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Community and Q&A

Furring strips or another layer of sheathing over foam?

brian_mcdonald | Posted in GBA Pro Help on

I am in the process of framing a house 2×6, 24”oc. The house will be sheathed and taped with the Zip system and 2-1” layers of rigid foam (staggered seams and taped). Over top of the foam, I planned on using 1×4 or ripping down ¾” ply for furring strips to both hold the foam and attach the vinyl siding too.
My framer suggested using another layer of ½” sheathing instead of the furring strips. The sheathing would be screwed (1 1/2”) into the studs the same as the furring strips. His thought is that it will be cheaper to use the sheathing then furring strips and returns the siding process “to normal” after it is attached.–I have large amounts of trim and areas under the porch/etc. that will get some type of stone.
My questions are:
1. Assuming I need a water barrier to protect the outer sheathing–Will I run into any problems installing a water barrier on the inside and the outside of the foam? (studs, zip sheathing, foam, exterior ply, house wrap) Do I still need to tape both layers of foam?
2. Will ½” sheathing with ring shank nails will be sufficient enough hold to keep the siding from flying off. Does the siding need to be attached to the studs or inner layer of sheathing in this application?
3. Are there any drawbacks to using sheathing instead of furring strips?


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  1. GBA Editor
    Martin Holladay | | #1

    I call your suggested method "site-built SIPs." You can do it if you want. Other builders have done it.

    It's also possible to simply install nailbase (rigid foam with OSB adhered to one side).

    You can install two layers of housewrap if you want -- you won't have any problems if you do -- but only one layer is necessary. Choose where you want your WRB to be, and install it there.

    You don't have to tape both layers of foam, but taping both layers won't hurt. I would tape at least one of the layers.

    Most vinyl siding manufacturers allow attachment to 1/2 inch sheathing, as far as I know. But if you have any doubts, contact the vinyl siding manufacturer.

    The main drawback to using solid sheathing instead of furring strips is that you lose the air gap between the siding and the sheathing (or WRB). However, if you are installing vinyl siding -- a type of siding that is well drained and well ventilated -- that concern doesn't really apply.

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