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Community and Q&A

Insulating a stud cavity with no sheathing

rspatnky | Posted in General Questions on

Hello all.  Thanks in advance for taking the time to entertain my question.  
I have a 1900 home that I gutted down to the studs.  I’d like to insulate the stud cavities, but there is no sheathing, just pine lap siding nailed directly to the studs.  I’m restoring the original siding without removing it first.  So, I have a stud cavity with the back of the siding exposed.
I’d like to leave a small air gap between the back of the siding and insulation, so the siding can dry to both the exterior and interior if needed.  
Question is, how do I, and with what do I, line the stud cavity.  My original thought is to line the stud cavities with Tyvek drain warp, and then do blown damp cellulose to fill the cavity. I figured if sealed correctly, the Tyvek will create a air and vapor barrier, and the small spaces between the back of the siding and the Tyvek will be sufficient for siding to dry or drain if moisture collects.  
Is this a silly though?  Any other options that might be better?  Thanks again for your time.  

RS

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Tyvek is not a vapor barrier, nor do you necessarily want a vapor barrier on the exterior side of the assembly. It can be detailed as an air barrier, but you need something stiffer to hold the insulation back. Tyvek Drain Wrap isn't rigid enough to maintain the air gap to the siding with the pressure of blown insulation or compressed batts. The air gap is an essential capillary break to keep wind driven moisture from soaking directly into the fiber insulation.

    Cutting strips of half-inch rigid foam board as spacers and using rigid foam board (at least an inch thick for batts, thicker if dense packing cellulose) as the exterior air barrier works. Using foil faced polyiso makes for easy and reliable air sealing of the seams. Cut the foam a half inch narrower than the stud bay space and use can foam or polyurethane caulk to seal the edges.

    How deep are the studs, and where are you located?

  2. rspatnky | | #2

    Thanks Dana. I'm in northern KY. The stud's are rough cut true 2x4. I had originally planned exactly with you said with the foam boards, but with 1/2in foam boards and using 1/2 inc spacers to prevent the boards form collapsing. That would leave me with only 3in left in the cavity, which with the boards themselves being R3, I should be able to get to R13 with blown damp cellulose. What are your thoughts? Thanks.

    RS

    1. Expert Member
      Dana Dorsett | | #4

      With 4" stud depth minus 1/2" for the air gap and 1" for the polyiso you'd have 2.5" for the cellulose, not 3". Half-inch polyiso for the air barrier would still be too flexible IMHO, and it has a lot less dew point margin at the foam/fiber boundary.

      With a nominal 2.5" of damp sprayed cellulose at R3.7/inch you'd have R9+ for the fiber layer, and at 1" you'd have R6 for the polyiso, R7 if you count the thermal performance of the foil facer adjacent to the half-inch air gap. That adds up to R15+, R16+ counting the foil facer & trapped air films.

      If you go ahead with half-inch polyiso (R3) +3" cellulose (R11) you'd be at R14, R15 counting the foil facer next to the air gap.

      1. rspatnky | | #6

        Thanks Dana. I really appreciate the advice.

  3. GBA Editor
    Martin Holladay | | #3

    RS,
    First of all, can you tell us your name? (I'm Martin.)

    Here is a link to an article that explains what you need to do: "Insulating Walls in an Old House With No Sheathing."

  4. rspatnky | | #5

    Hi Martin,
    My name is Ron. Sorry about that. I appreciate you linking the article. I'll have to sign up to gain complete access to it. Off the bat I can see that they beefed up the depth of the stud cavity. I'm trying to avoid this, as the home is very narrow to begin with (old shotgun style) and I'm trying to avoid narrowing it further. Maybe I'm hedging between a total of 4 inches, but I feel like every inch counts in a small space. Thanks.

    Ron

  5. Peter Yost | | #7

    Hi Ron -

    Check out this GBA resource as well: https://www.greenbuildingadvisor.com/green-homes/the-colorado-chautauqua-cottages.

    Peter

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