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