Fiberglass batt insulation is not a good way to insulate a rim joist. Moisture gets through and mice love to nest in it. I would remove the insulation that's there and replace with foam board insulation, ideally at least 3" thick (two layers of 1 1/2" works). Caulk around the edges or use spray foam if there's room. There are discussions at www.greenbuildingadvisor.com about this same topic that are worth reading. I've done two houses this way and it's fairly easy to do. The only problems I've had is getting around wires and pipes. That's where a can or two of spray foam is handy.

Hi Seabornman,

Thank you for your comment and the link you provided.

The rim joist bay seams if you will were the joist, exterior sheathing, and rim joist meet have all been sprayed with insulating foam stopping the air/moisture filtration.

Do you have any suggestions on what I might use to cover a section of the batt insulation?

Thanks

I think I would cut corrugated plastic covers and staple in place I've used this stuff for other uses and it's easy to cut and make tabs that fold to give you a surface to staple to. I'd probably caulk afterwards. The stuff I've bought from Home Depot is this

Depending on the type of work you'll be doing in the work area, plastic could be a fire hazard. Why not install drywall on the ceiling?