I brew outside in my detached uninsulated garage. First, I run a kerosene torpedo heater (construction, as Baron calls it) for about 30 minutes to take the chill out of it, but once the HLT and BK burners turn on the kero heater isn't really needed... 200k btu of propane burner does a good job as a heater, too.
For ventilation, I have 20 feet of ridge vent and soffit vents and keep two windows (opposite sides of the garage, to get some cross) open a couple of inches. Digital CO/Gas leak detector never goes above zero, and I have a backup CO (non-digital, just a noisemaker... yes, I'm paranoid) that has never gone off either.
I prefer 70 and low humidity over the cold, but I prefer the cold over 95 and humid any day!
edit: CO is nothing to screw around with, it will make for an unpleasant day... but I think people (myself included) tend to be a little overly cautious. 20 years ago most of us would have done this in a garage without a second thought... nobody had a CO detector in their house. Now we're just conditioned to be afraid of CO, and that's a good thing.