Retro fitting interlining to curtains - advice?

[TheYearOfTheCat]

I am buying some ready made curtains, and was hoping to sew some interlining into them. (We only have single glazed windows)

What I was thinking if doing was to unpick the hem, turn the curtain inside out, then sew some interlining to the top of the lining, then turn it all back the right way out.

Does this sound like a good idea? Would I need to sew the interlining down the sides, or would it hang better not sewn in at the sides?

Any advice would be much appreciated.

you can buy ready made liners that attach to the curtain hooks

Following on from the ready made linings - perhaps you could buy blackout linings which should provide some extra insulation. I wouldn't try to stitch in some interlining.