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Insulating window weight pockets.
I would not try to fill in the weight pockets with any insulation.
If there is a gap i would caulk it.
Insulating window weight pockets i am not 100 sure that just removing the casing will reveal the cavity.
It may be be covered with lath and plaster as well which would open up another can of worms.
Use a hammer and pry bar to carefully remove the wood trim from around the inside of the window exposing the sash weight pockets.
I did this on several windows.
Use foil tape to seal the joint between the window jamb and foam insulation.
Then cut strips of 2 foam board to fit into the wieght pocket where the weights used to be.
Check the fit of the brick molding.
Remove exterior flat casing to reveal weight pocket.
These houses typically have no water management details around windows doors which works while the wall is open but will cause issues with insulation.
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.
I restore these windows for a living.
Stack strips to completely fill the void.
That will address any issues with the weight pockets.