I live in the state of massachusetts,so it very cold here.when i take the old window out and the weight of the window what type of insulation should i put in the gap inside. thanks

labman

Are you replacing the windows? If so, I would use that instant foam stuff in the aerosol can. It is expensive and and messy to work with, but you can be sure of completely filling the gap between the window and rough opening with a quality insulation and plug any gaps cold air could infiltrate in. This is a big project, and you only have one chance to do it right.

You can stuff fiberglass or rock wool in the gaps, but it is hard to be sure of completely filling the narrower gaps and it allows air ti infiltrate through it.