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Community and Q&A

Insulating rim joist made of concrete block – 1960s home

Patrick45 | Posted in Energy Efficiency and Durability on


I am a first time homebuyer who purchased in December 2018. I live in Canada, just west of Toronto, ON. I am currently renovating my basement and I am looking for the best way to insulate my rim joist. The home is a 1960 bungalow with Brick exterior finish an 8in poured foundation and plaster wall on upper floor. My rim joists are sitting on the foundation (separated by a water barrier) and the space between the joist are filled with concrete blocks and parging. I have read some similar Q&As on this page but none seem to really describe my rim joist detail. I was initially looking to spray foam the foundation walls and rim joist but I am worries about creating condensation and possibly reducing the “breathability” of the floor joists. Any advise would be greatly appreciated.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Patrick.

    It's good to know that the builder installed a capillary break between the concrete and the floor joists. It is safe to insulate with closed-cell spray foam or rigid foam against the concrete wall and the CMUs between the joists. If you use rigid foam, use sealants at the seams and perimeter. If you build stud walls in front of the spray foam/rigid foam, you can add additional insulation in the stud walls. Some builders won't use fibrous insulation insulation in basements, others will if the walls are first insulated with spray foam or rigid foam and moisture issues are under control. It's a judgement call.

  2. Deleted | | #2


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