| When the return ducts in the cellar leak, they depressurize the foundation. This pulls cold, dry, outdoor air into the house. This inflates the heated areas driving warm, moist, indoor air through the walls and into the attic. |
Wet air forced into the walls soaks the sheathing and siding. Peeling paint and decay in the framing result. The cellar appears to be drier because water (and other soil gases) being drawn into the furnace. |
Warm, moist air forced into the attic heats the roof and causes condensation and ice.
Some holes in the return ducts are from poor workmanship. But often registers are purposefully cut into the return plenum to cool the heat exchanger. All holes in the returns are bad.