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Exterior insulation with zip sheathing

Dennis_Vab | Posted in General Questions on

I just placed an order for zip 7/16” wall sheathing. I was going to use their r6 sheathing, but it’s outrageously priced right now. $33 vs $70. 

I will be using polyiso, or xps for an r value of 10-12 exterior of the sheathing, possibly reclaimed material. 

How important is it to run two 1” staggered layers, vs just butting up 2” material?

My zip panel will be my wrb. Should I run building paper between the insulation and sheathing for better drainage? And should the foam seams be taped?

I forgot to add, stud cavity insulation will be batts, with craft paper as a vapor retarder. Although, if I am going to get r12 outside, I can skip the vr in climate zone 6. 

thanks!
Dennis

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Replies

  1. Expert Member
    Armando Cobo | | #1

    Even though you have your WRB in the Zip sheathing, using the rigid foam, along with the foam manufacturer's approved tape to use as WRB, it’s a smart idea. Sort of belt and suspenders. Using 2-1” staggered layers even better. Quality installation is key. Smart moves.

    1. Dennis_Vab | | #5

      Thanks Armando. I almost want to just rely on the zip, as it will require much more detailing. I also have to figure out hot to hide the exposed foam near the sill, while allowing the zip wall to still allow water to escape from the bottom.

  2. Jon R | | #2

    IECC code says you can't completely skip a vapor retarder. Latex paint might do it - but I wouldn't count on it:

    https://www.research-alliance.org/pages/perm.htm
    https://rockytopmat.com/product/58FB10-N

    > polyiso, or xp

    I'd use unfaced EPS or GPS. Higher perms (more outward drying), lower $/R.

    1. Dennis_Vab | | #3

      You’re correct, latex paint would be my vapor retarder if I did unfaced batts.

  3. Expert Member
    Michael Maines | | #4

    You'll get slightly better performance using two overlapping, staggered layers, but with cavity insulation as well, it's not as important as it would be if the foam was your only insulation. I generally use a single layer on walls.

    1. Dennis_Vab | | #6

      Thanks for the feedback. Would you tape the foam board seams? And will the zip system still be a good water barrier with foam pressed up against it?

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