How I finally got the sill plate sealed right - steps that worked

When I got around to sealing my own sill plate, I found that the biggest difference wasn't the type of caulk, but how well I cleaned the area first. Before pushing any bead of sealant in, I gave the entire joint around the foundation line a wipe down with some acetone and a rag. It surprised me how much loose dirt and fiberglass dust were just sitting in the grooves, which obviously compromised the seal. If you don't clean it out, even the best acrylic caulk will just shrink away from the gunk. For the rim joists, I usually prefer a low expanding foam like the one from Great Stuff because I can control the shot better than a rapid-cure version. I found that if you use a disposable foam backing sheet first, it gives you a little bit of cushion and also acts as a liner, making the application much easier. It keeps the expanding material from touching the actual wood frame, which felt alot cleaner overall. I kept the lines tight so I wouldn't have to cut back all the excess later. I spent an entire Saturday working on the lower level, and the difference in temperature along the walls was noticeable immediately. It’s amazing how much money you save just by stopping those sneaky drafts instead of having to invest in a total replacement of the windows later.

Still figuring this stuff out myself so apologies if this is obvious, but does the fiberglass batt that's allready stuffed in those bays stay, or does the foam go right over it? Mine has batts in the rim joist from the builder and they look gray and sad. I was going to pull them out and just foam the whole bay but I got worried about doing it wrong and making things worse. Also my house is a split level so half the rim joist sits on a slab, no basement down there, not sure if the same approach even applies on that side. Any thoughts before I start squirting foam around?

Before the foam cans came along we did those bays with cut pieces of rigid board, wedged in tight and sealed around the edges with a bead of caulk. Still my favorite method honestly, cheap and you can see what you got. Only thing I'd add to your plan is look at the outside of the rim while you're at it. Half the air leakage I've found on old houses was the joint where the sill sits over the masonry, and a little time on a ladder with a tube of sealant from the exterior side saves you from doing the whole job twice. Also check above any hose bibs or old dryer vents, those holes get hammered thru that area and nobody ever seals them.