Insulating a metal shop?

"A buddy suggested yanking the roof off of the building and putting the insulation on, then reattach the tin! I scoffed at that idea.....but now.....it doesn't sound so outer space!!!!!"

That's exactly what I did. My purlins are only 8" deep. So I had a contractor screw 4" extensions onto the purlins and then install the interior roof sheeting. They then pulled the roof sheeting off, installed all new insulation (12" in the purlins and 3 1/2" over the purlins), and replaced the existing roof panels using slightly larger new screws. Hasn't been a bit of trouble.