Finally got all the rim joists sealed this weekend

I tried somethig a bit diffrent on my side last winter. My rim joists were mostly standard twenty-four inch bays, so I decided against the custom foam board approach since my cuts were always slightly off. I ended up using the low-expansion spray foam exclusively, just covering the entire rim joist frame from top to bottom. It looked messy while I was doing it. I used the aerosol cans, not the multi-stage kits, because I was trying to keep the job simple and quick. If you try to seal every single edge separately, you end up wasting a lot of product trying to make the overlap perfect.

My biggest headache was ensuring the foam didn't squeeze out the bottom gap where it meets the subfloor. Even though I cut a small gap, the expanding foam always tried to fill the whole hole. I ended up having to use caulk around the bottom perimeter after the foam had fully cured to hold it in place. I also ran into trouble around the electrical junction points, where the foam tended to push wire insulation right out of its casing. I had to take small sections of the trim off just to apply the foam carefully to the joist only and then put the trim back over it. Overall, while the custom foam board might look tidier than the thick foam application, the spray foam really did change the sound of the house. The usual faint hum you hear through the floorboards vanished almost immediately. It felt like a huge improvement over just insulating the studs.

I took a completley different route when I was insulating my basement area. I found the idea of cutting rigid foam to fit every single irregular bay to be a massive headache. Instead, I decided to insulate the joists themselves using fiber batts, which is a common practice for cavity insulation. I stuffed the joist bays with the batt material, making sure it was tightly packed around any existing wires or pipes. This provided much better thermal mass than just covering the outside of the framing with foam. Once the batt was installed, I used a specific bead of canned foam only to seal the perimeter where the joist frame met the concrete foundation. The key was to treat the entire rim joist area as a heat bridge connection, not just as a void to be filled. I noticed a lot of cold air coming up around the main stack for the furnace, so I made sure to use the foam to seal that penetration thoroughly, which completely stopped the draft I was feeling.

My experiance suggests that simply filling the void with compressible material and then sealing the exterior edges is often a more effective and less labor-intensive way to go than trying to perfectly custom-fit rigid panels across the entire circumference. It felt more solid and the whole process took less time and effort overall.

Gotta push back on the batts comment. Stuffing fiberglass tight into the bays is exactly the wrong move at a rim joist. Compressing it kills the air pockets that give it any r-value to begin with, and fiberglass does nothing to stop airflow anyway, which is the whole problem down there. Wind washing right through loosely packed batts is why my basement floor stayed cold for years no matter what I stuffed in the holes. You need an air barrier first, foam board or sprayed, and then insulation behind it. A batt by itself is basically a filter, not insulation, in that spot.

One thing I'd double check is that gap you left around the flue. I've got an old boiler too and when mine was serviced the tech made a point of the clearance distance to the flue, said foam sitting too close to hot pipe is a real fire risk even if it feels fine. I ended up putting a small metal shield between the pipe and my board patches so nothing could drift closer over time. Worth checking what your boiler manual calls for before you glue anything down permanently.

The template trick works even better if you trace the bay directly, since every opening is its own wierd shape. I hold a piece of foam up against the back of each bay, reach around with a marker, and follow the edges of the wood, then cut about a quarter inch inside the line so the can foam has a real gap to grab. Cutting to a friction fit just means the board bridges over voids you can't see.

One habit that saved me: number each cut piece and the matching bay with a pencil on the concrete, because after you've crawled out for lunch they all look identical, and a piece cut for a wonky bay near the basement steps absolutely will not fit the square one at the far wall.