Sometimes I'm in a wooden shack atop some Conexs, sometimes I'm roaming about (or sitting) in a vehicle, if need be, or if my legs get stiff I'll walk about for a bit. At -40F, the heater will NOT keep up and keep ya warm, it'll just cause the engine to drop below "warmed up" level and cool off more. LOL And, even though the shack is insulated, sealed pretty dang well, and has heat... it'll still be cold in there with a down parka on.
When it gets down into those ranges, work "outside" is restricted, equipment use is cut back, crane use is scratched. Metal begins to do funny things (gets brittle) at those temps. Hydraulic seals do funny things. When it gets that low, I keep close by anyone having to be outside for a task... available for them to hop in and warm up after a few minutes of being outside. Bare skin freezes in a matter of seconds.