I have converted a barn to an in-law apartment. I was thinking of using the double bubble reflective material as my only form of insulation. I live in CT where days in January can stay in the single digits and nights be as much as 15 below zero.
I would not suggest you insulate the barn with just a FBBF product. Rather you should consider a hybrid insulation system consisting of fiberglass and the reflective insulation. The harsh winters are best insulated against with a mass insulation, and the reflective insulation will provide a good air and vapor infiltration barrier as well as a good reflector of heat in the summer. It’s only in severe hot climates that reflective insulation is a single option insulation.