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Community and Q&A

Foundation wall insulation

Evan_O | Posted in General Questions on

Hi folks,

I own a circa 1987 two story home in climate zone 6 with an unfinished basement with poured concrete foundation. 

Having already improved the rim joist area with XPS foam and Great Stuff since I moved in 18 months ago as well as having a contractor insulate the attic to R-60 and spray foam in a few knee walls, I’m now looking to take the next step and insulate the foundation walls. The goal is to lower heating costs and energy consumption. No plans to finish the basement but I want to insulate following best practices and to minimum code where possible. 

I plan to use Dow Thermax with fasteners or glue, tape seams and spray foam the sill and floor connection. 

My question has to do with those very tough to access foundation wall areas. I have a few spots with hard to move obstructions (ducting, water tank, etc) which will prevent me in some cases from getting 2.5” of clearance and prevent me from accessing sills and sheet tops in others. In the case of the clearance issue in a few spots I could always use a thinner foam (below code but that is less of a concern for me as). 

I understand there might be mold risks by not sealing the entire perimeter of each sheet however, such as in cases where I can’t reach the top side of a sheet to seal it. 

Would I be better off leaving exposed sections of foundation wall instead (for instance an 18” vertical area near the top of one section of basement wall where a duct is in the way) by trimming the sheets short and sealing them lower on the wall? I understand that poured concrete is permeable although is it so permeable that moist air from inside the basement would enter the slab and then to the other side of a sealed piece of foam in large volumes? Another alternative is sealing/treating the concrete in certain areas first where I expect such breaks in the foam might need to occur, in order to keep basement air/moisture from entering the slab in those areas in the first place. 

…Or I am worrying about something I shouldn’t worry about?

For what it’s worth, the basement humidity is kept at bay generally with a dehumidifier in summer and a HPHW running year round. 

Thanks for any thoughts/advice.

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  1. Expert Member
    BILL WICHERS | | #1

    I would use thinner foam in the obstructed areas. That’s what I’ve done in my own house. Any insulation on those areas is better than nothing, and is likely to prevent the condensation issues you’re worrying about.

    Hilti makes some great plastic fasteners to put the insulation up. Just drill a hole through the insulation and into the concrete, then push the anchor in and set it with a few light hammer blows. These anchors are much better than glue in my opinion, mainly because you don’t need to worry about pressing the insulation to the wall while the glue sets.


    1. Evan_O | | #2

      Thanks Bill. That’s helpful info on all accounts.

      There are still a few areas where even with thinner foam I won’t be able to seal off the top, since I can’t access the rim joist and sill there.

  2. GBA Editor
    Martin Holladay | | #3

    You can use thinner foam where necessary. Or for hard-to-reach areas, like 1-inch gaps between ducts and the concrete wall, you can consider using spray foam from a two-component spray foam kit.

    However you solve the tricky areas, just do your best, and don't worry about it.

  3. Expert Member
    Peter Yost | | #4

    HI Evan -

    Agree with Martin and Bill above but just want to reassure you that the rate at which vapor moves back into concrete and the inherent hygric capacity of the concrete means lose no sleep over this issue at all.


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