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Community and Q&A

Plastic Vapor Barrier with ZIP-R in Zone 6

katemichigan | Posted in General Questions on


My first question.  Thanks in advance.

My husband and I are building a post-frame workshop in Zone 6.  For our walls, we’re using 2×6 bookshelf girts, batt insulation, and ZIP-R (R-6) sheathing.  For our roof (4:12 w/ 24′ span), we’re hanging batt insulation between 2×4 on-edge purlins and we’re sheathing with ZIP-R (R-9).  Our attic is within the building envelope but not usable.  Of course we want more roof insulation, so we’re considering blown-in cellulose atop the interior ceiling.

Two questions, please:

1) We’ll be living in this building for a couple years while we build our actual home.  Should we attach plastic sheets to the inside of our walls and to the bottom of our trusses to prevent moisture from entering the walls/attic during the cooling season?  We’re concerned that the ZIP-R might not be vapor permeable enough and that we might trap moisture in the walls/attic (should it ever happen to enter into those cavities)?

2) Do you foresee any issues (e.g. moisture) in the attic if we have insulation on top (purlins/sheathing) and bottom (atop the interior ceiling) of trusses?  The trusses are 4:12 pitch with 24′ span, so only 4′ or so of height at ridge.

Thanks for your thoughts and guidance,


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  1. rockies63 | | #1

    I hope you are using raised heel trusses to get extra space for insulation over the exterior wall top plate.

  2. katemichigan | | #2

    We are. 7 7/8" heel, I believe. Plus the 3 1/2" between the purlins. Plus the 1 1/2" exterior foam on the Zip-R.

  3. creativedestruction | | #3

    1. You're right to be concerned, as interior polyethylene would drastically limit the drying potential of both assemblies. Instead use a class 2 vapor retarder like "Membrain" or "DB+" or "Intello".
    2. Yes, this could cause issues. Avoid half-in-half-out attic space, otherwise it will get cold enough for condensation at the inside of the ridge. If you're set on using ZipR you have a great continuous air barrier at the exterior. Condition the attic space (if not for storage or anything else run your ductwork through it) and install the vapor retarder on the inside face of the batt insulation, not at the bottom of trusses.

  4. Expert Member
    Akos | | #4

    ZipR is not rated for roofs, it is only for walls. It also a pretty expensive way to insulate a roof if you can use simple vented assembly with fluffy on the floor. You can't split the insulation though, so either it is all on the roof deck, or all on the floor.

    If you want to insulate your roof deck, add in 2x on flat cross strapping above your purlins so there is a vent path from eave to ridge. Also bump up the purlins to at least 2x8 to get some real R value up there.

  5. katemichigan | | #5

    Thank you all for your guidance. Here's what I've taken away:

    1) Poly sheeting: Use a class 2 vapor retarder to avoid trapping moisture in ZIP-R walls and a foam-insulated roof.
    2) Don't create pockets in an insulated attic. To insulate at the roof deck, hang batts at and directly below the purlins and install the class 2 vapor retarder directly below that insulation.
    3) ZIP-R isn't rated as roof sheathing. Use standard 1/2 ZIP sheathing for roof.

    Follow-up question: If we still want to add foam board to the roof (e.g. 2" Foamular 150), should we do so above or below the ZIP sheathing? Seems like we might as well do it above the sheathing, then put our 2x furring strips (for cold roof) atop the foam?

    Thank you again!

    Edit: Found this in our Michigan code: "The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation." Seems like it should be cold roof > sheathing > foam > batts > vapor retarder.

    1. Expert Member
      Akos | | #6

      If you are putting batts in the rafters, you have two options. Make it unveted assembly, which means enough foam above the roof deck for condensation control (50% of the assembly R value as rigid insulation, so a LOT of foam). You can push this ratio a bit with a smart vapor retarder on the warm side carefully detailed as an air barrier. This is might still be a fair bit of foam. Top venting is still a good idea in snow country.

      You can also build it as a vented assembly. You would need a clear vent path from soffit to ridge and baffles to space the batts 1.5" off from the roof deck. In this case the foam can go on the inside and replace the vapor retarder. In this assembly, there is no limit to the thickness of the foam but generally want it to be 15% of the batt R value to minimize thermal bridging from the rafters.

      1. katemichigan | | #7

        We were planning on a vented roof. Metal panels over furring strips over sheathing. The 2x4 furring strips would give us a 1 1/2" gaps from our vented soffit to vented ridge. We hoped that would whisk away any warm air or condensation that happened to make it through the sheathing.

        Would that still be considered an unvented assembly based on your examples?

        So the total assembly would be:
        Metal panels
        1 1/2" vent (soffit to ridge) using 2x4 furring strips
        ZIP sheathing
        XPS foam (2")
        Batt insulation (between purlins)
        Batt insulation (hanging under purlins)
        Vapor barrier (hanging under batts)
        Conditioned attic (between trusses)

        1. Jon_R | | #8

          Note that there are roofs that have some kind of ventilation and roofs that have ventilation that meets code requirements and so don't need to follow R806.5. The two should never be confused, even if they can both be called "vented roofs".

          See IRC R806.1 (or local equivalent) for the latter.

        2. Expert Member
          Akos | | #9

          There is a big difference between what you are proposing and an actual vented roof. You could make the above work if you used a very permeable sheathing such as fiberboard, but that is not a roof I would walk on. The CDX/OSB is not permeable enough to allow sufficient drying. About the only way to make it work is with a very good warm side air barrier and dense packed with cellulose witch allows for some moisture buffering.

          A proper vented assembly needs a gap bellow the roof deck and the insulation bellow. You also not want much more than a thin layer of unfaced EPS. Plus if you are adding more batts bellow the rafters, just make those thicker, way cheaper. Cross strapping under the rafters/purlins works about 95% as well as continuous insulation for reducing thermal bridging, no need for anything fancier than that.

          So in your case:
          -1.5" vent gap (brings in cold air to prevent snow melt in heavy snow country)
          -roof deck (ZIP/OSB/CDX)
          -1.5" vent gap (roof vent, this could be 2x4 on flat over your purlins)
          -batts between purlins
          -cross strapping (say 2x6 on edge)
          -batts between strapping
          -vapor barrier

          You can also skip the cross strapping/batts and go with a layer of rigid insulation under the purlins. This would need to be one of the rated polyiso products. This could be taped as your warm side air and vapor barrier.

          1. Jon_R | | #10

            +1 on good interior side air sealing with testing to prove it.

            Note this R806.5 requirement:

            "No interior Class I vapor retarders" (aka vapor barrier). A Class II is required in Z5-Z8.

  6. katemichigan | | #11

    Thank you, everyone. I've learned a tremendous amount, and it seems that I should go back to the drawing board (a bit) for venting our roof assembly below the sheathing.

    I suppose both vents (atop and below sheathing) connect at the soffits and ridge line?

    Edit: One more question, please. If we have a 1.5" gap (flat 2x4 perpendicular to purlins) on both sides of sheathing, do we still add blocking to support all edges of the sheathing? It seems like blocking would... block... the vents.

    1. Expert Member
      Akos | | #12

      A great resource is:
      They have a lot of great information about any building science question you might have.

      I was looking at my reply above, I don't think the vented assembly needs to be also top vented provided you have a good warm side air barrier. As long is not enough warm air is escaping into the vent space to cause the snow to melt, a single vent is enough. To be safe you can also bump up the size of the vent space to allow for more flow.

      The top venting is a good idea for unveted assemblies in heavy snow country.

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