I'm trying to improve my home heating efficiency and am in the process of sealing my forced air ducts. In the process, I have noticed that the cold air returns from the 2nd floor to the basement are just the stud space between the outside wall and the inner drywall (no sheet metal liner). Was this pretty standard practice back in 1966 when this house was built? This return system seems like it would suck in a lot of cold air, plus there is no insulation in this wall cavity. Has anyone dealt with this before? Is this not a big deal in terms of heat loss? Thanks in advance for your thoughts.