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Double Stud wall Retrofit

B_G_ | Posted in General Questions on

Looking for some general feedback on my plans, as well as answers to a few specific questions below:

I am in the process of converting half of a large detached garage into a 400 sq ft studio apartment for full-time living. The project lies in Zone 3C, north of San Francisco and the existing structure is a traditional stick build, with T1-11 plywood wrapped in hydrogap and finished with hardie panels.  Most of the foundation is concrete blocks, stacked 3′ high because the structure was partially dug into the hill side.

Because of the concrete blocks, my plan is to build a second wall on the interior side, creating a cavity for plumbing, electrical and insulation. This will eat into the living space but seems like a good idea to create a separation from the cold concrete and make the space more comfortable. For insulation I plan to use Rockwool batts, as they are easier to fit as well as fire/sound resistant.

I do not know what kind of barriers were installed when the slab was poured, but after installing some drainage around the perimeter of the structure, there was no water infiltration this past rainy season (and it was a very wet one).  I plan on using Radonseal to epoxy the cracks and seal the top. Once I am ready to install flooring I plan to use Dricore subfloor panels, with cork flooring on top.

 
Questions:
1. Does my general approach seem valid? Any glaring oversights on my part?
 
2. Is my plan to build a second interior wall overkill? I could also simply furr out the concrete.
 
3. Would installing 1/2″EPS over top of the existing concrete block wall, before building out the second stud wall help limit thermal bridging?
 
4. Because the windows are already installed (outies), would it be bad practice to build window bucks that don’t go under the window flashing? I was going to follow this guide for creating my window wells:
 
5. Is filling both wall cavities with insulation overkill in zone 3C? Temperatures in my area rarely dip below freezing, and summers hover around 80.

Really appreciate all the wisdom shared on this website, thanks in advance for your time.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    B_G_,

    1. Yes, it sounds good.

    2. I would build that second wall- if simply so you don't have to deal with a ledge part way up.

    3. You need a layer of foam board on the inside of the blocks, not to stop thermal bridging, but rather to stop moist interior air from reaching the foundation and condensing. It needs to be treated just like any other basement wall: https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall

    4. "would it be bad practice to build window bucks that don’t go under the window flashing?"
    I'm not sure what you mean here? All the window flashing is on the exterior. The JLC detail looks like it will work fine.

    5. Your choice. That's an awful lot of insulation for where you are, but certainly does no harm.

  2. B_G_ | | #2

    Thanks for the quick answer Malcolm!
    Sounds like I was on the right track - your link for basement insulation is exactly what I was looking for.

    For question 4, maybe I can clarify. The exterior walls are already framed and the windows installed into the rough openings, so there is no way for me to slip the plywood under the windows when framing out the extra deep jambs that connect the double stud walls. Seems I should be able to caulk that joint and get decent sealing, but I was wondering if that created any problems I am not foreseeing.

    1. Expert Member
      MALCOLM TAYLOR | | #3

      B_G_,

      Thanks for the clarification - and yes, I'm sure you can just seal whatever you are using (plywood, or membrane) to the window frame.

  3. B_G_ | | #4

    Thanks again Malcolm,
    While I have you on the subject, do you think that it would be worth building a double stud wall as the partition between the garage and studio? My original plan was to go with a single 2x6 wall instead of a 2x4, so I can shove a bit more insulation between the conditioned and unconditioned spaces.

    1. Expert Member
      MALCOLM TAYLOR | | #5

      B_G_,

      You don't need the double walls anywhere, they are just really a byproduct of having to cover the thicker block wall below. I'd stick with your plan and use a 2"x6" wall between the studio and garage.

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