If it's a 1900 house with original windows, you almost surely have
counter-weights in channels on either side of the window. They are
connected to a rope/chain over a pulley to the window. This means you
can't fill the side channels with insulation. I remember on an episode
of This Old House where they did it. I think the solution was to get
rid of the counter-weights and install a modern control mechanism. Of
course, that is a lot of work.