Just got through a stretch of sub-zero temps here. Days at or below 20. I kept my shop in the mid 50's with a propane "Big Buddy" catalytic heater.
But my garage is insulated and drywalled. It wasn't always. I decided a long time ago that it wasn't practical to heat an uninsulated shop in those kinds of temps. Besides the cost and the fumes, there is the increased fire hazard.
Maybe you should consider a different winter project - insulating that garage. It's one of the cheaper home improvement projects, and pretty easy too. Makes a huge difference in the quality and amount of winter "shop-time" you can have. Even with temps hovering around zero, I never see the garage thermometer go below about 35. Doesn't take too long to get it up to 50, and I can hold it there with the heater on a lower setting. I don't have condensation problems.
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