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.

wrote in message
oups.com...
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?

ty