insulation around windows

here's my problem - circa 1900's house with single hung origional windows - in excellent shape. inside casing is wide painted pine (6"). if i remove it's gonna split so it not an option. i know there is a cavity around 4 sides of the window around 4" wide. anyone know what to do around insulating this - one idea was to drill holes in framing and blow the stuff in then plug it. is this the correct choice?
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Single pane old windows are likely less than R-1 with high air infiltration, insulating the cavity will have no effect on your utility bill. Storms will help a bit but new dual pane Low E Argon will help perhaps 15% winter and summer.
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yes, the storms are already there. i have started the process of replacing the windows with "inserts" - couple a year. the inserts i have put in do not solve the problem of the space around the casing.
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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.
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It sounds like you have the weight and pulley system. That cavity is to hold the weights. If you are changing, to the insert style, you can remove the pulleys, at the top, and though the weight access door (usually held in with one screw) you can remove the weights. Than with a stick you can push insulation up till you see it in the pulley hole.

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