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Advice on proposed zone 6 wall assembly

itsmyname | Posted in GBA Pro Help on

We are building a cape in zone 6, Waldoboro Maine. I am considering the following wall assembly and have several questions regarding the components. Forgive the length of this, but I don’t know how to make this any briefer.

The assembly is Prefinished FiberCement smooth lap siding, 1″ of XPS rigid foam covered with Rain Drop by Greenguard (grooves for draining), 7/16″ OSB sheathing covered with Crinkled building paper by Dupont, and 24″ on center 2×6″ studs with cavities filled with dense packed blown-in cellulose insulation. While not ideal from an R-value standpoint, I will take special care to prevent the majority of potential air leakage.

I chose FiberCement siding because its 15 year warranted finish will preclude frequent painting and with its trim looks wood-like. Does the Rain Drop by Greenguard provide sufficient channeling for air drying behind the siding? The siding manufacturer specifies 1″ max. of rigid foam unless 3/4″ lath strips are applied over the studs for nailing. I want to avoid the extra depth of the wall beyond the OSB so that the fiberglass casement windows don’t require additional framing to fur-out the opening for support.

For the rigid foam in Zone 6 I realize that a minimum of 1 1/2″ is recommended to keep the OSB sheathing warm enough, so that dew point is not reached on the face of the OSB. Does, however, a high enough perm rating on the foam allow for enough drying to the exterior to prevent moisture damage to the interior of the wall? I was surprised to find that Dow’s 1″ XPS rigid foam has a perm rating of 3.5 (ASTM E96). This is vapor semi-permeable not semi-impermeable as many people suggest for 1″ of rigid foam. Does this dry enough to the exterior, so that I don’t have to instal a vapor retarder to the inside of the building envelope? Does the sheathing product Zip System help in preventing moisture damage within my proposed assembly?

I must admit a low-tech 12″ wall is looking more appealing, but I am not ready to deal with the different challenges such a wall presents.

Thanks for your help!
Tom Ruben

Update: the rigid foam I had initially suggested, 1″ XPS shiplap by Dow with perm of 3.5 is not available in the U.S. I don’t know the perm value for their regular tongue and groove panels , but I suspect its more in line with Owen’s perm. I guess the higher perm available to Canadians is for the extra cold and potential dew point problems.

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  1. jklingel | | #1

    "I must admit a low-tech 12" wall is looking more appealing, but I am not ready to deal with the different challenges such a wall presents." •• You may be over-worrying that. With some searching, I'll bet you get it figured out, if you decide to go that way. First, do a heat loss analysis to see what wall makes the most economic sense. A double stud w/ dense packed cellulose sure may. And, if you already know your exterior foam is too thin, a plan B is in your future anyway.

  2. GBA Editor
    Martin Holladay | | #2

    1. Your exterior foam is too thin: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    2. I think your decision to install fiber-cement siding on top of rigid foam without furring strips is a mistake. There are many benefits to the furring strips: the air gap will help the wall dry in case of water entry, and the air gap will help your paint last much longer. Don't be afraid of furring strips or the details necessary to install windows in such a wall. You can do it.

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