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Insulating a Basement with Plank Sheathing

Beach1212 | Posted in General Questions on


I’m in zone 5A, and am trying to decide how to proceed with updating the insulation of my 100 year old house.

I think the house has loose insulation in the studbays and I’m not planning on changing that right now. The basement is unfinished however, so I’d like to put spray foam in those walls to seal up all the gaps, to stop air, bugs, etc, and for insulation.

The wall is currently vinyl siding, potentially some house wrap under that (but I’m not sure how well it’s done), then wood siding, and then shiplap wood plank sheathing.

My questions are:
1. Is this a good idea?
2. If I spray foam the inside of the plank sheathing, will I cause moisture issues with the sheathing being able to dry properly?
3. If in the future I replace all the siding (vinyl and wood) and add taped house wrap, or liquid-applied water barrier, or similar, will that cause moisture issues given the spray foam?
4. Should I go with closed cell so it can be my vapor barrier? Or should I go with open cell to act as an air barrier but still allow vapor to pass? Or should I do a thin layer of open cell and then fill the rest of the cavity with fiberglass batts?
5. Am I missing any other considerations? Any other advice is greatly appreciated!


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  1. Beach1212 | | #1

    Reading further on this, I'm wondering if spray foam is the right choice here. The possible issues are:
    * It will only be used in some walls, since other parts of the house will never be opened up to apply foam.
    * It may cause issues with vapor permanence, particularly later when I redo the siding and add exterior insulation (creating a foam sandwich in some places)?
    * It will only allow the sheathing to dry to the exterior. Up till now, it has been able to dry to both sides.

    I've also discovered that the rest of the house probably doesn't have any insulation at all.

    Maybe for now I should just put mineral wool batts (without kraft paper, to avoid absorbing moisture from the interior due to lack of a great air barrier) in all the opened walls?

    Then later on, when I'm residing the whole house, I can do the following:
    1. Remove the vinyl and wood siding, down to the bare sheathing
    2. Leave the wood plank sheathing as-is? Or remove it and spray foam the wall cavities from the outside? Or replace the sheathing with taped Zip-R?
    3. On top of the sheathing, add:
    ....a. drainable housewrap (creating an air barrier, by either taping the housewrap or by adding a fluid-applied sealant)
    ....b. 1"+ of external XPS foam, to keep the sheathing warm
    ....c. rainscreen furring strips or mesh or drainage mat
    ....d. new siding (clapboard or vinyl)

    Does anyone have any advice or thoughts on this?


    1. GBA Editor
      Kiley Jacques | | #2

      I’m giving your question a bump. If I understand correctly, you want to use spray foam to air-seal your basement, not as the insulation proper. In an existing house, adding continuous rigid foam board to the interior of the basement walls is a solid strategy outlined here (along with things to be aware of and address before adding insulation): Three Ways to Insulate a Basement Wall.

  2. Beach1212 | | #3

    Thanks Kiley.

    I'm trying to figure out how to insulate the whole house consistently. The house (zone 5A) currently has no wall insulation. I currently have access to the basement wall cavity from the inside, but I won't ever have access to the 1st/2nd floor wall cavities from the inside.

    In the future, I will be replacing all the siding, and that may give me an opportunity to use exterior foam insulation on the whole house (but I'm not 100% sure of what assembly I should use).

    But for now, I'd like to do something with the basement walls that will not cause a problem later when I do the exterior foam. I'm hesitant to use spray foam in the basement, in case it will end up interacting badly with the future exterior foam (e.g. creating a foam sandwich in the wall assembly). I do definitely want to add air sealing wherever possible, though.

    I'm really looking for some advice on what the best wall assembly will be, given I have to work with what I've already got.


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