brewing in a bag outside in winter

I'd imagine direct fire from a propane cooker? I mash in a rectangular cooler and if needed throw a sleeping bag over it. I lose at most 2 degrees in one hour. With BIAB in a boil kettle you'd need to likely apply heat to maintain temp as the kettle probably doesn't insulate as well.

 

When I did BIAB, I wrapped the kettle in an old sleeping bag.

 

I did the same thing last winter, it was in the low 40's in my garage. Cut the fire, wrapped in a sleeping blanket (15F sleeping bag). The only problem was that I was so concerned I was going to lose heat, I forgot to stir so my attenuation suffered a bit but not so much that a dash of LME can't fix. Over the course of an hour my 7.5 gallon pot lost about 3 degrees.

 

I BIAB outside in the winter...I leave the flame on and its really not that hard to maintain.