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Community and Q&A

Basement insulation

AlanB4 | Posted in Green Building Techniques on

I live in a century old brick basement wall house (above grade 2×4/siding/plaster and lath) and have been thinking about insulating the basement when i have money someday. I read the Basement Insulation Systems Building America Report – 0202 from building science’s website

which was very informative but did not address the suggestions i got from local insulating companies last year. They suggested R20 of 2lb spray foam directly on the brick covered with monokote for fire resistance. My concern is the bricks are probably a century old, they are painted with two layers of presumably oil based paint, some have efflorescence deposits but no liquid water leakage. I have no idea what type of brick they are and if covered with insulation they are likely to freeze in winter and may crumble which would obviously be a huge structural problem. Is this likely to happen?

These are my thoughts (please correct if i am on the wrong track), right now they stay warm since they are uninsulated (unfinished basement at room temperature), they are surrounded outside by wet soil (frost line is about 4ft) and the water inside the brick and adjacent to the bricks probably never freezes because of heat coming from inside the house. If they are insulated they would be at virtually the same temperature as the soil, which should be above freezing most of the time but since the frost line is 4ft they may have occasional freezing spells if we have a month or two stretch of temps well below 0F

I’m in southern Ontario and granted it does not go below 0F very often but last winter we did have a stretch of below -5F for quite a while.

What do you think the risks are, i would like to insulate it but i don’t want to increase the risk of structural damage which would be hideously expensive to repair (and very hard to discover being hidden by two layers). The basement will never be finished, since the ceiling height is slightly less then 6ft but i am interested in reducing heating costs

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  1. mfredericks | | #1

    I think the local insulating companies could be on the right track if the walls are uneven. Because using flat rigid foam sheets on a bumpy wall is difficult. But if cost is an issue and your walls are pretty flat, I expect it's possible to use the rigid foam approach. This is discussed further in this article.

    Your concerns with keeping the brick cold is a good question. I won't pretend to know the answer but I happened to come across another article on BS website that might help you.

  2. user-1120647 | | #2


    You are correct to be concerned about brick freeze-thaw damage and the foundation of your house is no place to take risks. If you wish to insulate from the inside, you need to make sure the bricks are not getting wet from the outside. The only way to do this is by digging around the outside and applying damp proofing and a drainage mat. Even with this approach, there is a small risk that rising damp from the footings could be an issue.

    If you are going to dig anyways, you should proabably use the lowest risk solution and insulate from the exterior so the bricks stay warm and dry. It's an expensive approach, but by far the most robust and lowest risk.

    Alternatively, you could use exterior insulation on the above grade portions of your foundation wall and apply a 'skirt' of horizontal insulation (sloped away from the house) just below grade to keep the frost from penetrating the soil around your foundation. This would still involve some digging, but much less than full exterior insulation.

  3. GBA Editor
    Martin Holladay | | #3

    Trevor has given you good advice.

    There is some risk to installing interior spray foam insulation on your brick walls. If the bricks are dense bricks fired at a high temperature (in other words, high quality bricks), and if you have taken measures (to the extent that you can) to divert rainwater away from your foundation, then your bricks will probably be fine, even if you insulate on the interior.

    If the bricks are of poor quality, and the soil is very damp, this approach is riskier.

    In any case, exterior insulation (using closed-cell spray foam, XPS, EPS, or mineral wool panels) is always the safer way to go.

    For more information, see How to Insulate a Basement Wall.

  4. AlanB4 | | #4

    Thanks for the replies, there is no way i can afford to dig outside the foundation, and i don't know how to determine if they are high temp dense bricks. I am 99.9% sure there is no waterproofing outside the foundation and the soil is very damp :(
    This might be a stupid question but how can i be sure there are actually footings under this house, the basement floor is concrete but the walls being brick, could it have been built without footings? I'm guessing the house is a century old.

  5. charlie_sullivan | | #5

    If you've got a summer weekend free and are feeling energetic, it's probably feasible to hand dig a hole outside to see if there is a footing. I'm not sure what you would do with that information though. Especially since, if there wasn't one, you might weaken the support of the bricks in trying to find it.

    But more to the point, Trevor's suggestion of exterior above-grade insulation plus a horizontal skirt just below grade is a good economical way to elminate freeze damage risk.

  6. iLikeDirt | | #6

    Yes, it's an excellent idea. Basically you insulate your full-size foundation as if it was a shallow-frost-protected foundation. It will require some digging, but not as much, and it'll probably keep your basement drier too.

  7. Expert Member
    Dana Dorsett | | #7

    The freeze thaw spalling risk falls off pretty rapidly with warmer climates, but also with the amount of roof overhang (limiting the direct wetting of the brick). If you're in US climate zone 4 or lower the risk is quite low. In zones 6 & higher it's more of a problem, but still manageable. Most of southern Ontario is comparable to US climate zone 5, or the warm edge of zone 6, where it just depends. But it's still not a serious risk if bulk water is managed well.

    If you can dig down even 2' around the perimeter and install 2" of EPS up against the foundation, up to ~6" above grade (finishing the exposed EPS with a cementicious purpose-made finish such as QuikCrete Foam Coating) , you'll still be fine with R20 of 2lb foam on the interior, as long as the exposed exterior brick is not being chronically wetted and can dry toward the exterior.

    Even safer would be to install 2" exterior EPS from at least 2' below grade up to the foundation sill (cut in some flashing that extends over the EPS and give it a bit of slope at that cut to direct bulk water outward. That is sufficient exterior R in a zone 5/6 climate that an R15 batt-insulated 2x4 wall on the interior (without any vapor barrier) or 3" of continuous rigid rock wool would give the brick an average mid-winter temp above freezing, and provide an extremely generous drying path for any moisture wicking from the footings toward the interior. The thermal performance of the combination of R8 exterior foam and the R15 studwall 24" o.c. would have comparable or higher overall performance than R20 closed cell foam on the interior, and would lower the moisture content of the brick while keeping it warmer.

    If your local climate is more like the cold edge of zone 6, you'd probably want to either add 3-4' of wing insulation (shallow frost-protected foundation style), or go all the way down with the EPS, not just a couple feet.

  8. AlanB4 | | #8

    Interesting ideas Dana, a 2ft exterior EPS would be doable as a DIY, i am near Toronto Ontario, and one of the nice things about interior R20 2lb foam is the ability to skip the framing and drywall for a huge labour savings. I may just do the 2ft below grade EPS on the outside and leave it at that for now, would it provide much savings just on its own?

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