The Natural Philosopher wrote:
[snip]

Thick curtains are in every way better than DG, except for condensation.
The windows WILL get ICY.

The analysis is ONLY valid if your curtains trap the cold air
efficiently. Unfortunately, the current fashion is for curtains to hang
on a rail 3-4 inches outside the window recess, so the air trapping is
very poor. It's even worse if there is a radiator under the window and
long curtains that hang over it directing the heat towards the window,
not the room.

Great original post, BTW.

TL