Depending on your skills — and if working in small, dark areas is not a problem — then yes, it’s easy to insulate the rim joists. A typical floor joist is a 2- by 10-inch joist set 16 inches ...

When floor joists are installed, a board, either a 2×10 or a 1×10, is nailed across the ends of the floor joists to support them in an upright position until the sub-flooring can be installed.

Insulate the rim joist by cutting pieces of two inch rigid foam insulation to approximate size to fit between the floor joists, and spray-foam them in place against the rim joist.