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Advisability of Continuous Interior Rigid Foam

BillStyles | Posted in Building Code Questions on

Hello. I have been considering continuous interior rigid foam as an option to increase the efficiency of my 1950’s home in southern Ontario, Canada in zone 5. 2×6 construction, 16″ on centre. Brick exterior with a weeping gap between brick and the sheathing/stud walls. 2′ roof overhang, so wind-driven moisture on the exterior walls is rarely an issue.

Exterior continuous rigid foam insulation is not an option. The old brick look is not something to be covered, apparently.

There has been an article or two here and there, and the odd mention in other threads about interior continuous rigid foam use. However, I am having trouble finding any definitive answer as to whether it is advisable. It seems wise in the sense that it decreases thermal bridging from the studs and is also a vapour barrier, which is required anyway but has the added bonus of insulation. Partition walls can be dealt with, as the overall gains would likely compensate for any thermal bridging there. Horizontal furring on top is a consideration.

However, my main issue is not the practical application, but theoretical side. Some threads have pointed to the possibility that it would cause increased condensation in the wall cavity since we do heat the house for more or less half the year. This means the exterior walls are cold and form condensation where it meets the warm rigid foam, correct? So basically, the thicker the foam the better, so we would need no less than 2″ foam to keep the interior wall cavities from reaching the dew point, if I am correct in understanding?

If it is semi-permeable, is that better to allow some breathing?

My knowledge of how condensation works inside walls is limited, mind you. I am a DIY’er.

Another option I’ve considered is rigid foam strips along the studs to reduce thermal bridging (Bonfiglioni, I believe) and spray foam in the wall cavities. This seems the most tested in my mind as spray foam is known for its effectiveness in cavities and the rigid strips would minimize thermal bridging.

I don’t foresee electrical and plumbing utilities as an issue with interior rigid foam as we plan to do all (or mostly) exposed utilities as stylishly as possible because hidden utilities seem unnecessarily annoying and troublesome.

I am mostly musing at this point, as the inefficiency of the house is tolerable but eventually making it more efficient is the goal.

Thanks for any insight.

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  1. Expert Member
    Malcolm Taylor | | #1


    Luckily the problems with too thin foam only apply when it is on the exterior. That's when the interior face of it can be too cold and get moisture condensing on it. Foam run on the interior of a wall stays warm enough that you don't have to worry about it.

    Deciding what to do to improve old houses is always a judgement call, depending on how far you want to go.

    The low-hanging fruit is to work on the biggest energy losers. They are typically air-leaks, which are most usually at the rim joists, ceiling, and windows. Attic insulation which is easy to beef up, basement walls, and the old windows. The advantage of stopping there is both financial and the lack of disruption to your house. Maybe that's doing enough?

    If you want to take off the drywall on the exterior walls, you can improve their performance in a few ways. You can fur out all the studs and plates so that they become the equivalent of 2"x8"s. Run the furring horizontally instead to create a Mooney Wall which limits thermal bridging, or do what you suggest and use foam either continuously or as a Bonfigioli wall. With all these options I'd suggest using either batts or dense-packed cellulose in the cavities, rather than spray foam.

    You may find this article useful:

  2. DirkGently | | #2

    My 2004 i was motivated to remodel 2nd floor of my 50's era cape style house which is 100% brick siding due to heat gain during summer months (brick oven house). I knew zero about proper air sealing back then....but did stumble on info of using rigid foam inside just prior to installing drywall which i promptly installed on walls and ceilings. It worked excellent at keeping summer heat out and house was much more comfy in winter.
    There are many, many things i would do differently nowadays on that same project....but even still it worked out well for me. Turned my house from energy hog to econmical.

  3. BillStyles | | #3

    Thanks for your replies. Malcolm, you make a compelling case, especially after reading that article. Interesting take on retrofits I never really considered. I'll have to look more into a Moomey wall, since it doesn't currently make sense in my mind how putting wood on top of thermally bridged wood decreases thermal bridging.

    DirkGently thanks for your experienced input regarding physically accomplishing the proposed idea. Good to hear it is effective should I ever go ahead with the project. How thick of foam did you use and how did you find attaching drywall?

    1. Trevor Lambert | | #5

      You have reduced contact from the furring to the studs, owing to the fact it's running perpendicular.

      1. BillStyles | | #7

        Makes sense!

  4. GBA Editor
    Martin Holladay | | #4

    You might want to read this article: "Walls With Interior Rigid Foam."

    1. BillStyles | | #6

      Thanks Martin, I had already seen this but forgot about it! A good read and very relevant.

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