Rim joist
Also called: band joist, rim board, band board
The outer band of framing that sits on top of the foundation wall and caps the ends of the floor joists. In a crawl space or basement it's a major source of air leakage and one of the cheapest places to insulate and seal.
The rim joist is where the floor system meets the outside wall. It’s thin — one piece of lumber between the crawl space and the weather — and it’s full of seams: the joint with the sill plate below, the subfloor above, and every joist end along its length. Air moves through all of them.
Sealing it is straightforward: caulk the seams, foam around penetrations, and either fit a block of rigid foam into each joist bay and foam its edges, or have a pro spray closed-cell foam across the whole band. Materials for a DIY rigid-foam job run $1–$2 per linear foot; a pro spray-foam rim joist is $400–$1,500.
For the effort, it’s one of the highest-return steps in any crawl space project — batts between the joists do little if air is streaming in around them.