Just staple a plastic sheet over the insulation. They have it at the lumberyard. They consider it overkill here, but it will help with heating and cooling, by stopping the air leaks. It is NOT critical, just helpful. They seal the plastic joints with Tuck tape (not Duck tape) designed to stick and stay. I have seen them use 6 mil plastic but any thickness would be better than none, and it mounts with staples.
IF you can't, it is not a killer problem. It probably saves 5% of your heat loss. I will have to use it, because I need to use styrofoam on what I am adding, and it doesn't seal as well as the fiberglass. :D