Apart from the obvious that one is for the base, and the other for the top layer how are they different?
If you are talking about attic insulation, the base layer will have a vapor barrier next to the ceiling then insulation on top of thatIn a lot of houses built in the last two decades they put down a layer of fiberglass insulation with a kraft facing that is put facing down next to the ceilingOn top of the fiberglass they will blow in cellulose or some form of loose insulating materialSome use a solid foam core insulation as a base to put either fiberglass or loose fill overSince the foam is solid it will act as a vapor barrierThe best vapor barrier would be plastic sheeting put on the ceiling joists before drywall is installedHowever, obviously, that is impractical if the ceiling is already in placeHope this helps.
Chocolate Ice Cream with Chocolate Chunks.
