I'm trying to insulate the attic in a 100-year-old house. The bays between the joists are 20" wide, but I can't find any rolls of batting at that width.

My first thought is to buy 23" batting and slice off the extra 3" with a utility knife, but that seems like it might be both messy and time-consuming. I'm looking for 6 1/2" deep fiberglass insulation, the higher R-value the better.

Any ideas on the best approach are greatly appreciated. For instance, is there any way to get batting custom cut? Thanks in advance