Insulating a metal shop?

Mine is wood structure 2X6 walls R20 Insulation R40 in the ceiling. My Biggest concern is windows. I have the best glass you can buy but the cheap glass in the overhead doors ( I have 4 doors) lets the heat in really bad. Talked to 4 different door companies and they all say they have crappy windows in the doors. Winter time is fine, Summer time windows face South and West, It is Brutal on a hot day