Air sealing can be accomplished from the interior and the exterior of your house. While air infiltration is common around doors, windows, and electrical outlets, there are also gaps and openings that only can be seen and sealed from the crawlspace, basement, and attic.
Look for pipe, wire, and utility penetrations through floors, ceilings, walls, and through any surfaces that separate heated and unheated areas such as hatches to attics, crawlspaces, and kneewalls.
If you are unsure about where the leaks are, or whether you may overtighten the house when sealing leaks, you can employ the expertise of an energy auditor to assess the house using a "blower door." A blower door is a piece of diagnostic equipment used to locate air leaks and determine the tightness of the home.
Caulk around plumbing and utility penetrations through the building shell from both the exterior and interior of the home. Caulk around vents, but do not seal off planned ventilation (such as for dryers). Add foam seals or gaskets behind cover plates on outlets and switches (especially on exterior walls after shutting off the power).
Air leakage around doors and windows together constitute about 17% of the infiltration in your house. This can be resolved by weatherstripping all exterior doors, moving window sashes, caulking around casings that often cover unsealed gaps between door frames, window frames and structural members or the rough opening. Caulk around the frames, interior wall surfaces and frames, and exterior siding surfaces.
Another source of unheated air entering your home may be your heating system ductwork (both supply and return). Closely inspect for disconnected, split, or loosely fitted ducts. These should be reconnected and duct seams and holes should be sealed with a special duct mastic tape made just for this purpose, or with a foil tape. Other products like duct tape will lose effectiveness over time. Leakage of this type can be responsible for around 14% of residential air infiltration.
Another problem spot, if you have access, is in the crawlspace or unfinished basement where the house wall framing sits on the foundation. This is called the sole or sill plate. It is often not sealed tightly and can provide around 25% of air leakage into the home. Remembering how pressures work in a building, you may be able to feel air moving into the basement at the sole plate.
Here are a few links offering free .pdf files that address the air sealing process:
Air Sealing - DOE
Energy Savers - DOE
Montana Energy Savers Guidebook
Air Leakage Control - Canada
