Timothy Murphy wrote:

Can sash windows ever be made completely air-tight.
I live in an old building with very non-air-tight large sash windows,
and have fitted secondary glazing.
But I have often wondered if I could replace or renovate the sash windows
in such a way that the heat loss through them is negligible?

fairly straightforward job. Remove the sliding sashes (remove beading
and lift out), route a groove around 3 sides, and slot suitable brush
type draught excluder into the slot. Refit windows. You'll want to
repaint after doing this.

This stops any rattles too.

NT