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Adding double stud wall during remodel?

AlexD2022 | Posted in General Questions on

Hi all,
I’m in Salem, Oregon and converting a workshop that is attached to my attached garage (one wall shares 15′ with the garage and ~5′ with the house) to a home office/movie theater and am wondering what the best approach would be to try to get this room to be comfortable year round without putting in a mini split.

The room is about 7’6″x20′ with 2×4 walls and a vaulted ceiling, however I’ll be adding a ceiling at about 7’6″. The walls outside to in are clapboard siding, hopefully some kind of wrap but I haven’t had a chance to try to pry back any siding to check, a type of particle board with some kind of asphalt facing on the interior side, and r-11 paper faced fiberglass. Another detail is the garage wall has ~12″ tall stem wall, the 20′ exterior wall only has a 12″ stell wall for a few feet before transitioning to the mudsill just being on the slab, there’s no exterior doors and two windows.

My plan right now is to put tyvek up on the inside face of all my walls as an air barrier and then build an interior 2×4 stud wall and use r-15 unfaced batts then drywall over that. For the ceiling I’m going to use the new attic space as storage so plan on 2×8 joists filled with fiberglass batts then 1-2″ of EPS with taped seams and then 1/2″ plywood on top. Additionally I plan on putting in new windows and the smallest ERV I can find. This seems like a fairly economical approach to trying to get a room that can be used year round without any kind of heat or cooling but I don’t know if there’s anything else I should be concerned about? Or if there is an alternative approach that might work better for my situation?

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  1. AlexD2022 | | #1

    From doing a bit more reading it seems like the biggest concern is condensation on my exterior facing sheathing. Is covering the interior face of the outermost stud wall with tyvek and detailing it in an airtight manner enough to avoid condensation in the PNW?

  2. mr_reference_Hugh | | #2

    You should consult a designer or the building permit department in your county but here is some information for you to consider and help you formulate some questions for when you talk to these people.

    You will see below the 3 types of vapour retarders that Tyvek is not considered a vapour retarder.

    Class I: Less than or equal to 0.1 perm [e.g., polyethylene];
    Class II: Greater than 0.1 perm but less than or equal to 1.0 perm [e.g., kraft facing];
    Class III: Greater than 1.0 perm but less than or equal to 10 perm [e.g., latex paint].

    Tyvek has a vapour permiability of 54 perms.

    You would may need a vapour "retarder" that is specified in the building code that governs construction in your geographic location. I believe that you are required to install a class 2 with some exceptions. To read the vapor retarder 2022 building code requirements/amendments for your state, go to "R318.1 Vapor retarders" using the link provided below. The code appears to indicate that the type II vapor retarder must go on the "warm-in-winter" side of the walls.


    This GBA article is a bit older but is specifically speaks to climate zone "4c marine", which I believe is the same zone as you live in.

    The article speaks about the problems of adding an interior *vapor* barrier where one is not required.

    Drywall is commonly used as an *air* barrier but you need to know how to properly air seal it. Because you - apparently - need a class II vapour retarder (for you to confirm), you would likely not use Tyvek on the inside. You can use another vapor retarder like Intello X or Intello Plus [Permeability: VAPOR VARIABLE from 0.13 to over 13 perm – best in class (factor 100)] but note that rigid materials are often easier to "seal" thant products that come in a role.

    Rigid materials, as a simple example, allow you to firmly press on tapes you apply; use sprayfoam to seal around pipes or electrical boxes (anything electrical use the right type of sprayfoam).

    For the roof/ceiling, your code appears to require a class II vapor retarded on the "warm-in-winter" side of the ceiling, just like the code requires for your wall.

    So the plan you had does not appear to meet the building code.

    1. AlexD2022 | | #3

      Thanks for referring to the code. Reading into the last code book it looks like you are correct that I would have to go with a class II vapor retarder (I don't mind using intello, was just hoping to save some money). It looks like from what I've seen in the discussion in that article from Martin that vapor diffusion is not nearly as much of a concern as air leaks moving moisture into the wall.

      Regarding building code, it looks like I can mostly follow code by upgrading to a class II retarder (intello), and carry the intello over the bottom of my joists from the middle of my stud bay (taking a generous definition that in between the two stud walls counts as warm in winter).

      However I am more concerned about ensuring that I both build an efficient (well insulated and air sealed) wall that will not cause any moisture problems. In a perfect world I would have the money to tear off all my siding, add an air barrier and exterior insulation, unfortunately that project will have to wait for if I ever get a big windfall. So excluding doing exterior work, is this approach of having an air barrier (and vapor retarder) in the middle of the wall (sandwiched between r-11 and r-15 fiberglassbatts) going to cause any foreseeable problems? I suppose there are at least two follow up questions too that I'm not sure how to answer on my own - is putting the intello right ontop of the fiberglass paper facing OK? Do I need to be concerned in the sections that I have a concrete stemwall the bottom two feet of the wall?

      I understand there can be many different ways to accomplish the same goal, but I sure do wish there were a bunch of prescriptive methods for building walls.

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