My question and answer is this: is there any insulation in it at all from the factory? I have no way to tell on my M725 as I never have drilled out any of my rivets to look inside. I live in northern N.Y., and I learned that with a thin area to insulate, spray foam has a much higher R-value over fiberglass. But, it is a lot more expensive per cubic foot. And, not necessarily a DIY project. You should let the pros do it. That is how I insulated my summer place that has 2x4 wall construction.