Tony Bryer writes:
(snip)
Yes, but curtains, mine away, are not trapping air behind them to any
extent. You've got gaps at the cill and at rail level a continuous gap
(with most curtain rails anyway).

Mmmm, true. Sometimes when I see insulating/thermal curtain and blind
installation described, it turns out that some come with a magnetic
strip you install to have the edge snugly meet the wall or frame. How
available this is, I don't know: I don't think I've seen it in real
life.

-- Mark