In reply to
Thanks. The body actually radiates a lot of heat. If you stood naked in a field on a cool day with no wind, more than half of your heat would be lost by radiation. The key to blocking heat loss is putting solid objects in the way, and that's one of the things that down does very well: there is almost zero heat loss by radiation through a thick load of down.
Down blocks all the heat loss mechanisms very well, and that's why it is so effective. There is no natural convection inside a load of down because the air gaps are too small, and forced convection (e.g. wind) is usually minimal because of face fabrics over the sleeping bag or jacket. Conduction is where thickness is important, and obviously down-filled things tend to be really thick, minimising heat losses by conduction.