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Huber Zip system R-6.6 sheathing and minimum foam R-values

MJDesigns | Posted in Energy Efficiency and Durability on

In zone 5 (central OH), the minimum R-value of rigid foam for 2×4 construction is R-5 and R-7.5 for 2×6 construction. What would be the required amount for a 2×4 staggered stud wall on 2×6 plates? Would the Huber Zip R-6 (R-6.6) sheathing be adequate since it would only be in contact with the 2×4 studs staggered to the exterior?

Is the product comprised of multiple thinner laminations of OSB & rigid foam or single thicker layers of OSB with rigid foam at the interior surface? Reason I ask is that I heard that it was bad to create a sandwich of foam surrounding traditional OSB sheathing (ie. rigid to the exterior and closed cell spray foam at the interior). If multiple thinner laminations, would it be OK to apply either closed cell spray foam to the interior (ex. 2″ layer, followed by 3 1/2″ of either blown cellulose or blown fiberglass such as JM Spider’s product) or just apply either the blown cellulose or blown fiberglass by itself within the entire 2×6 cavity? Appreciate your thoughts …. thanks.

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  1. Foamer | | #1

    The r-values required for rigid foam are to ensure that you have the proper amount of insulation outside of the potential condensing surface (inner surface of rigid foam), relative to how much you have inside of it. It must always be warm enough to prevent condensation. In our area (zone 5, northern Ohio) we need at least 30% of the total r-value in the assembly to be on the outside of the condensing surface. What matters is not how the wall is framed but how much r-value is in the cavities. Since you are considering a double studded wall anyway, a simpler approach would be to forget about the exterior foam in favor of regular sheathing. You will have a slight penalty in the plates but your double studs will eliminate most of the thermal bridging. Use spray foam for insulation and air sealing or, if you prefer, a combination of closed cell foam and cellulose/blown fiberglass as you suggest. You can tweak the wall thickness and insulation choices to arrive at the r-value that you want.

  2. MJDesigns | | #2

    Thanks for the follow-up and explanation. That specific Zip product most likely makes more sense for someone interested in framing a traditional 2x4 stud wall and using it in lieu of regular sheathing to provide the thermal break at the studs. In that case, do you know if closed cell spray foam is still appropriate for use with the Zip R-6 product? Net result would be just under R-30 (3.5" of closed cell spray foam and R-6.6 for the Zip sheathing).

  3. GBA Editor
    Martin Holladay | | #3

    I'm sure that some builders have used your suggested approach successfully, especially if the OSB is very dry on the day that the spray foam is installed. However, I'm not a fan of the foam-sandwich approach to walls -- and remember, it's unnecessary.

    Why not design a wall that can dry inward? You could choose to insulate your stud bays with cellulose if you wanted, or even open-cell spray foam.

  4. MJDesigns | | #4

    Hi Martin,
    Does the Huber Zip R sheathing actually promote sandwiching of the OSB layer(s). Attached is their "sell" sheet. When you look at the picture of the product across the top, it looks like "sandwiching" of multiple laminates of OSB & polyiso, yet the illustration below it gives the appearance of 3 distinct separate layers ... zip water resistive barrier, OSB nailbase for trim and rigid foam up against a stud which wouldn't actually sandwich the OSB, right? Looks confusing without actually seeing the product up close.

  5. GBA Editor
    Martin Holladay | | #5

    I agree with you -- the photo appears to show a multi-layer sandwich. As I wrote, that doesn't necessarily mean that there's any problem with the product. Just be sure that everything is very dry before installing any spray foam -- or choose a different approach if all of these worries concern you.

  6. Foamer | | #6

    Thank you for the attachment - that was helpful. I think what we are seeing are three panels pulled out from the stack. Each panel consists of three layers (vapor permeable outer skin, OSB, polyisocyanurate foam). If I am correct, you are good to go as far as the OSB is concerned.

    You should still pay attention to the balance between the r-value of the sheathing and the cavity insulation that I talked about in my earlier post. If your goal is an r-30 assembly, you will have to add low-perm insulation to the back of the sheathing foam before you build up your cavity insulation with a permeable insulation. Or you can do what we do here in Cleveland and use semi-closed cell spray foam for both vapor control and insulation value. That approach has the benefit of being just one application rather than two and it is a lot more economical than using all closed-cell foam. Fell free to contact me if you have specific questions about that ([email protected]).

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