One of the elements of energy loss in homes that is often forgotten is the infiltration of hot/cold air from the attic and crawlspace through gaps in the ceiling or floor.
Since the attic and crawlspace are opened to the atmosphere through various vents, there is usually a air pressure differential between them and the living space. The pressure differential is also driven by the comfort system duct leakage. The result is air movement in or out of the living space into the unconditioned crawlspace and attic.
In the summer, air from the attic is often at a super heat condition from the roof load and close to outside temperature in the winter. Crawlspace air is usually cooler but can create air quality issues. That is why it is a good idea to seal those air gaps whenever possible to eliminate those issues. Air gaps in the living space can be found at recessed light openings, plumbing/electrical penetrations, walls with missing top plates, and other unsealed wall joints.
Since many of these air gaps can be buried when insulation is installed, we recommend a thorough sealing of them before insulation work is done.
More information about the sealing can be found at the Department of Energy's Energy Star website.