Ductwork

According to the us department of energy,

Typical duct systems lose 25 to 40 percent of the Heating or cooling energy put out by the central Furnace, heat pump, or air conditioner. Homes With ducts in a protected area such as a basement may lose somewhat less than this, while some other types of systems (such as attic ducts in hot, humid climates) often lose more. Duct repairs could be the most important energy Improvement measure you can do if your ducts are in the attic.

1. A. Leaky duct connection
2. B. Return leaks
3. C. Furniture blocking register
4. D. Leaks at furnace and filter slot and duct tape failure
5. E. Fallen duct insulation
6. F. Supply leaks
7. G. Kinks in ductwork restricting airflow

Return Leaks
Leaks in an HVAC return can suck unconditioned air into the HVAC system, increasing the cooling and humidity load, particularly if the unconditioned air drawn in is excessively damp. The unintentional airflows put the living space under a positive pressure that can drive cool, conditioned air into the building cavities. The same effect occurs when the return duct has been undersized — an all-too-typical design flaw.

Supply Leaks
Leaks through supply ducts into unconditioned space may result in negative pressure in the living space, causing unconditioned air to be drawn into building cavities.