Insulating Basement with Partially Cast-In-Concrete Joists

A homeowner in Calgary with a 1960s basement asked for advice on a tricky insulation problem: floor joists that are partly cast into the concrete foundation, a common older-home detail in that area. Because exterior insulation wasn't an option (the house has asbestos stucco and an attached deck), the plan was to insulate from inside — adding a vapor barrier, rigid foam board, a stud wall with mineral wool batts, and drywall on the basement walls, while air-sealing the gaps where the wood framing meets the concrete.

An experienced builder who replied said this embedded-joist setup was standard for decades in that region and, in a dry climate like Calgary's, tends to hold up fine even though it would rot quickly in a wetter place. His advice: sealing the gap between the foundation and subfloor with spray foam is a good move and helps protect the already-cold outer rim joist from moisture. There's no real benefit to relocating the old wood blocking near the foundation — leaving it in place avoids disturbing the structure above. He also said the wood dries out fine on its own over time, so a gap or no gap between the foam board and the joists doesn't much matter. His main recommendation was to air-seal the joist spaces but leave them without added insulation, calling that the safer choice for protecting the wood long-term.

For homeowners with similar older foundations where joists are partly embedded in concrete, the takeaway is that regional climate matters a lot in deciding how cautious to be, and sealing air leaks can matter more than adding insulation everywhere.