Dropped ceilings and kitchen soffits, ductwork and plumbing chases, attic accesses and pull-down stairs, recessed light fixtures, holes in mechanical room closets, and wiring penetrations through the top plates of walls represent major connections between the attic and conditioned space. Many times unseen holes or pathways, called bypasses, occur at key junctures in the framing (such as at attic-to-kneewall transitions) and permit large quantities of air to leak in and out of the home.

Major leakage sites in the floor can be found around the tub drain and the numerous plumbing, HVAC, and wiring penetrations through the floor decking and bottom plates of walls. In walls, the band joist (for two-story homes), window and door rough openings, and penetrations through the drywall and exterior sheathing are primary leakage sites.