In this video, This Old House general contractor Tom Silva explains how to insulate window weight pockets.
1. Use a hammer and pry bar to carefully remove the wood trim from around the inside of the window, exposing the sash-weight pockets.
2. Cut strips of foil-faced rigid foam insulation to fit into the weight pockets; stack strips to completely fill the void.
3. Use foil tape to seal the joint between the window jamb and foam insulation.
4. Replace the window trim.