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Community and Q&A

Insulation Madness and Confusion Questions

Geoffrey Cook | Posted in Energy Efficiency and Durability on

Ok..passed our inspections and ready to insulate. Climate zone 6, two story, balloon frame, vented attic, 6″ ceiling joists.

I have 3″ rigid foam on wall interior between the wall studs. (I bought them cheap and they were cut to 16″ already) Exterior is 1″ Douglas Fir with Typar then Cedar Shingles:

Questions on wall insulation:
1. Can I just shove fiberlass between foam and edges to “air seal”?
2. If not is there a caulk that will work or is spray can foam my only choice?
3. I was planning on blowing cellulose but the budget isn’t going to allow it, I never really factored in the huge cost of insulweb and the budget is pretty tight right now so I’ll go ahead and install fiberglass to fill in the remaining 3-5″ of wall space as we furred out the walls. The code is calling for a vapor/poly barrier. With the (outside to in Foam/fiberglass/gypsum plaster) wall structure is a poly going to cause problems? The concern by the building inspector is moisture migrating to the fiberglass. Thanks I hope to get going on this today or tomorrow.

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  1. Geoffrey Cook | | #1

    To Clarify I am in Zone 6B. Also If I am forced to poly, which thickness would be the most beneficial for my scenario and is there a link on how to properly install the poly as I will be putting it on my ceiling which will be fiberglass R30 with a generous layer of cellulose blown in on top after the rock is up.
    Thanks again!!

  2. GBA Editor
    Martin Holladay | | #2

    It sounds like you have chosen the cut-and-cobble approach. With this method of insulating, it's essential that you seal the perimeter of each rectangle of rigid foam against air leakage. Fiberglass is not an air barrier, so fiberglass won't work.

    Most cut-and-cobble practitioners use canned spray foam for air sealing, but it's possible to use caulk if your gaps are small. To learn more, see Cut-and-Cobble Insulation.

  3. Geoffrey Cook | | #3

    Great Article...Thanks Martin,

    Do you have any thoughts on the Poly Barrier on the walls after the Air Sealed Rigid and Fiberglass are installed?


  4. GBA Editor
    Martin Holladay | | #4

    You don't want to install any interior polyethylene. To learn more about this issue, see:

    Vapor Retarders and Vapor Barriers

    Do I Need a Vapor Retarder?

  5. Geoffrey Cook | | #5

    Getting close to a solution... What are your thoughts on vapor barrier paint over plaster...then insulweb, cellulose, foam? Cut and cobble is fun!! I have to have some sort if vapor barrier per code...the hi tech plastic looks great but will add many dollars to the project.

  6. GBA Editor
    Martin Holladay | | #6

    As long as you are willing to accept all the disadvantages of cut-and-cobble, your system should work.

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