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Community and Q&A

Effects of Insulating a Basement

user-7127335 | Posted in GBA Pro Help on

We own a house built in 1923 in Montreal that has poured concrete foundations and are looking to refinish the basement. We removed the existing drywall and framing in order to inspect the foundation and to add insulation (none existed). The concrete foundation wall shows some signs of moisture (flaking parging, some efflorescence) but is generally in good shape considering its age (no cracks, water penetration, very solid). We had a foundation expert come by to inspect and he said that the reason the foundation was in good shape was because there was no insulation and so the heat from the house has kept the concrete wall from freezing in the winter. He warned that if we insulate the wall (intending to use  spray foam) we would essentially be blocking that heat and subjecting the concrete to freezing. This would lead to it’s deterioration if we did not take measures first to protect it from the exterior moisture (rubber membrane, drainage board, french drain etc.)  Such a project would be a major expense that we cannot afford to undertake at this time. So my question is – under these circumstances would it be better to insulate the wall or not? I understand that the foundation company does have a vested interest in selling their services here but I’m hoping there may be impartial research behind this issue and would appreciate any advice or references.

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  1. Gregorythebuilder | | #1

    Hey there,

    Is your basement completely below grade? Do you have any pictures that you can post?

  2. user-7127335 | | #2

    Hi Greg,

    The basement is 3 feet above grade and 5 feet below grade. Here are some exterior and interior pictures. Thanks for your followup.

  3. user-5946022 | | #3

    Looks like a parged CMU foundation.

    I'd be inclined to trust your expert. It sounds logical.

    I'd also ask the expert their opinion of only insulating, from the inside, the portion of the foundation that is above grade and below grade within about 1' of grade.

  4. Gregorythebuilder | | #4

    Its kind of hard to tell what type of foundation this is. It clearly looks like its CMU block on the outer portion of the wall, but it looks like a concrete wall was poured against the CMU blocks from the inside? - figuring that based on it looks like 2x6 or 2x4 form boards were used for a pour rather than a slurry coat used when parging CMU. Cant really tell though.

    I agree with CL's comment about trusting your local expert. And I would be less weary about the foundation company trying to sell their services. Its rare that a legit construction company is just trying to sell something to make a sale when they dont think its necessary.

    Asking what is better to insulate vs not insulate is difficult to answer because your overall needs play a huge factor. For instance it looks like you are converting your basement to living space. Is your basement currently extremely drafty, using a lot of energy to heat, uncomfortable, wet? - If so allocate finances to address to issue (before using the area for living space), get your concrete waterproofing systems in place and insulate the wall.
    Is the space currently relatively warm/dry/efficient? - Don't risk insulating it.

    Just my opinion here, and hopefully that helps! I also want to add that im a builder in Seattle, Washington where we dont deal with freezing temperatures so I dont have direct feild experience with effects of moisture/water freezing in concrete. Hopefully more people here will chime in who build in cold climates.

  5. Expert Member
    Akos | | #5

    House that old, the foundation is most likely stone. Around me (Toronto), it is very common to insulate similar houses with SPF on the inside. Its been about a decade that this started and I haven't heard of major failures.

    Provided you have dealt with any ground water on the outside, I can't see a way that the SPF will damage the stone. With insulation on the inside, the stone will be colder but as long as there is no liquid water there, it won't cause any damage.

    If you want to be extra safe, you can install dimple mat against the foundation up to grade and spray foam over that. The dimple mat should be connected to an interior drainage system like (

  6. user-7127335 | | #6

    Thank you all for your replies and advice. I'd like to clarify that the foundation is poured concrete- not CMU. The parging on the outside look likes stone blocks but that is just decorative.

  7. Expert Member
    Peter Engle | | #7

    Is the building from the 1920's or so? That looks very much like concrete cast using lumber forms. The stone looking exterior may have been cast in steel forms. That's pretty old concrete, but I still doubt it would be damaged by freezing. I absolutely agree with your local guy about drying it out, but only to the degree of keeping bulk surface water away - gutters, downspouts with long extensions, grading away from the building as much as possible. That's always a good idea. But there's probably little risk of damage if you insulate the interior. Concrete structural and flatwork outside does just fine in wet and freezing conditions, so long as it's not soaked with ice melters.

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