Air sealing is important, not only because drafts are uncomfortable, but also because air leaks carry both moisture and energy, usually in the direction you don't want. For example, air leaks can carry hot humid outdoor air into your house in the summer, or can carry warm moist air from a bathroom into the attic in the winter.
Most homeowners are aware that air leaks into and out of their houses through small openings around doors and window frames and through fireplaces and chimneys. Air also enters the living space from other unheated parts of the house, such as attics, basements, or crawlspaces. The air travels through:
• any openings or cracks where two walls meet, where the wall meets the ceiling, or near interior door frames;
• gaps around electrical outlets, switch boxes, and recessed fixtures;
• gaps behind recessed cabinets, and furred or false ceilings such as kitchen or bathroom soffits;
• gaps around attic access hatches and pull-down stairs;
• behind bath tubs and shower stall units;
• through floor cavities of finished attics adjacent to unconditioned attic spaces;
• utility chase ways for ducts, etc., and
• plumbing and electrical wiring penetrations.
These leaks between the living space and other parts of the house are often much greater than the obvious leaks around windows and doors. Since many of these leakage paths are driven by the tendency for warm air to rise and cool air to fall, the attic is often the best place to stop them. It's important to stop these leaks before installing attic insulation because the insulation may hide them and make them less accessible. Usually, the attic insulation itself will not stop these leaks and you won't save as much as you expect because of the air flowing through or around the insulation.