It's 3 degrees here today. Forecasted high is 13. Last night I switched over to steelies and snow tires on "Black Betty"--my wife's black sedan. Heater kept the temp at 55, outside temp was 30, still with open rafters to the roof vents. Kept a window slightly cracked open for venting also. As I posted previously, I was able to turn the heater down to barely on after 15 mins. Thinking about getting one of the vertical column electric heat units to see if it would maintain temp after running propane unit. Noticed some fogging of the car windows this time. Decided to just put up some 3 mil plastic (with an opening in the middle for venting) as a temporary ceiling for now, until I can go look at a good truss design that I could duplicate as someone else suggested.
What kind of heater are you running? I was surprised that with a little ghetto insulation as I put in it made a world of difference. Holds the heat pretty well. I can't imagine if I had new construction with much better overall insulation and build quality.