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Community and Q&A

Cut-and-Cobble Rigid Foam Board Between Interior Beams

OneSmooveAlpha | Posted in GBA Pro Help on

Can I use these rigid foam insulation boards in between the beams on an interior wall?

I’m a newb and haven’t done my own insulation before. I had to take down a wall because of a roof leak that generated mold on the wallboard. I am also removing the insulation that was underneath and I’m wondering if replacing it with the rigid foam boards in between the beams is ok.

Also, how do I appropriately affix the foam board within the beams?

Thanks.

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Current thinking is to avoid the "cut and cobble" method, which I think is what you considering. It would be helpful to know your climate zone and the construction of your wall (all materials outside to inside).

  2. GBA Editor
    Kiley Jacques | | #2

    The “cut-and-cobble” method—said to be coined by GBA Expert Member Dana Dorsett— does indeed have its detractors (though I have spoken with many builders who do it regularly). You might be interested to read Martin Holladay’s take on it: Cut-and-Cobble Insulation.

  3. Expert Member
    Zephyr7 | | #3

    Rigid foam installed by cut and cobble a problem in UNvented roof assemblies since it can't be guaranteed to stay air tight over the long term. In VENTED roof assemblies it's not a problem, although it is pretty labor intensive.

    Cut and cobble in walls isn't usually a problem either if you have a good air barrier, but it's very labor intensive. Usually people doing cut and cobble insulation installations have saved a lot of money on the rigid foam so they have a financial incentive to use it. If you're doing a new installation, I would personally consider mineral wool here and use the rigid foam on the exterior of the wall where it will be of most benefit.

    If you are set on doing a cut and cobble installation between the studs of a wall, the traditional way to seal the cut rigid foam pieces in place is to use canned foam around the edges. What you do is cut the rigid foam so that there is a 1/4 to 1/2" or so gap all the way around (enough to fit in the canned foam straw or the tip of a foam gun), then you fill that gap with canned foam. You'll be using a LOT of canned foam for this, so using a gun makes it a lot easier. Note that it's better to have the gap a little too big than a little too small when using this method since it's easier to fill an overly large gap with canned foam than a very small gap.

    Bill

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