My son has a home that is 100 yrs +. His heat bill here in the Midwest was $400 per month. Last summer we blew in 6 inches of insulation in his entire attic area, and it barely helped. Must assume that the heat is escaping thru the walls themselves. Windows do not appear to be leaking. Old slat walls - and no idea how best to insulate. Appreciate some knowledgeable input!