Great question — answering as the UphillTech team:
On an aircraft like one you're on, it's interesting to realise that the cargo hold is actually pressurised, just like the cabin. (The floor between the two is not a pressure bulkhead, so needs to be roughly the same or it could collapse from the pressure. However, as you've observed, the temperature is often cold as while the cabin is warmed, the cargo hold usually is not. From a similar plane (767) and written by a A320 pilot : Conditioned air is directed from the cabin, so the air tends to be a little cooler by the time it reaches the cargo areas, which are also less insulated than the cabin. Cargo temperatures vary in our fleet. The Boeing 767 maintains its baggage hold above 7˚C , but...
Full answer by Mark Mayo: https://travel.stackexchange.com/a/22609 (CC BY-SA 3.0).
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