Butane stops vaporizing at -1C (31F), isobutane at about -10C (10F). Propane's boiling point is even better, at about -40C/-40F, but it self-cools and doesn't develop the required pressures to run a torch.

I know this because my otherwise dependable camp stove is a 3-season affair. For winter camping, you basically need a white gas system (liquid fueled, manually pressurized or gravity fed).

I suppose I'd reach for an acetylene torch in a cold workshop.