All proper roofs are, to some extent. There is an air gap in addition to the rock wool. The air can (very slowly) escape at the ridge line. However, if warmed air would want out it would melt the snow/ice at the ridge. That did not happen. It would also melt the snow on the steel shingles because it would warm them. That did not happen either.
On the garage roof stuff did melt a little. The garage has no insulation in the roof and I worked on my bicycles and other stuff in there. So my body, some machines and the occasionally opened door into the house to get in and out generated heat.
I guess the roof insulation can't be all that bad.
Nope. Build dates range from late 60's to a few years ago.
Yes, but most of all longer. We had years where we had to fire up the wood stove in May, one year even for a day in June. That wasn't the case in the late 90's, you could sit outside in a T-shirt in March back then.
Nope. Temps are the same as usual. In our case 68-70F in the living room and 62-65F in my office.
Most of them don't but I do. However, I already did when we used two cords.
If there is a global warming then we ain't gettin' our fair share :-)