In this video, This Old House general contractor Tom Silva explains how to insulate a window with sash weights.
1 Use hammer and pry bar to remove the molding from around the window.
2 Cut a strip of foil-faced rigid foam insulation to fit snugly into the space between the side jamb and plaster or drywall.
3 Spray some maximum-expanding polyurethane foam sealant into the space at the bottom and top of the window's side jamb.
4 Press the strip of foil-faced insulation into place alongside the side jamb, flush with the wall.
5 Repeat the above procedure for the opposite side jamb.
6 Cover the seam between rigid-foam insulation and wall with adhesive-backed foil tape.
7 Plug the hole where the rope meets pulley with a small strip of rubber carpet padding.
8 Replace molding around window.