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Community and Q&A

Vapor Barrier – Why not a wall service Cavity?

kiwiscott | Posted in General Questions on

I’ve read a lot of discussions on this forum about the ideal wall layout but most discussion don’t include a service cavity in the wall. I know there’s a small cost involved and you lose ~4inches of floor space but surely a service cavity means that the vapor barrier will be better protected over the 50+ lifespan that people are trying to build to. 

Some of the comments on the articles talking about service cavities talk about how the ‘other’ trades love to work with them and they must offer benefits longer term — say adding new cabling throughout a house lighting etc. 

I’m about to tear down several room to their studs, to remove breaking plaster and add insulation, and I’m wondering if there’s a downside? 


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  1. Expert Member


    Just to clarify: vapour-barriers don't need to be continuous, so if you need one (or your code requires one) in your climate zone, you don't need to worry about penetrations for services. That's different for air-barriers. They need to be continuous and sealed., so yes it is a good idea to either bury them in the wall, or use the exterior sheathing. If you are renovating, and don't have access to the sheathing, then they can be vulnerable, and benefit form being protected by a service cavity.

    The question then becomes one of proportion. Most walls or ceilings end up having one or two electrical boxes (which are pretty easy to seal) with a couple of wire runs, and exterior walls rarely have plumbing in them. Is it worth building a whole new wall to house what could end up in many places being nothing at all?

    That said, there is no downside. If you do build a service cavity, it's a good opportunity to increase the insulation in the walls. Make sure any vapour-retarder doesn't end up being too far towards the outside. As a very rough rule of thumb, , if there is more than 1/3 of the insulation inside the vapour-retarder, that can cause it to accumulate condensation.

    1. kiwiscott | | #2

      Thanks - that’s pretty much aligned with what you said elsewhere :).

      Not having to be continuous is the key thing I don’t really understand. Does that mean vapor will try to pass through the material it’s closest to … and in theory because of the air barrier (external to the vapor barrier and 2/3 of the interior insulation) there’s little air movement to worry about to cause a vacuum to the outside?

      (Sorry badly worded but hopefully you get it).

      1. Expert Member
        MALCOLM TAYLOR | | #3


        Yeah, I guess my posts are fairly consistently against the idea when it comes up :)

        Air mainly passes through holes due to pressure diffeences, so much like the way a bucket full of water with a leak would soon empty itself, an air-barrier which is not continuous will let an awful lot of air through. Vapour diffuses through the entire surface of any material, so a vapour-barrier that covers 99% of a wall will stop 99% of the vapour.

        In your situation I'd imagine you will likely be using a combined air and vapour-barrier (whether poly or a variable perm membrane) so it will need to be continuous in any case.

        One option you might consider, which is sort of half way between none and a full blown second wall, is strapping the inside of the studs with 2"x2"s. That provides a service cavity, protection for the air/vapour barrier, and a thermal break, without taking up much space. Depending on how far you wanted to go you could also fill that cavity with foam or mineral wool boards.

        1. kiwiscott | | #4

          Quote: “Yeah, I guess my posts are fairly consistently against the idea when it comes up :)”

          It’s good to know this is opinion based though and not science based! Theres nothing inherently wrong with an additional service cavity or a 2x2 strapped wall.

          Quote: “In your situation I'd imagine you will likely be using a combined air and vapour-barrier”

          Plan is to retrofit the outside by removing the existing siding then adding an air barrier to the existing sheathing, something like Majvest 500 SA, then some exterior insulation (zone 5a so 2inches of something), strapping then siding.

          Internally - use rock wool or perhaps a cellulose batt to fill 2x4 walls and then add intello or similar vapor barrier.

          I’m doing best efforts here to be comfortable for the long term. Sadly the costs are so high this will probably take a little while.

          I could use my service cavity space for insulation if I wanted but then I’d really just be expanding the interior walls depth and not protecting the vapor barrier.


          1. Expert Member
            MALCOLM TAYLOR | | #5


            "It’s good to know this is opinion based though and not science based!"

            Definitely. As I said in my first post, I see no downside. However once you add an effective exterior air-barrier, and additional insulation, the justification for building a service cavity to protect what is only an interior vapour-retarder largely disappears.

  2. Expert Member
    BILL WICHERS | | #6

    There isn't really much of a downside to a service cavity aside from the extra work to build it and the space it eats up. There isn't much bang for the buck here though, because chances are you'll never, or almost never, actually use that service cavity for anything. If you're expecting to do wiring changes in the future, running conduit is actually a better option, and it won't eat up extra space. If you want to maximize flexibility for the future, put in a vertical chase somewhere to allow you to run cabling between floors -- but be mindful of fire spread concerns. I wouldn't personally put in a service cavity everywhere, I'd only put access in a few key places, and I'd allow for basement/attic as a "service cavity" for the upper and ground levels.


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