GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Double-Stud Walls with Recycled Fiberglass Batts

jack_of_some | Posted in Green Building Techniques on

Hi All-  I am a happy homeowner handyman  electrician type- I am tackling a garage conversion.  I plan to insulate the slab with 2” Polyiso sheets.  2×4 sleepers tapconned into the slab and advantech subfloor.  6 mil vapor barrier under everything

The existing walls are 2×4 with fiberglass batts.  I plan on adding another 2×4 wall to the interior ( 1/2” gap between the two walls) and insulating the cavities with fiberglass batts.  Upstate NY Zone 6

Being conservative- I’d like to use the existing batts- they are well installed and in fine shape.

Am I asking for trouble if both layers of the double 2×4 wall have a facing/vapor barrier?

Existing exterior wall is 1/2 OSB sheathing, Tyvex and vinyl siding

Thank you for your considerations.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    BILL WICHERS | | #1

    The normal kraft facer is a vapor RETARDER, not a vapor BARRIER, and shouldn't be a problem. If you have those "batts in bags" where the fiberglass is in a sort of plastic tube, that could be an issue. I would use the faced batts on the interior side of the wall, with the facing on the warm in winter (indoor) side, then unfaced batts in the exterior side of the wall assembly.

    I see you are intending to insulate the slab with polyiso. I'm assuming you mean the polyiso will go ON TOP of an existing slab? That's doable, but a bit risky. Polyiso will absorb water, so you don't want to use it in an application like this if there is any chance of water getting in at any time. EPS or XPS would be a better choice here, with higher density EPS preferred since it's a greener material.

    If the insulation is going UNDER the slab, then polyiso isn't an option at all, and you have to use one of the other types, with XPS being the best choice in this particular application.


    1. jack_of_some | | #3

      Thank you for your insight Bill. I’ve been reading some 5-10 year old articles here on GBA warning of creating a “cold sheathing” ,that can become a rot spot , by filling a double wall assembly with insulation and not putting foam on the exterior of the OSB.

      But there seems to be some other opinions in some other articles I Have read here as well. Any thoughts on the chances that a retrofit double 2x4 batt or cellulose wall is risky. Retrofit meaning I am not demo’ing the exterior wall


      1. Expert Member
        BILL WICHERS | | #5

        You'll probably be OK since you have drying to the exterior too (OSB/Tyvek/vinyl siding is vapor open, since the siding is essentially vented).

        The time you're most likely to run into issues changing insulation in an exterior wall is when you have a masonry exterior, or if you have something used as exterior cladding that is a vapor barrier.


  2. Deleted | | #2


  3. PAUL KUENN | | #4

    Any chance you are willing to use 1/2" foam board between your walls. That could be taped before you put up the second wall as an air barrier (caulk at the bottom). With vinyl siding, you don't have to worry about rotting exterior sheathig as it will always dry to the exterior as long as it stays dry from the outside.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |