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Does leaving a service cavity on the inside of a double stud wall make sense?

John Ranson | Posted in Energy Efficiency and Durability on

Would there be anything wrong with the following assembly? From exterior to interior: Siding, furring, rigid insulation, sheathing (air seal and WRB), 2×4 stud wall, gap, netting stapled to inner studs. 2×4 stud wall, and finally drywall. The outer 2×4 stud wall and the gap would be filled with cellulose, but the inner 2×4 stud wall would be empty to allow easy wiring and plumbing.

This design seems to have some clear perks. Removed thermal bridges. You get a service cavity. The inner wall supports the netting, so it won’t need excessive stapling.

It’s clear the service cavity makes the wall 3.5″ thicker than a wall of the same R-value without the cavity. That I’m okay with.

I have two big questions.

1. Is there is any risk of the netting stretching or sagging and allowing the insulation to fall?

2. Other than extra thickness, what are the downside to this assembly?

–John

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    John,
    The main problem with your plan is not the service cavity. The service cavity is fine.

    The problem is your desire to install rigid foam on the exterior side of a double-stud wall. You don't want to do that. This article explains why: Exterior Rigid Foam on Double-Stud Walls Is a No-No.

  2. John Ranson | | #2

    Sorry, I was actually considering mineral wool for the exterior. I forgot to mention it. I want the wall to dry both ways.

  3. John Ranson | | #3

    I'm not married to the idea of having mineral wool on the exterior. It seems like a good idea to keep the sheathing warmer. If you think it would be equally durable with just a rainscreen and siding, that seems more economical.

  4. Expert Member
    Malcolm Taylor | | #4

    John,
    Why not insulate the inner wall? The plumbing and electrical go in before any insulation so they don't hinder the installation.
    The advantage of a discrete service cavity in double walls is usually that it keeps the pipes and wires inside the air barrier, but yours is the exterior sheathing so the whole wall is inside it.
    It's worth noting that your assembly doesn't identify a vapour control layer, be that a vapour barrier or vapour retarder, which some codes require.

  5. John Ranson | | #5

    My reason was ease of future renovation. However, upon reflection, the extra cost of the larger space used probably greatly outweighs the small decrease in future cost of renovation.

    --John

  6. Expert Member
    Malcolm Taylor | | #6

    John,
    If the service space is important to you, keep it. Just fill it with batt insulation. You can then decrease the size of the gap to offset the walls thickness if you'd like.

  7. Robert Hronek | | #7

    I just dont see the point of a service cavity. Aline the plumbing on an inside wall. How many times do you really need to get in to the wall for electrical.

    It seems a big added expense in case you might want to do some wiring. Spend the savings on insulation, exterior insulation.

    I also dont see the dry in both directions argument for most climates. Put enough exterior insulation that you dont have to worry about the condensation point, . Let it dry to the inside. Make the home very tight so exfiltration/infiltration are not an issue.

  8. KEVIN ZORSKI | | #8

    Replace the netting with a good air barrier of some sort, nix the outside foam/ mineral wool. The gap provides the thermal break. Cellulose or mineral wool batts are a lot less expensive than mineral wool "boards". Insulate the inner wall with mineral wool batts - easy to remove, make changes and replace if necessary. I think that double wall with the air barrier in the middle is a very resilient wall, that allows for no foam, thermal break, and condensation control. Best of luck to you with your project.

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