Not actually sure myself, but I see you're in Colorado.
I figure I'm taking a chance (because I'm still demo'ing) and have to be in bus by September 1st... 20° is pretty extreme lows for my area, I can adapt and have a plan B for the colder than normal temps (staying in the T-Shirt shop I work at) Also, for immediate use I'll insulate and close off the bedroom and hibernate in there mostly, for short term.
I was in a 10x20 insulated (ceiling and walls only) storage building last winter... I survived most times with 1 Quartz heater
On the coldest nights, I kicked on my little milk house heater
One of the things that helped the most was a 10" oscillating fan to circulate the warm air from the ceiling.
I'm probably not much help, just letting you know what worked for me.