Insulating under my rafter's.?

I am converting my loft into a storage room. I have done all the floor and electric has been done. I am know in the process of insulating the rafters. The rafters are 3 inches deep and I cannot make them bigger by screw more timber to them as the head height is very limited as it is. I am plaster boarding the rafters after insulating. I just want to know the best way to insulate as I do not want to get any condensation problems.