We have used the sauna in the -20F weather (actual temp—not wind chill). The sauna heats up to a nice 160F in about 1.5 hours in such conditions and has no problem hitting 200+F. It retains the heat very well, even in the changing room with the hot room door closed. Most of the time, we end up opening a window or two in the changing room once the hot room gets up to temp because it gets pretty warm even there.
Overall, having used the sauna in one of the coldest days of the year, I can honestly say I’m very happy with the insulation. Furthermore, you can tell the roof remains cold as there has been no ice dams and the roof stays uniformly covered with snow regardless of the sauna use.