It is well known among energy professionals that the largest facilitator of heat loss in homes is air leakage to the outside. Heat moving on air (convection) can be swift and is typically more cost-effective to reduce compared to stopping the heat loss through solid objects (conduction) such as walls. There are many points of heat loss associated with air leaks (called thermal bypasses) that are often not obvious to the homeowner. Some of these include pipe chases that run from the basement to the attic, interior dividing walls, air space surrounding the chimney, electrical penetrations, air ducts, recessed lighting, the rim joist (which rests on the sill plate along the foundation) and the band joist (which caps the ends of floor joists supporting second and third floors).
Historically, and even today, many new-home builders and those who do remodeling don't recognize thermal bypasses and the importance of making sure that construction minimizes heat loss through these bypass air leaks. This is why the new building codes are important and why weatherization programs that offer rebates are focused on air sealing existing homes.
The best approach to air sealing a home is to begin with a blower door test, which allows you to determine how leaky your home is, and to combine this test with an easy infrared scan to identify where the largest leaks are located. The blower door test also provides critical information regarding air exchange rate of a home to make sure minimum ventilation requirements are met for occupant health and safety.
Once you have identified house leaks, you can use a variety of materials and techniques to seal, or weatherize, them. Spray foam, rigid foam board, sheet metal, caulking (including high-temperature caulk), and duct mastic are some of the materials used to block holes and seal cracks.