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Community and Q&A

Service cavity in retrofit without losing floor space

mrkawfey | Posted in Energy Efficiency and Durability on

I am working on an interior renovation and the details of our walls can be found in my other post
Controlling Vapor Drive Inside Soon-to-be-Renovated Walls – GreenBuildingAdvisor
I am seriously considering adding a service cavity which would significantly reduce the risk of air infiltration. It would also allow us to move plumbing and ductwork inboard which has some huge advantages as well. 

However, this question is about a specific detail that I haven’t seed discussed. In a couple of locations we just can’t afford to lose the floor space so I wanted to see if anyone had feedback on the idea of converting a 2×6 wall to a pair of 2×4 walls with the vapor barrier/retarder in between? 

Essentially this process:
Remove the 2×6
Install a 2×4
Insulate the bay
Install a vapor control membrane
Install a 2x true-2″ -or-
Install a 2×2 bottom and top plate and a second 2×4
Run services in interior cavity
Insulate this cavity
Install wall board

As long as the inside cavity is never in condensation in the winter and the outside cavity is never in condensation in the summer would this work?


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  1. Expert Member
    BILL WICHERS | | #1

    In post #8 here:
    I describe a simple way to build a "mini" service cavity if you only need it in a few spots.

    If you're planning a service cavity along the entire wall, I don't really think that's worth the effort. It's not that difficult to air seal around services in a wall, and you really don't want plumbing in an exterior wall regardless of how you build it. The same goes for ductwork. Keep plumbing and ductwork in INTERIOR walls as much as possible.

    Is there any specific reason you're stuck running a lot of services in an exterior wall?


    1. mrkawfey | | #3

      That's perfect! Exactly what I was looking for, thank you. Problem solved.

  2. Expert Member


    I'm not at all sure it's worth it, but I'm also not sure it's feasible. The existing studs are fastened to the sheathing. You could carefully cut them free with a reciprocating saw, but how would you make the necessary connections to the sheathing for the new 2"x4"s?

    1. mrkawfey | | #4

      Yeah, you saw the problem before I did, but that's exactly why I asked. Thanks for the reply.

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