Properly insulating and air-sealing rim joists (band joists) takes patience, so most builders simply stuff in some fiberglass ...

A: Rim joists are the boards at the end of all floor joists on the first and second floor. They literally are a “rim” around the house.

Rip two three foot lengths of 3/4 inch thick plywood or OSB wide enough to span the distance between floor joists in the repair area.

The OSB was just covered with shiplap wood siding. If the builder had installed a water barrier correctly, there’s a good chance the flashing leak would have only rotted out the wood siding.