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Community and Q&A

Roxul Comfortboard rainscreen

user-6603947 | Posted in Green Building Techniques on

Our home is located in Lincoln, NE (Zone 5). Existing wall construction is typical 1/2″ drywall, 2×4’s @ 16 oc with fiberglass batt insulation, 1/2″ sheathing (mostly fiberboard, some areas are OSB), house wrap & vinyl siding. We will be replacing our siding due to hail damage, so I thought it would be a good time to add exterior insulation to the walls.

For the exterior insulation, I am considering either EPS, Poly, or Roxul Comfortboard 80. I like the environmental impact and drying capability of Roxul; however, in terms of cost, EPS ($0.08/r-value) or poly ($0.12/r-value, using R-4.5 cold-weather derating) are both cheaper than Roxul ($0.18/r-value).

My understanding is that with EPS or poly, I can attach vinyl siding directly to the face of the foam insulation. All installation guides I have seen for the Roxul product have used furring strips. Are furring strips required with Comfortboard if I am installing vinyl siding, or could I attach the siding directly to my underlying sheathing similar to EPS or poly? If furring strips are required, I don’t think I can justify the additional time and expense over the EPS or poly.

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  1. GBA Editor
    Martin Holladay | | #1

    Just so you know -- when people refer to "poly," they are usually talking about polyethylene sheeting. I think you are talking about polyisocyanurate -- and the short version is "polyiso," not "poly."

    When I interviewed to building scientist John Straube for a GBA article called Installing Mineral Wool Insulation Over Exterior Wall Sheathing, he told me, “We installed vinyl siding over 3-pound-per-cubic-foot Roxul on a number of walls, including Eric Burnett’s house, about 10 years ago with no issues.”

    So it seems possible to install vinyl siding over mineral wool without furring strips if you want to give it a try. Before you go this route, though, make sure that you know where your WRB is, and how to integrate your WRB with your window flashing. Think through your window flashing details carefully before deciding that you want to try this approach.

  2. charlie_sullivan | | #2

    What's on the inside of your fiberglass? If it's foil-faced, the lack of good drying capability to the interior is a possible reason to use vapor-permeable insulation on the exterior: mineral wool is the best in that regard although EPS has a little vapor permeability.

  3. user-6603947 | | #3

    In this area, I would expect the fiberglass may be kraft paper, but I doubt it is foil-faced. However, I did have to deal with a small mold issue in our basement because the builder installed polyethelene on the interior side of the above-grade walls. Local code supposedly required a poly barrier in above-grade basement walls when our house was built in 2000; it was not common practice to continue the poly to the upper floors. I will cut a hole in the 1st floor laundry room to verify there is no poly behind the drywall.

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