One way to seal up the ring joist/joist/sill area is to spray foam the ring joist to sill plate joint from the inside, add some fiberglass insulation, and finish with rigid foam board. Cut the rigid insulation a half inch or so undersize all around and wedge it in place with small chunks of scrap foam board. Spray foam to seal. The vapour barrier can be stuck with acoustic sealant and taped to the foam board.
Greg
