For heavy floor-length curtains to insulate a window, what type of material is best?
I’ll tell you what would really insulate – use quilted cotton fabric and lining. Hem both and put in side hems if you can sew, hire it done if you can’t. If you use clip rings and a rod, you have a very dense cover for your window that can look rich and warm at the same time. If you can buy off-white quilted fabric, it goes with everything, gold rings and rods it still looks classy. Try tie=backs during the day, with tassels on a cup hook, to let the light in. Make sure you hang the rods well above your window, and add extra at the sides if you can, so when you pull your curtains open they stack off the glass.
