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Community and Q&A

Insulation behind stucco

jdclassen | Posted in Energy Efficiency and Durability on

I have a customer who has requested 3″ of closed cell foam in the walls of his 1920’s house. He has removed all lath and plaster and exposed everything. My issue is that the Siding is Stucco (in good shape) I have no idea what the building paper under the stucco is like. It makes nervous to spray the closed cell , or do any other insulation for fear that water intrusion will absorb into the wood and not have the opportunity to dry out. I was thinking about a dimple-mat installed first, and then Mineral Wool insulation.
I have installed plenty of insulation in stucco homes in years past, but now, seeing the potential issues, I am not sure this is a good idea.
what would be the best assembly for efficiency and durability?
Current assembly is Stucco, Building Paper, 1x sheathing, 2×4 walls.

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  1. Expert Member
    Dana Dorsett | | #1

    Start by seeing if you can't convince him as to just what a waste 3" of closed cell foam is when thermally bridged by R1/inch framing:

    A full 3.5" cavity fill of R4/inch 0.7 lb-1.0 lb open cell foam would have comparable thermal performance to 3" of R7/inch 2lb foam on a whole-wall basis. If it's an full dimension rough sawn 2x4 the 4" of 0.7lb open cell foam wall with R16 at center-cavity would outperform the 3" closed cell/R21 wall due to the thermal bridging.

    At 4" most 0.7lb foam is still ~10 perms- 2-3x as vapor permeable as standard interior latex paint on wallboard. If that's too vapor open for the climate zone & stackup a layer of 2-mil nylon (eg Certainteed Membrain ) between the wallboard & foam would be less than 1 perm when dry, but more than 5 perms at humidity levels that matter.

  2. GBA Editor
    Martin Holladay | | #2

    If there is some doubt about the WRB and window flashing details, you might consider dealing with the stud bays in the same way that builders deal with walls that lack wall sheathing. For more information, see this article: "Insulating Walls in an Old House With No Sheathing."

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