What is the best way to insulate embedded joists in a basement Montreal Home?

We advise against spray foaming any rim joists that are wet set in concrete because they are highly vulnerable to moisture movement coming through the concrete, so they could rot, leaving you with a big problem.  Its not often that we recommend ‘not’ insulating portions of a house, but yours is one of those cases where we do.

You will experience a bit more heat loss, but that would pale in comparison to the cost (financial and ecological) of having to rebuild the floor structure of your home.

Spray foam on rim joists of old homes that are NOT embedded in concrete is one of the few places we regularly recommend using it, as any material with spray foam on it is not recyclable or re-useable and will no doubt end up in a landfill. And one last tip – there are a couple of manufacturers of spray foam that have switched to blowing agents with much lower GHGs (green house gas emmisions), you can read about that here -

Where to find spray foam with eco-friendly blowing agents.

One other tip that would save you money would be to swap out XPS for EPS insulation. It has a slightly lower R value per inch, but it’s about half the price and has a fraction of the global warming potential, you can read more here on our page about choosing the best rigid insulation panels.