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Insulation assembly

pmartens | Posted in General Questions on

I’m working on an apartment that was broken out of our house 30 years ago, it has a vented truss space above ~11” of fiberglass. The fiberglass is a layer of foil faced in the bottom cord of the truss with an 8” layer of unfaced above that. 

we will be spray foaming the walls because they are only 2×4 and I want more than an R13 wall assembly. 

My question is, can we put a 1” layer of spray foam on the inside surface of the fiberglass?  We would lose the radiant benefit of the foil facing but would gain about R7 and would air seal the area inside.  To accommodate this I would add 2x4s running perpendicular to the existing bottom cord of the truss for the new drywall to be attached to. This would be a sort of reverse flash and batt trying to use most of what is already here.

I know that there may be an issue with point of condensation which is why I’m asking.

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  1. Patrick_OSullivan | | #1

    > we will be spray foaming the walls because they are only 2×4 and I want more than an R13 wall assembly.

    Open cell foam is going to get you right to that R-13 number as well. If you're thinking of installing closed cell instead, don't. It's not worth the cost increase in that assembly.

    It's all documented right here:

  2. pmartens | | #2

    We do have 1” of blue board outside of the original siding beneath vinyl. That has been there for some time and but the vinyl has a lot of life left.

    The studs will produce the same thermal bridging regardless of insulation material, I can see that open cell would give another 0.5” of material but at half the value per inch of the closed cell.

    I will reread the article tonight to see if I can gain better understanding of the issue.

    Any insight on the original question?

    1. Patrick_OSullivan | | #4

      So you're pulling the ceiling down, putting up the strapping, and then spraying between the strapping?

      Trying to picture this...

      1. pmartens | | #5


        Ceiling had to come down anyway due to old water damage and several lousy repair attempts

  3. andy_ | | #3

    If you're looking to add R value to a 2x4 wall, you might want to look into the Bon-fig, aka Bonfig, aka Bonfiglioli wall technique. It's basically strips of one inch higher density EPS and 1x3 (or 3/4" ply strips) nailed to each stud. It turns your cavities into the equivalent of 2x6 but with a thermal break on each stud. has an article on it

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