mekineer,
The bag's outside temperature is low, quite close to the air temp in the tent. Thats the personal experience by touching it.
I believe in reality the bag (all its layers put together) is "radiation-proof", and there is no heat transfer from the body to the tent by radiation straight through the bag, but only by convection and by breathing vapor-loaden air (maybe very little by radiation from one layer inside the bag to the next).
Why do I believe this?
Because, when (assuming) the total heat loss of a human body from inside the bag is, say, 1/3 by radiation 1/3 by convection and 1/3 by evaporation, why dont all bag manufacturers focus on all 3 items?
They focus on convection the most, a little on Evaporation (for very extreme conditions), but they leave the radiation aside.
Do you really believe, all bag manufacturers around the world are so stupid to overlook any significant heat loss by radiation? No, I dont believe so.
We had sleeping bags in the past that had a perforated space blanket inside, and were advertised as being very "warm" by having very low weight.
Total crap. Didn't work when they were new, and didn't work when they were used, exept the space foil had crumbeled to pieces soon and thus provided some quiet sleep.
So this leads me to the conclusion that a typical (down) bag blocks direct heat Radiation, and all that counts in the total heat transfer is convection and radiation off the outer surface of the bag (plus some evaporation).