Radiant Barrier Insulation
I did the regular fiberglass bat insulation on the walls and ceiling in my shop. It made an amazing difference. It can be in the 30's outside and I can work in a sweatshirt inside using just a 30K propane heater. I do use two big ceiling fans to push the heat down from the shop's peak. I'd do it again in a heartbeat. It's in the low 40's at the moment and I'm headed out to the shop for the night. In 15 minutes it will be very nice inside. Cheap and effective. I did use some white tyvek-looking stuff over the bats which sealed against dust and dirt, as well as was very reflective for lighting.