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Minimum thickness of exterior insulation – when NOT using foam.

brad17 | Posted in General Questions on

Hello folks,
We understand the logic behind this article regarding calculating minimum thickness of exterior rigid foam insulation in terms of preventing wet walls. 

Our question is: In zone 6, if we have R23 Rockwool batt in the cavity, and use Rockwool Comfortboard 80 for exterior insulation instead of rigid foam, would R6 be adequate here instead of the R11.25, given that this assembly would be more able to dry?

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  1. PAUL KUENN | | #1

    I believe code for our zone 6 is R10 as anything less will increase dampening the sheathing. Plywood could probably handle it but certainly not OSb. I like to keep the exterior close to 2/3rds of the R value.

    1. brad17 | | #4

      Thanks, Paul. For 2021 IECC it is either 13+10 or 20+5, so we are assuming 5 ci.

  2. Deleted | | #2


  3. lacroixb | | #3

    I'm reading and the formula isn't about actual thickness, it's R-Value that is later converted to thickness. I would stick to the same R-Values, which with Rockwool Comfortboard, will mean it's thicker than a comparable R-Value of rigid foam. One other thing to keep in mind... in cold weather, it's the INSIDE of the sheathing that will condensate and have to dry toward the inside - the exterior insulation is meant to minimize the temp difference at that surface, therefore minimize actual condensation risk. In the winter, having a vapor-open exterior insulation won't have much affect of drying ability of the sheathing-to-interior wall side of things...

    PS: I'm far from an expert on this, so take my comments with a grain of salt...

    1. brad17 | | #5

      This makes sense, thank you.

  4. PAUL KUENN | | #6

    The higher the R value on the interior, the more the sheathing temp decreases and leads to interior condensation. When the R value is higher on the exterior, the sheathing is kept warmer.

  5. Expert Member


    The ratios are for low-perm or impermeable materials like foam. As Martin has said,
    they do not apply to exterior mineral wool because it does not impede outward drying of the sheathing - and it is only due to foam inhibiting drying that there is any worry about the sheathing in the first place. The interior moisture doesn't typically condense on the interior face of the sheathing, it is taken in by sorption. As long as it is being released to the outside at the same rate, the sheathing stays safe.

    I do remember Ben Bogie saying he is still bit worried about dense mineral wool boards impeding drying, but have never heard him elaborate on whether that was just his fear, or he had some other reason for the concern.

    1. brad17 | | #8

      Thanks, Malcolm. This resonates with what we were thinking. It makes sense to us that sheathing would potentially dry in two directions assuming there is enough permeability on both sides. Incidentally, we were thinking to use henry blue skin on outside of sheathing. Do you think we would require something like Intello Plus between studs and GWB?

      1. Expert Member
        Akos | | #9

        There is generally no need for full peel and stick on the outside unless you have some questionable details that will dump a lot of water onto the sheathing. Taped sheathing with a quality tape plus standard housewarp is more than good enough and much cheaper.

        If you are looking to do full peel and stick, just make sure it is vapor open, ie VP100/Vp160 not the stuff for foundation waterproofing.

        1. brad17 | | #10

          Thanks, Akos.

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