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Community and Q&A

ZIP R for exterior remodel

Mike Woods | Posted in Energy Efficiency and Durability on


I am considering ZIP R to replace my siding, sheathing, and windows of a 1970s home in Saint Paul, Minnesota. I have an interior wall vapor barrier that was put in place when the house was built (poly). The exterior is currently fiberboard 5/8″ sheathing with insulated (fiberboard backed) Aluminum siding.

If I add ZIP-R (looking at up to R13 potentially) am I going to have a moisture problem given that the permeability of ZIP-R looks to be <1 perm creating the dreaded moister sandwich?

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  1. Expert Member
    Dana Dorsett | | #1

    It it 2x6 framing, or 2x4?

    Is the 5/8" fiberboard structural, or is there let-in bracing or shear panels or some other bracing on those walls?

    1. Mike Woods | | #2

      2x4 framing. I am unsure about the structural bracing of the walls. Is there a way I could determine this without tearing open the wall?

      1. Expert Member
        Dana Dorsett | | #3

        You might take a look at the fastener spacing and fasteners on the fiberboard.

        If it's 1-3/4" - 2" galvanized roofing nails 3" on center along the edges of the sheets, 6" on center along each stud for the rest of the sheet the fiberboard is surely structural.

        If it's more than twice those spacings it's probably not structural.

        If it's somewhere in between it might have been intended to be structural.

        Let-in bracing can sometimes show up infra-red imaging if you're looking for it, but it's subtle.

        Shear panels are often well-fastened plywood (in lieu of fiberboard) spanning two or two stud bays, most often at the ends and exterior corners of walls. Sometimes they are on the conditioned space side of the studs. They are sometimes inset into the framing to avoid a step in the wallboard or exterior insulating sheathing.

        The walls might also be braced with flat steel X bracing under the wallboard or under the fiberboard instead of let-in bracing or shear panels.

        With 2x4 framing R9 ZIP-R would be adequate for dew point control in your climate, but Huber doesn't have a fastener spec for anything thicker than the R6 to meet structural requirements, But R6 wouldn't quite make it for dew point control. It would be fine to install flat steel X-bracing in lieu of structural sheathing, but have an engineer spec the bracing.

      2. Expert Member
        Zephyr7 | | #6

        One of the new fancy stud finders can usually see let-in bracing if you're careful using the device. You have to have a solid surface, so the siding would have to be off so that only the sheathing was on the wall. Let-in bracing will look like a diagonal "stud", which you can find by running the stud finder back and forth and tracking the diagonal brace, if it's there.


  2. Expert Member
    Zephyr7 | | #4

    If you're going to have the exterior siding, sheathing, and windows off, it wouldn't be all that difficult to use a razor knife to cut the poly vapor barrier out of the wall cavities if it's a problem. You'll have the wall open from the exterior, so you'll be able to get into the walls pretty easily.


    1. Expert Member
      Dana Dorsett | | #5

      Good point!

  3. Jon R | | #7

    Below link is good generic vapor retarder/barrier advice.

    I'd add EPS foam (any amount >= 1 perm) on the exterior, replacing some sheathing with plywood if you need more structural support.

    1. Expert Member
      Dana Dorsett | | #8

      That's right!

      If the existing cavity insulation and fiberboard is in good shape just leaving it and installing a couple inches of unfaced EPS would do it. Even a 3" most Type-II EPS would be just under 1 perm. The 5/8" fiberboard is worth about R1.5, and the fiberboard itself is fairly water tolerant, so with R8+ of EPS on the exterior it'll stay pretty dry and stable with or without the polyethylene sheeting in place.

      If the wall is air-leaky due to the seams holes/penetrations of the fiberboard, that may be fixable with a fully adhered permeable weather resistant barrier (Grace Vycor enV-S, Henry Blueskin VP 100, Delta Vent SA, etc.) .

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