I have an old house with double hung windows.  I am thinking of
replacing the window sashes with with a replacement kit...new sashes
and runners.  I am trying to decide if this is only a little better
than replacing the old aluminum storm window/screen combos with new
ones in conjuntion with using the 3M type window film in the winter
time.  I think most of the air infiltration comes in from the un or
under insulated area around the window jamb behind the casing where the
weights and pulleys are located.  My windows are alomost all 35x63.
Those replacement sashes are expensive.  I can buy a lot of 3 M kits
for that money.  Pros and Cons ?  Thanks