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Wall service cavity insulation

entropic | Posted in General Questions on

Question is asked with climate zone 6 in mind.

Most wall service cavity examples seem to be made with 2×4’s “on the flat” perpendicular to studs, making 1.5” deep horizontal service cavities between the vertical stud bays and drywall. Is there widely available insulation that could be added to these horizontal service cavities? I’m having a hard time finding 1.5” batts. Would anyone recommend turning the strapping on edge to make the service cavity 3.5” and therefore more accommodating of conventional batts? Would that be problematic for mounting drywall?

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  1. Expert Member
    BILL WICHERS | | #1

    You usually don't put insulation in service cavities -- the insulation would tend to defeat the purpose, making it much more difficult to pull in wires and the like in the future. Normally "service cavities" like you describe are built over other insulated walls, such as over rigid foam insulating a basement foundation wall. In these cases, where the service cavity is over something that's already insulated, there is no need to put any insulation into the service cavity itself.

    If you have a different kind of assembly that really does need insulaton, I would use 1.5" EPS here, or 1.5" polyiso. EPS would give you about R6 or so, polyiso about R9. EPS has the advantage of being cheaper.


  2. Expert Member
    Akos | | #2

    An in-between would be to use 2x3 on edge. You can use cheap R8 rolls to insulate. 2x3 also have enough depth that most device boxes don't protrude into the wall cavity behind.

    To make drywall simpler, I would make sure the 2x3 at 4' height is placed properly to support the drywall edge, you don't want the drywaller to have to cut 1/2 off a full sheet on the long edge. You can also go with standup sheets (common in commercial build) but your taper might complain.

  3. walta100 | | #3

    The point of building the service cavity is to have a space to run your utilities that does not interfere with your air barrier and insulation.

    What is the R value of the wall behind the service cavity?

    Seems to me the small service cavity given all its obstacle is going change the wall by maybe 10%.

    Say from R19 to 22 If the goal is a R 24 wall build 2x8 wall.


  4. entropic | | #4

    Thanks for the rapid replies. The rest of the wall assembly is still TBD, but chief contenders are 1) 2x6 24” O.C. with mineral wool batt plus 3” exterior mineral wool board or 2) double stud (two 2x4 16” O.C.) with 3.5” gap using mineral wool batt in all three layers (no reliable dense pack subs in my area). Both assemblies would have an smart vapor retarder and a rain screen.

    I think my reasons for wanting the service cavity is to improve chances for a grade 1 installation of batt insulation in stud bays and to protect the interior air barrier (smart vapor retarder) from accidental damage with hanging pictures. I suppose I wasn’t really thinking about ease of future wire pulling as a reason for the service cavity.

    I realize 1.5” (or 2.5” with AKOS TOTH’s suggestion) may not hold a lot of R-value, but with service cavity strapping being perpendicular to studs, it seems like another chance to further reduce thermal bridging. I figured if a widely available and relatively cheap batt/roll insulation exists that would fit, it would be low effort and cost to add. In the case of 2.5” R8 batts, it could even reduce the thickness of exterior mineral wool board in option 1, potentially reducing the cost of exterior insulation and fasteners (shorter length).

    Maybe I’m off base or just sweating the small things that aren’t that impactful. I’m happy to get any experienced advice you have to offer. In planning stages for next home, which will hopefully be a PGH. Thanks.

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