Air sealing can be a major problem for many home owners. There are many things that should be done to prevent air sealing but one place you should inspect is Duct/Piping Shafts. Spaces can be left unsealed where ducts, pipes and wiring serve as links between conditioned and unconditioned space. An example of this would be the space left unsealed around HVAC vents and light fixtures.
Use the proper sealing products such as caulk or handheld canned spray foams to seal any cavities that may allow heat transfer. Finding and sealing wall penetrations associated with mechanical ventilation systems, electrical openings and plumbing pipes will pay dividends to you and your customers for weatherizing a home for unwanted air leaks.