Hi all,

I’m wondering if there is a way that I could improve the insulation at the rim joists of my 1966 home in Toronto, Canada (Climate Zone 5a, I think). I have experienced cold drafts in the winter.

A previous owner finished the basement already, probably back in the 1980s or so. The ceiling near the walls is removable (drop ceiling) so there is some access, however the walls are built up to the joists so it’s a bit tight.

The walls, unfortunately, were finished with polyethylene vapour barrier behind the drywall and fiberglass batt-filled wood framing (classic diaper), however the good news is that the basement doesn’t smell and the wood is in good shape, leading me to believe that there isn’t a mold or rot problem. I noticed there is black tar paper between the concrete foundation and insulated wall so perhaps this is what’s saving it. Also, I have a feeling that any moisture that occurs inside the assembly evaporates out of the gap at the top of the assembly. I run a dehumidifier in the summer since the basement is humid in summer. However, I have never seen any sign of bulk water entry.

I’ve begun removing the old paper-faced batts from the rim & end joists and air-sealing the gaps in the wood framing of the joist cavities using Dynaflex 230 caulking.

There is a black plastic sill gasket between the foundation wall and the sill plate, which I learned is a good thing. I’ve filled any large gaps I could find (and access) between the sill plate and foundation using foam backer rod, placed between the concrete and the sill gasket. I plan to further seal this seam with silicone caulk if in the places where I can access it.

However, I’m stumped as to what insulation to use as I don’t want to prevent any inward drying that seems to have been successfully happening. If I was to use rigid foam I believe I could be making things worse by sealing things off. I would like to stop any cold drafts if possible, though.

I also have a weird void space at the front and back of the house as shown in the diagram. It is not accessible other than the 3/4″ gap at the bottom. Is there anything I can do with this that is relatively straight forward for a DIY homeowner?

In the worst case If I can’t improve this assembly affordably, I’m thinking of just putting new mineral wool batts or boards into the cavities and calling it a day, hoping that whatever air-sealing I did would provide enough of an improvement.  However, the gap at the top of the wall would remain open to the inside (through the mineral wool).

I know this is not the ideal setup, but hoping to make some improvement without a huge expense.