I did the same thing as Paul when I built my shop. I have 2" styrofoam and then 6" of insulation in the walls and 24" in the attic. I keep my heater at 68* during the day when I am out there and keep it 55* at night. The wood shop only gets heated when I turn the vent on. It was 42* in there the other morning when it was 7 below. In about a hour after turning the vent on it was 67*. I keep the other part of the shop warmer because I also do clock repair out there and can't let it get to cold. I am using a 60,000 BTU Hot Dawg.
Bernie
Never put off until tomorrow what you can do the day after tomorrow.
To succeed in life, you need three things: a wishbone, a backbone and a funnybone.