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Service cavity insulation

raul4817 | Posted in General Questions on

I will be framing a service cavity soon to run emt conduit. My construction is currently 2×6 with rockwool batts. I was planning on using foam board or intello plus as my primary air barrier. I will frame a 2×3 service wall in front of that to house electrical and hang drywall. My question is is there a concern with placing a 2.5″ batt of mineral wool in the cavity. I will stagger my studs so I can reduce the thermal bridge. So essentially the air barrier will be sandwich between two batts of wool.  Is there an advantage to using foam board taped as my air barrier vs the intello plus as my air barrier? I am in climate zone 5.

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  1. Expert Member
    Akos | | #1

    Intello has no insulation value. For the same price/sqft you can get 3/4" foam which would bump up your wall a bit (from R28 to R33). If you get foil faced product, it can also serve as your vapour barrier.

    There is nothing wrong with placing batts in the service cavity, my home is built similar way but with dense glass as the air barrier.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Raul.

    You could use either rigid foam insulation or the Intello membrane as your air barrier and as Akos pointed out, either could provide the vapor control you need as well (with foam, you'll have to choose specifically for the permeability you are looking for.

    You may also consider installation as a deciding factor. It is important that your air barrier is continuous and installed judiciously. When you consider the installation details of rigid foam vs. a membrane, what do you, or your builder feel more comfortable working with.

  3. raul4817 | | #3

    Akos and Brian,

    Thank u both for the insight. I will be using the foam board. Price is fair and install should be easier for me since I don't have a second pair of hands, the extra bump in Rvalue is a bonus as well. I will not be using anything faced since I've learned enough via GBA that I dont want or need a vapor barrier in my zone. 3/4" Eps or 1/2" film faced xps are the options both readily available and mostly vapor permeable. I do understand the negative environmental implications our U.S. born xps can cause, unfortunate since it is a more workable and superior material imo and seems to take a taping a little better. My biggest concern was the sandwiched air barrier. I was going to frame the service cavity regardless, I figured I might as well stuff some extra fluffy in there. My primary reason for the cavity was to keep the integrity of the air barrier isolated far enough away from the drywall that will undoubtedly get punctured with photos, shelves, etc.
    Thanks again

    1. Expert Member
      Akos | | #4

      There is no issue with the air barrier in the middle of the wall. The challenge with that layout is ensuring air barrier continuity from the foundation all the way up to the ceiling.

      As long as those details are good, the layer of foam should work.

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