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Do you need a rain screen between ZIP and wood fiber board?

Mixed_Beans | Posted in Green Building Techniques on

I had in my mind to build a wall assembly that  is a 2×6 > ZIP > Exterior wood fiberboard > strapping ? cladding. I had someone today say I need a rains creen between the ZIP and wood fiberboard. I don’t believe I’ve ever seen this done with any exterior insulation. Is there a reason to do this? If not why?

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  1. Expert Member


    The place for a rain-screen if you are going to install one is over the wood fiber board. It makes no sense to introduce air behind the exterior insulation, or to have the backside of the cladding tight to it losing the benefits of having a cavity to help dry it.

    1. Mixed_Beans | | #2

      That's what I thought. He was pretty convinced about where the rain screen should go. His concern was that you would be trapping water between the insulation and the ZIP but I think because wood fiber board is permeable it should dry just fine.

  2. Expert Member
    Michael Maines | | #3

    The only reason you might include a drainage plane is if the exterior insulation does not have enough R-value to prevent condensation, with the possible result of liquid water forming on the outer face of the sheathing: Wood fiber is vapor-open and moisture will dry to the exterior, but including a gap would remove the moisture more quickly. I wouldn't bother, but that's the only supporting evidence I can think of for why you might do it.

    1. Mixed_Beans | | #4

      Thanks. TimberHQ calculated R22 of TimberBatt in the 2x6 and 2.5” TimberBoard (~R9) on the outside of the Zip. Is that enough outsulation for this assembly in Zone 5A-6?

      1. Expert Member
        Michael Maines | | #5

        Using the IRC prescriptive code for what assemblies are allowed with a Class 3 interior vapor retarder (e.g., latex paint on drywall), a proxy for insulation ratios reasonably safe from condensation at the sheathing, with 2x6 wall in CZ5 you need at least R-7.5 on the exterior and in CZ6 you need at least 11.25. (See table R702.7.1: That table also shows that in CZ5, you are safe with vented cladding over wood structural panels (including ZIP). On the border of zones 5 and 6 you might be cutting things a bit close but that's where wood fiber being vapor-open helps. If you can add more exterior insulation, or a variable permeance membrane at the interior, you should be completely in the clear.

  3. Expert Member
    Joshua Salinger | | #6

    Christine Williamson articulates this well, I believe in one of her BS Fight Club posts (although, I could be wrong about where I heard her detail this). Essentially, one wants to leave enough room for bulk water to drain out should it get between the sheathing and the exterior insulation- if the exterior insulation is hydrophobic, such as the plastic foam options. In this case, hydrogap or similar wrinkled housewraps would do well. The hit on the thermal is negligible at this 1/8" spacing and the benefit to not trapping water is WAY more important. I have heard the the roughness of Zip can account for enough drainage, but I am not totally convinced (maybe?). However, if one is using a hydrophillic (or hygroscopic) material such as wood fiberboard, then any water that finds its way between the insulation and the sheathing should be allowed to redistribute and dry to the exterior or interior (depending on specific assemblies, i.e. if the interior wall is vapor open). Of course, if there was a consistent leak this wouldn't be great as the safe storage capacity of wood fiberboard is good, but not great. Adding the 1/8" gap would only make the assembly more resilient, but it would admittedly be an extra cost that would be for a 'belt and suspenders' safety margin. I land on the idea that the 'roughness' of the Zip combined with the hygroscopic materials of the wood fiberboard would be a safe assembly and would do this on a custom home we build (or one for my family). If one really wanted to make it bulletproof, add the 1/8" gap.

    1. Expert Member
      MALCOLM TAYLOR | | #7


      I can undertand Christine's reasons for using a small gap between the sheathing and exterior foam, but agree with you and Mike that it doesn't add much with wood fiber insulation.

      Conceptually I wonder at how deep do we need to go before we can assume water will not penetrate a wall? With a rain-screen cavity you have a primary (the cladding) and secondary (the WRB) layer of protection separated by a vented capillary break. Does it make sense to propose a third (the dimpled house-wrap) just in case? At some point we need to have confidence in these layers doing their job, and I'm not sure it makes sense to continue adding largely redundant back-ups.

      1. Expert Member
        Michael Maines | | #8

        Malcolm, a rule of thumb I have heard Dr. John Straube relate is that of the rain the hits the face of a cladding, roughly 1% will get through the cladding. When the cladding is installed directly on a WRB, roughly 1% of the water that got past the siding will get past the WRB.

        With a rain screen (meaning a gap of 1/4" or more), he said that of the rain that gets through the cladding, 1% of that will make its way across the rain screen gap and reach the WRB, and 1% of that will get through the WRB.

        Of course those percentages are very rough approximations, but provide an order-of-magnitude way of thinking about it.

        1. Expert Member
          MALCOLM TAYLOR | | #9

          Thanks Mike. Even it it is off by a bit, it's an interesting way of looking at it.

        2. Mixed_Beans | | #10

          So, adding outsulation to the mix we're now at 1 % gets through cladding, 1% gets through rainscreen, 1% of that gets through the outsualtion (but probably less when you are talking about 3" of outsulation) and 1% of that gets through the WRB. That sounds like it is certainly not worth the extra rain screen. I would bet that the assembly dries out long before water gets to the WRB and spends any time there.

          1. Expert Member
            MALCOLM TAYLOR | | #11


            That sounds like a reasonable conclusion to draw.

        3. jollygreenshortguy | | #12

          So is the fiberboard the WRB in this wall section? I don't see any explicit WRB drainage plane.
          I wonder about its appropriateness where the rainy season and generally high humidity levels last over half the year.

          1. Mixed_Beans | | #13

            Yes. The Zip sheathing.

            ZIp > wood fiber board > rain screen > cladding

      2. Expert Member
        Joshua Salinger | | #15


        When I visited Switzerland a few years ago we went to a multifamily building that had 6" of rockwool attached to the framing on the exterior-- no sheathing/WRB or anything else. The interior had the gypsum board, which was the air barrier. Apparently, the thinking is that there is no way bulk water will get through 6" of drainable rockwool. Probably something to be said for this, but I don't know that I would have the guts...

        The point is that I agree, there is likely exceedingly little water that could make it through all these redundant layers.

        1. Expert Member
          MALCOLM TAYLOR | | #17


          Yeow! It may well work, but I couldn't sleep at night if I built that wall assembly.

  4. john_gilchrist | | #14

    703.7.3.2 says a "rain screen", an air space at least 90% open and at least 3/16" would be required anywhere in the eastern half of the US.

    1. Expert Member
  5. DamionL | | #16

    If you check out Matt risinger on YouTube he did a video where he poured water between zip sheathing and polyiso that was installed to the zip with no gap and there was still enough room in between due to the non smooth nature of the osb and zip coating to allow water to drain if it was to get behind the insulation.
    Plus since your insulation and zip is vapor permeable, any incidental water that gets there should diffuse out.
    If you were doing foil faced polyiso on say ice and water, a small gap may be better since any water that gets in between those layers cannot diffuse anywhere. You should be fine.

  6. Mixed_Beans | | #19

    Good info everyone. Thanks for the insight.

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