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Foil or not on exterior insulation?

IRFL | Posted in General Questions on

On a existing house climate zone 5 we are removing the exterior stucco and adding R-5 exterior insulation, plywood sheathing, Blueskin and cedar shingles. Would you recommend the exterior insulation to be foil faced (Vapor barrier) or un-faced to allow for some vapor permeability?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Flemming.

    Do you know if the house has an interior vapor retarder? If, for example, the walls had interior poly, it would be advantageous for the exterior insulation to be vapor-permeable--since the walls can't dry inward, they will be able to dry to the exterior. On the other hand, if there is no interior vapor retarder (other than the painted drywall), foil faced insulation is fine because the walls can dry inward. In the latter case, you'll want to make sure that you have adequate exterior insulation to manage the dew point temperature at the sheathing. Take a look at this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. IRFL | | #2

    No interior vapor retarder. Half the walls are plastered with blown in fiberglass insulation and the other half are currently gutted and will be insulated with dense pack cellulose and a smart air barrier on the inside (Intello or Membrain). In other words the wall should be able to dry inwards.

    1. Expert Member
      Dana Dorsett | | #3

      It's fine to use foil faced goods for the exterior insulation layers of that stackup, but it's not necessary. The additional sheathing layer of plywood adds another ~R0.5 exterior to the structural sheathing, the cellulose will buffer the seasonal load, an the smart vapor retarder on the interior lets the cellulose & structural sheathing dry readily toward the interior.

      It's worth installing a mesh type rainscreen underlayment between the Blueskin and shingles, which will improve the longevity of the shingles and protect the plywood layer by promoting faster drying, and a partial caplillary break. (eg: 1/4" Obdyke Rainslicker or Rainslicker Plus).

      1. GBA Editor
        Brian Pontolilo | | #4

        Agreed. And yes to the rainscreen for cedar shingle installation. Good point to add.

  3. IRFL | | #5

    Great feedback THANKS, and the rain screen is part of the plan.

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