Roger wrote:

I'm not convinced that the marginally greater radiation from warmer
walls would have any significant effect (see final para for the reverse
effect) but to the extent that it does it makes diMMs conjecture even
less tenable.

I'm having difficulty following your argument. I agree that the
difference in surface temperature, at least in the steady state, is
pretty small. If you compare two walls, one with a U-value of 0.4
W/(m^2.K) and the other with U = 2.2, assuming 21 deg. inside temp and
-3 outside, and using the usual value of 0.06 m^2.K/W for the resistance
of the internal boundary layer, the difference in the inside surface
temperature works out at only 2.6 K, according to my back-of-envelope
calculation. However that 2.6 K difference is about 15% of the
temperature difference between the couch-potato-body and the wall, so it
will have a fairly significant effect on the heat flux. With
intermittent heating in the poorly insulated place the difference will
tend to be larger, due to the lag introduced by the thermal mass of (for
example) solid brick walls.

The lower the air temperature (at which the body feels comfortable)
the greater the temperature difference between it and the warm body
and hence the greater the heat loss.

By "warm body" here I presume you mean the heat source. In "the
greater the temperature difference between it and the warm body" does
"it" refer to the air mass or the (human) body? If the latter, it's at
a fairly well-regulated 37 deg. or so and the air temperature doesn't
affect the heat flow very much from the radiation point if view.

Have you considered the fact that in a poorly insulated room there is
much more radiant energy about that in a well insulated room?

Well, no, my argument was constructed on the principle of there being
less. Why is there more? It does depend on the heating source of
course. A good blazing fire can make you feel quite warm in a very cold
room, so in some cases you may be right, but so what? - it doesn't alter
the radiation to the walls argument.

--
Andy