The Natural Philosopher wrote:

When hanging naturally our curtains are about 1cm thick. I can't begin
to consider that is worse than a 12mm gap between two pieces of glass.

Over our patio door/window we have net curtains and double-lined thick
curtains. To prevent condensation due to the 'reverse chimney effect'
we also use a sammy snake across the bottom of the curtains, when
drawn for the evening.

One cold night last winter, 0degC outside, I placed our weather
station's outside thermometer betweem the window and the net curtain,
and sat back to watch.

Eventually, the inside thermometer was reading 22degC, but the one
behind the curtains wasn't far off 0degC, about 2 IIRC.

Those curtains were - and are - doing a fine job.

--

Frank