# Insulating a cantilevered rim joist while keeping the rodents out



## LooksLikeANail (May 26, 2015)

I know there are many threads about insulating rim joists (including this one, which has construction similar to mine), but my situation presents an additional challenge.

My 1980 home has a finished walkout basement which I'm remodeling, in part because it's always very cold in the winter.

The load-bearing walls are concrete block, outside of which is a brick facade that rises to meet cedar siding. There is a 1" air gap between the block and brick walls. In order to accommodate the brick facade and air gap, my rim joist cantilevers a short distance beyond the sill plate.










Unfortunately, this cantilever creates a "rat run": Any rodent able to slip between the siding and brick can crawl over the sill plate and into the basement ceiling, where they were nesting in the (now removed) fiberglass batts. That's daylight in the photo below:










My dilemma has been figuring out how to insulate this area and also keep out rodents. I'm concerned that rats and mice will chew through spray foam, and that filling the 1" air gap between the block and brick with spray foam might be ill-advised (I assume it's there for moisture evaporation via convection).

One idea I had was to build boxes in the rim joist bays using galvanized step flashing. This would keep out rodents, and would also "hold" any closed cell foam sprayed over it, preventing it from filling the air gap between the inner and outer walls:










Aside from being tedious (and in a few places almost impossible), is there any reason I shouldn't use this approach? I'd prefer to err on the side of overkill if it means never having to deal with rodents in the ceiling again. But if anyone knows a better way, I'm all ears.

Thanks in advance for your help.


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## Nealtw (Jun 22, 2017)

That should work but after insulation it should be solid blocked with joist material


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## chandler48 (Jun 5, 2017)

Or you could lay in 2x8x(space between joists), pressure treated, flat and fasten to the joists. Then foam and insulate the bay.


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## LooksLikeANail (May 26, 2015)

chandler48 said:


> Or you could lay in 2x8x(space between joists), pressure treated, flat and fasten to the joists. Then foam and insulate the bay.


That's an interesting idea. If I'm understanding you correctly, then the 2x8 wouldn't actually be touching masonry anywhere, right? In which case would pressure-treated wood be required, or just extra insurance?


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## 3onthetree (Dec 7, 2018)

I like the 2x8 laying flat. It would probably have enough fastener holding power just to the sill plate so you wouldn't need to contort against the rim to try and toenail it there. There should be, and looks like, a min 3/8" gap between the brick to wood, so no touching mortar/brick just like the joists. Then air seal on top of it and insulate the joist space. The blocking is required as the drawing Neal has, but in this instance since the cantilever is only 4" I think you can leave it out and no one's the wiser.

Obvious to say but get those exterior gaps filled ASAP so mice don't inhabit the air space behind the brick.


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