Hello again all,

So we are moving on with renovations to our 1931-era house. As a result of our renovations we have essentially created "interior" 8" cmu walls. These walls used to be the outward-facing walls of the basement but over the 80 years various additions have altered their location.

My question is, what do I do for insulation on these walls, if anything. My conundrum is based on the fact that the footing of these walls is still on bare earth and there is not any sort of thermal or other break at the base of the wall/top of footing.

This means that there is a clear channel for vapor to travel through the footing, up the wall and into the basement. Is this something to be concerned about or am I over-thinking here?

In addition, one wall is partially an exterior wall while the rest is interior.

My intent is to finish the rest of the basement with R10 XPS followed by 2x4 with cellulose dense-pack, open cell at rim joist.

Any thoughts?

Thanks in advance.

Most of the footings are now well inside the thermal boundary of the additions, and unless you've insulated the slab and footings everywhere else and have a very high performance standard you're trying to meet there's nothing really worth insulating on what are now load-bearing partition walls.

It may be worth insulating the first 5-8' of the wall that intersects the new exterior wall, but it may also be possible to cut the old CMU wall at that intersection and insert insulation there. (That would be my first choice, if it doesn't

XPS is blown with HFC134a, which has ~1500x the greenhouse gas potential of CO2. Using 2.5" of EPS (blown with pentane at only ~7x CO2) is a lot friendlier to the environment. EPS is at least as moisture-tolerant as XPS, with a long history of being used for flotation in marine environments.

Cellulose is often problematic in basements, and a disaster if the basement has water/drainage issues. Second choice would be rock-wool, followed by dense-packed new-school fiberglass (Spider, Optima), which won't wick or hang onto water to anywhere near the extent cellulose does. Third would be open cell foam.

Whatever you fill the cavities with, put an inch of rigid EPS or XPS foam between the bottom plate of the studwall and the slab as capillary & thermal break. Compression strength doesn't matter- the studwall only holding up the gypsum & insulation, not the house.

Thanks for the input Dana. I'll look at EPS for the walls as well as rockwool for cavity. I suppose it would be possible to route channels in the EPS to put the 2x4 wall in and go with something more like 5" total with 2.5" being non-routed. My labor... Will do on the bottom plate.