One large concern is air infiltration.
Years ago I worked for a contractor and installed floor heat in an existing metal building. Anytime the wind was blowing the heat wouldn't keep up. We went back a couple times and of course the heating system worked fine. We kept asking about insulation, and the building contractor kept claiming there was something wrong with the heating system.
One day they called and complained again about being cold. I went out there and I could stand in the middle of the space and feel air moving. I went over to the bathroom and grabbed a piece of toilet paper and ran it around the bottom of the walls. The TP would blow right away from the wall.
Fortunately, they put up birch plywood panels on all the walls. The contractor that was responsible for the job came out, removed all the wall panels, removed the fiberglass insulation and had all the walls and ceiling spray foamed, then they put the panels back up.
So, I would be tempted to spray foam it all to make certain it is sealed up well. when it was built it should of had wind barrier, Tyvek put up, if they had known it was going to be finished and heated.
