelope intrusions acting as WRB and air barrier, 3″ EPS offset seams, rainscreen with light-weight cladding.

A more expensive alternative i was considering would be to replace the polyethylene with a fully adhered membrane all around, glued to the polyiso, but i would prefer to save on the membrane costs.

I’d love to hear any thought anyone has on this assembly.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Brian Pontolilo | | #1


    First of all, we'll need to know where you are located. Second, why are you over-complicating things by using materials like poly sheeting where they are not meant to be (as a WRB)? Third, why not build a wall that is proven to work? Here are a bunch of good options: Walls that Work.

  2. Ben47297 | | #2

    Thanks for the question. I'm in eastern Canada, zone 6 with about 4,500 HDD (18 degrees c) and 800 mm annual rainfall.
    The reason behind the assembly is to try to avoid the cost and other impacts of unnecessary products - wood-based sheathing in this case, potentially.
    Since I am planning to use 2 layers of exterior foam anyway, and simple bracing solutions exist,
    maybe plywood sheets are overkill?
    The space between the two insulation sheets can physically support a membrane and protect it from temperature extremes.
    I was thinking polyethylene because of price and durability, and because the faced foam boards are low permeability so drying to the exterior is already out of the question. Can anyone help me think of a better membrane to use?
    The point you rightfully bring up is that the assembly is not common, and it is surely a downside, but I am curious to hear more feedback about it if anyone has any ideas.

    1. Expert Member
      BILL WICHERS | | #4

      Understand that wood sheathing is often structural and provides racking resistance to the wall assemblies. That sheathing Isn’t just there as an air barrier. Why not just use regular house wrap? Why poly sheeting?


      1. Ben47297 | | #6

        I was thinking, for price and durability, and as I wrote, permeability plays a small role because the foam boards are faced. What would be the reason against it?

  3. Expert Member
    Akos | | #3

    You can use standard housewrap on the outside of the foam. This also lets you use paper faced roofing plyiso for the whole exterior insulation, which would be cheaper than your plyiso EPS combo.

    EIFS is also installed sometimes directly over studs, I believe in that case the WRB goes right on the studs under the foam.

    It is still good to go with multiple layers with staggered seams and tape them as your air barrier.

    That is a lot of exterior insulation, hitting studs through that much foam is hard. Make sure you do your math on your heat loses to justify the cost and complexity. 2" to 3" is much easier to work with.

    Even something like cement board is considered light weight cladding, there is no issues with weight or screws sagging, make sure to follow the manufacturers recommendation on screw spacing for the strapping.

    Generally, you only want to use as much foam as you need in your climate for condensation control, see bellow for ratios.

    For higher R value, installing more fluffy insulation between studs is always cheaper than rigid insulation.

    1. Ben47297 | | #5

      Thank you for the detailed reply. Could you please clarify:
      What is the benefit of housewrap over poly for this application? I believe permeability play no role.
      I like that the air barrier is sandwiched between two rigid layers for mechanical protection. It would also make penetrations easier to detail with a thinner layer of foam to the interior of the wrb. Do you see a disadvantage with that?
      I'm working under the assumption that EPS is cheaper than polyiso - why do you say it would be cheaper to go polyiso all the way? I was thinking to use the polyiso where it performs better, on the hotter side, and EPS towards the exterior.

      1. Expert Member
        Akos | | #7

        If you go with foil faced foam, you can use the foam itself as the WRB, you don't need anything extra. I don't like this option though, WRB should be shingled, less chance of issues down the road.

        The price difference between ply and house wrap is not that much, I don't see why use it here. This is not a technical argument, more of, use the rated product for the rated application. Poly could work, but why use it?

        Usually exterior foam installs still have a small amount of drying towards the exterior, for more robust walls it is good to pick foam with some permeability (unfaced EPS or permeable polyiso). With these you want a permeable WRB, which you get with house wrap.

        Around me, for $/R value, roofing polyiso is the cheapest. There is a bit of temperature de-rating in cold weather but over a whole year it still outperforms other insulation. If you can find it reclaimed, even cheaper. Paper faced polyiso is somewhat permeable but harder to tape.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |