Insulating window weight pockets i am not 100 sure that just removing the casing will reveal the cavity.
Insulating window weight pockets.
Cut strips of foil faced rigid foam insulation to fit into the weight pockets.
I would not try to fill in the weight pockets with any insulation.
I restore these windows for a living.
Remove sash weights for upper sash the ones closest to exterior wall and replace pulleys with the pullman device that fits into the pulley hole.
Then cut strips of 2 foam board to fit into the wieght pocket where the weights used to be.
That will address any issues with the weight pockets.
This old house general contractor tom silva shows an energy efficient way to insulate old windows.
We use a router to grove the sides of the sash so that the sash can move along an interlocking metal track.
See below for a shopping list and tools subscribe to th.
Remove exterior flat casing to reveal weight pocket.
I did this on several windows.
Use a hammer and pry bar to carefully remove the wood trim from around the inside of the window exposing the sash weight pockets.
These houses typically have no water management details around windows doors which works while the wall is open but will cause issues with insulation.
If there is a gap i would caulk it.
Stack strips to completely fill the void.
Use foil tape to seal the joint between the window jamb and foam insulation.