I read Rehd's response and he presents one valid strategy. However, I find it luxurious to use the whole garage.

My set-up is a drywalled double garage. I know the ceiling is not insulated, and do not know if the walls have fiberglass insulation. 3 sides are exposed to the outside and one side is attached to the house. I replaced the old wood garage door a few years ago with one of the light doors which I believe are mostly made of insulation - at least it feels very light compared with wood. I do have an outside standard walk through door and one door that leads into the house.

I use a freestanding kerosene heater, which cost me $99 on sale and is rated at 23000 btu. It does not require ducting, as it is made to be used in a house. I would recommend a little air leaking in to help ventilate, as does the manufacturer.

When I turn the heater on, I keep it at high for an hour or so to warm up the garage. Then I turn it down to near its lowest setting. On windy days with the temp at about 15-20, the garage heats up nicely, and it is comfortable to work in a tee - about 70 degrees. On warmer days, like this last week when the outside temp was not below 40, by mid afternoon I needed to turn it off or open the door or it starts going above 80 degrees in there!

This has the advantage of really drying out the garage as well. I have always had a problem with tools getting soaked with condensation and rusting in the garage. This warming of the garage does the trick of drying everything out and keeping the tools in great shape.

Course, the cars all need to get their windshields scraped every morning! There is no such thing as a free lunch.

Rick