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Minding the gap with rockwool over exterior sheathing?

hertzr | Posted in Green Building Techniques on

Based on people’s experience – should I plan on incorporating a small gap between the sheathing + WRB and exterior rockwool panels for my Cape Cod’s retrofit in CZ 4A? Am referring to Building Science Corp’s recommendation to include a small gap between foam board and sheathing (link below).

Does this thinking also apply to rigid mineral wool panels such as Rockwool’s Comfortboard IS 80? Or could a flat house wrap or membrane work just as well as something with bumps or crinkles?

I’m torn on mineral wool’s ability to reduce hydrostatic pressure at the face of the sheathing/WRB, acknowledging that the wall assembly will dry to the exterior on non-wet/humid days. Thanks!


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  1. Jon_Lawrence | | #1


    The gap the Joe is referring to is between the cladding and the foam, not between the foam and the sheathing. The same goes for Rockwool. If you are using OSB or plywood, you need to apply a WRB ( which can usually double as your air barrier), then the Rockwool, then the "gap" (usually 1x4 or 2x4) and your cladding. If you are using ZIP, then the Rockwool goes directly over the ZIP. I am also in Zone 4A and have 2 layers of 1.5" Roxul on my house over ZIP with 2x4's for the furring. We used 7" FastenMaster screws to secure the furring to the studs. You need to pre-drill the furring, otherwise it is very difficult to get the heads to sit flush and keep all of the cladding in the same plane.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Richard,

    Sometimes it is recommended to use a drainable housewrap to give OSB sheathing a bit of outward drying potential with certain exterior foam insulation that is not-so-permeable. Mineral wool is vapor open though, so you should be fine with a flat housewrap. As Jonathan mentioned, you still should have a rainscreen gap between the mineral wool and the siding.

  3. GBA Editor
    Martin Holladay | | #3

    I agree with Brian: because mineral wool insulation allows outward drying, you don't have to worry.

    Joe Lstiburek's advice in the "Mind the Gap, eh?" article -- and it's worth noting that contrary to Jonathan's comment, Joe was indeed talking about a gap between the sheathing and the rigid foam, but only in certain circumstances -- mostly applies to homes that have both exterior rigid foam and either (a) closed-cell spray foam between the studs, or (b) a challenging cladding like stucco or adhered stone veneer.

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