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Community and Q&A

How do I build a wall within an exterior wall to add insulation value?

XJhHRrLYHF | Posted in Energy Efficiency and Durability on

I want to at least double the insulation value of my exterior walls(existing). Do I have to rip out the drywall completly or just bust through it in a lot of places. The existing wall has no vapor barrier on the inside or tyvec on the outside. Its just a 2×4 wall with 3 1/2 ” bats and 3/4″ dow board insulation on the exterior and vinyl siding. I plan to put a vaper barrier in when I build the wall. Was thinking of another 2×4 wall off set from the existing studs giving me an 8″ plus wall, depending on the gap between. Any thoughts? Thanks!

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  1. Brett Moyer | | #1

    If it were my home, I would remove the drywall to expose the existing walls. You will want to check off the following:
    - No moisture damage from exterior/plumbing leaks,
    - The condition of the existing insulation (you may want to consider a full cavity blown-in cellulose)
    - Proper sealing of exterior penetrations (windows, electrical/gas lines, etc)
    I would include at least a 2 inch gap between the 2 walls.
    And as far as your plan to include a vapor barrier... don't do it, unless you are in an extremely cold climate (Fairbanks, AK)
    What you want to focus on is an AIR barrier. This can be accomplished through an Airtight Drywall Approach (ADA)

  2. Riversong | | #2

    Ditto on avoiding the vapor barrier. At most, a vapor retarder (1 perm) is all that's required. And its location depends on your climate.

    With only "filterglass" in the wall cavities, no interior air barrier and a vapor resistive exterior layer, you may have mold and other water damage on the sheathing or framing. The fiberglass may be full of mouse droppings and/or dead mice and should probably be replaced. Cellulose is the best option for such a renovation.

    You need to tell us where you live - what climate zone - if you want appropriate advice.

  3. GBA Editor
    Martin Holladay | | #3

    Many people in your situation find it less disruptive to add insulation on the exterior of your home. That way you won't lose floor space — and it's easier to live in the house while the work is underway.

    You might want to remove your exterior siding and add 4 or 6 inches of exterior foam. The exterior foam will do a better job of handling thermal bridges, especially at the rim joist areas, than your plan.

    It's fairly easy to remove and reinstall vinyl siding without damaging the siding.

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