My cutoff is usually -20 since the propane doesn't seem to flow too well at anything colder, but I gotta brew when I can squeeze it in, so figured what the heck. By the time I milled my grain the temp rose to about -20 so it was all good.
Except my burner would not stay lit, guessing the propane tank was still a bit cold, so I grabbed the trusty space heater and extension cord, rigged it up beside the tank in the woodshed and cranked it up on high. Felt like it was blowing cold air but after babysitting the burner for about 10 mins it finally stayed lit, and I was at mash temps a few mins later. If theres a will theres a way.
Put the usual comforter over the kettle once mashed in, and I figured since I had the heater there, may has well stick it under the blanket. This worked great! Checked on the mash half way through, after a good stir I had only lost 1 degree C.
By the time I finished mashout the temp had rose to a balmy -15, figured the boil would be nice and smooth, of course I ran out of propane with 30 mins to go.
Luckily I always have a spare tank, but like the idiot I am I left it outside between the woodshed and the house, and I had shoveled the roof off about a week before, so it was buried under about 3 feet of hard packed snow. Dug the damn thing out, took the trusty propane torch and melted all the ice off the nozzle etc., and I was up and running again about 25 mins later. Gotta love brewdays like that... Heres a pic of the boil in progress, and the thermometer showing a balmy -15 now, 
had my finger crossed that my neighbour wasn't going to call the fire department thinking my shed was burning down from all the steam.All in all the brewday went well, hah, oh ya and I left the kettle out in the snowbank to chill when I went to my friends to watch the superbowl and when I came back it was too cold to pitch, so ended up siphoning into the fermentor and setting it by the fireplace for a few hours to warm up. Few more than I meant to actually since I fell asleep on the couch as per usual, when I woke up it was a few degrees too warm but it was like 3am and I had enough so I pitched anyway hah. Was bubbling away nicely 4 hrs later when I woke up tho, so all good...
. If I lived in your climate, I'd have a still.