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Community and Q&A

Renovating and installing new cement board

Rossi Masonry | Posted in GBA Pro Help on

Installing new cement board. Insulation is 1/2-inch blue XPS.
Barriers go under insulation or over? Hardie / CertainTeed contradict each other.
Also, can I get certain types of material that I should use?
I’m in Foxboro, Mass.
1/2″ XPS is due to 3/4-inch window allowance, with getting into additional trim thickness.
Tape all seams in insulation and in barrier?

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  1. GBA Editor
    Martin Holladay | | #1

    Your questions are cryptic. I assume that "cement board" refers to fiber-cement siding, not cement board underlayment for a tile shower.

    I'm guessing that "barrier" means housewrap or water-resistive barrier (WRB).

    If you are installing rigid foam on the exterior side of an existing wall, your biggest problem is that 1/2-inch foam isn't thick enough. In your climate zone (zone 5), you need at least 1 inch of XPS (R-5) if your studs are 2x4s, or at least 1.5 inch of XPS (R-7.5) is your studs are 2x6s. (Of course, you don't have to use XPS. You can either skip the foam entirely, or you can use a different type of rigid foam.)

    If you use the thin foam that your are planning to install, your wall can have moisture problems. Read all about this issue here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. user-2310254 | | #2


    You can install the required foam and the fiber cement siding while accommodating your windows by adding side-built aluminum trim around the window opening. This is what I did, and it was not difficult. Check out this blog for specific steps:

  3. Expert Member
    Dana Dorsett | | #3

    If the house was built with a poly vapor barrier on the interior side under the gypsum do NOT use foil-faced rigid foam on the exterior, as that will create a moisture trap.

    With an interior vapor barrier XPS would be fine at 1" (about 1.2 perms of drying capacity) under VENTED (rainscreened) siding. Unfaced EPS under vented siding would be good up to about 4" (but I'm sure you're not going there.)

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