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thermal bridging in spray foam wall

SMTDIY | Posted in General Questions on

Five years ago I replaced a full exterior wall of an old house (year of building unknown, guessing at 70 years, likely someone’s old 3-season chalet converted to 4). We built the new wall using 2×4 and used full cavity spray foam (I’m not sure if it was open or closed cell- it is very rigid). There is Tyvek, then Maibec wooden siding. On the interior there is drywall and paint.
There were also ceiling issues caused by inadequate insulation and a low shed roof over this area of the house (was possibly a porch that had been turned into a bathroom), so after re-building the wall, there was still moisture at the top of the wall, on the ceiling, and obvious roof melt causing ice dams. In late 2019, fixed this issue by adding an “insulation heel” using custom built trusses and basically building it over the existing roof and filling it with more spray foam insulation. No more ice dams! However, the top plate of the wall, where it meets the new “heel”, is still showing signs of interior moisture at several points along the wall. I can feel cold on the walls in those spots on cold days. It is possible that the spray foam guys missed some small areas and there are pockets of empty roof cavity there. At any rate, there is thermal bridging happening and I would like to address it. My concern is that there is moisture on the inside of the drywall caused by either a gap in sprayed insulation or an inadequate “fit” between the old roof assembly and the new. 
Here comes the question…can I simply spray foam the exterior of that zone where old and new meet (currently remains un-sided)? Or is there something to do on the inside? 
I am located in southern Québec, zone 4 (for plant hardiness, sorry, can’t find if that’s the same for insulation or not!). 
Thanks for any advice you can offer!

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  1. user-2310254 | | #1

    Before doing anything, consider hiring a RESNET rater or similar professional to assess the situation. This is probably a good situation for using infrared imagery to get a better understanding of how the structure is performing. (See here for more info:

    If you are in southern Quebec, you are probably in climate zone 6. (See here for more info:

  2. Expert Member
    BILL WICHERS | | #2

    You shouldn't be having condensation issues between the drywall and the inside face of the spray foam. If you are seeing condensation here, it's probably due to an air leak somewhere or possibly a too-thin layer of spray foam. As Steve mentioned, an IR camera would be a big help to see what's going on without having to open up that wall.

    It is possible the spray foam may have shrunk and seperated from the framing in one or more spots. This is unusual, but it can happen. You can just fill any gaps with canned foam if you find any. An IR camera will find these spots too -- they'll look like cold bulges on the sides of studs in the camera's view.

    I'm not clear on where the areas meet. My guess is you can PROBABLY just add spray foam in there, but it would be helpful to see some pics or a drawing to be sure. What you want to be sure to avoid is blocking any vent channels for your attic that might be in that area -- you don't want to seal off a vented attic by mistake since that can cause mold problems.


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