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Closed-Cell Spray Foam in Basement with Limited Exterior Exposure of Above-Grade Foundation

JohnJones171 | Posted in Energy Efficiency and Durability on

Hello all. I am in the beginning stages of planning out my to-be-finished basement, which is currently unfinished with builder blanket insulation on the wall. Currently leaning towards closed cell spray foam to R20, same in the joist bays. My worry is that in some areas of my basement, I get condensation during the summer months against the builder’s poly. There are no cracks, there is no standing water after rain, it appears to be simply condensation against the air-conditioned poly. It is especially noticeable during prolonged heat waves, where some dripping down to floor can occur. Exterior is properly graded, downspouts directed away.

I know spray foam should solve this problem, as it will prevent the humid air/vapor from contacting the poly. I also know that it is fine for cement (poured concrete foundation) to remain wet indefinitely, and the foundation wall preferably should not be allowed to dry to the interior. However, my question surrounds the fact that I have very little exterior foundation exposed. Being a home built in the late 1970s, most of the exposed exterior foundation is anywhere from 1-2 inches in the problem areas. It appears I cannot dry inwards due to the spray foam and I have little drying ability to the exterior. So two questions:

  1. I have yet to do the investigative work to know if I have a capillary break under the sill plate. If not, does this pose an issue, as there is little exterior drying potential. The areas are mixed in terms of sun and shade; one is exposed to plenty of sun and faces south, the other faces north and gets very little shade due to gardens and such. I doubt jacking the house up to install a capillary break would be in the cards.
  2. Does this situation pose any problems for foundation spalling due to freeze/thaw cycles if the concrete cannot adequately dry to the exterior?

I’m Toronto based, so I believe that is probably equivalent to Zone 5-ish.

I apologize if these are somewhat newb questions but I would rather not make a mistake here…appreciate any input. Thanks in advance.

Edit-I am exploring all my options, green considerations included, spray foam is just one option I am considering. But the questions would apply to a rigid foam install as well.

John from Toronto

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  1. Expert Member
    Peter Engle | | #1

    If the walls are pretty flat concrete, you can also go with rigid foam. Recycled polyiso foam is cheap and gives good bang for the buck. Use foil faced foam and tape the seams to prevent moist interior air from contacting the cooler foundation walls and condensing. My favorite treatment is to hold the polyiso up a few inches from the floor and filling the gap with XPS, as the polyiso should not get wet. This gives you decent flood protection.

    My go-to wall assembly is to install 2" of rigid foam with taped seams, with 2x4 studs in front filled with R13 fiberglass batts. This makes an R25 wall without breaking the bank, plus you can run your wiring and plumbing through the studs and fasten drywall to the front.

  2. JohnJones171 | | #2

    Thanks Peter. My plan, if I was to do rigid foam, would have been the same with 2" of rigid foam, except I'd use Roxul instead of Fiberglass. Same setup though. That being said, am I way overthinking things with my question of either 2inch of rigid foam or spray foam and the ability to not dry at all to the exterior (limited above grade foundation exposed). I could have sworn I read a post from Dana Dorsett years ago that mentioned something similar as a concern, but not 100% on that.

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