Peter wrote:
I think you are kidding, at FL200, -30C, at night
Glider pilots do it with no heater. OK, not at night, but at FL350 and -40 to -50C!
But they are wearing an aircraft to keep them warm!
SCNR
It all depends on how much draught there is, I guess. A high performance glider will not have drag inducing holes in the fuselage, and flying on sunny days with a canopy will also help – so clothing will have to offer thermal protection, but not quite of the Eskimo variety.
On the other hand, the Seneca I flew once or twice had cold air inlets that did not seal properly – we called them “Ankle Freezers” for a reason…
You know what they say…“there’s no such thing as bad weather, only inappropriate clothing”.
I had a onesie designed for driving a skidoo at 100km/h in -60°C and I was snug as a bug in a rug at a mere -40° with no windchill. 
Operating the avionics in ski-gloves was something of a challenge, mind you. 
I can testify that the heater in a Chieftain gives-up at about 16000ft. After that, it gets might clod, mighty quick. I think the coldest indicated cabin temperature I’ve seen was -37 or so at 24000ft. Fortunately, the task was cut short as then hard drives in the onboard computers had given up. :)
