In buildings, stack effect is the phenomenon whereby warm air rises in a house and cold air enters to replace it. It is sometimes referred to as the chimney effect because it is also the driving force behind a fireplace draft up a chimney. In most houses, it is the single largest source of heat loss and air infiltration.
While some airflow through your house is desirable, it is preferable to choose where that fresh air comes from, rather than an uncontrolled and unknown volume removing your (hard earned!) conditioned air.
How it works
When indoor and outdoor temperatures and moisture levels are unequal, there is a difference in air pressure, which in turn drives air movement in an effort to equalize. On a cold day, for example, the warmed indoor air is more buoyant than the colder outside air. This causes the warmer inside air to rise up to the top of your home, and then out of your house through gaps and cracks around electrical fixtures, access hatches, plumbing stacks, ductwork, etc. At the same time, an equal amount of cool air will enter lower in your house to replace it. The same thing happens on a hot day, just in the reverse order.
Stack effect is continuous with greater effects on extremely cold or hot days, but there is a way to disrupt its energy wasting flow - seal the air leaks in your house.
Start high, then go low
Since hot air rises, and most households spend 2x-3x more per year on heating than air conditioning in our area, start air sealing with the attic floor for the biggest return on your efforts. Next, head to the bottom of the house before you chase down the leaks in between.
Caulking, expanding foam, and weatherstripping are simple and effective air-sealing methods with a quick payback on investment. Caulk and foam are for cracks and openings around anchored features, such as holes for pipes or recessed lighting, and door and window frames. As a rule of thumb, caulk anything thinner than your pinky, foam everything else. Weatherstripping is for sealing parts that move or need to open, such as the doors and windows themselves.
Finding the leaks
Besides visual inspection of all areas where two different building materials meet, a common technique of DIYers to detect air leaks is the incense test, which is exactly what it sounds like. Light incense and hold it near places where you suspect a leak, like baseboards, electrical outlets and switch plates, The smoke will be disturbed where there is leakage. Also, in the attic look for dirty or discolored insulation, a sign that air is moving through it.
If you would rather go the professional route, hire a certified Residential Building Analyst to conduct an energy audit with a blower door test. A blower door test, which depressurizes the home, reveals leaks. A Building Analyst can also provide recommendations on air sealing and other energy efficiency upgrades, and recommend contractors to perform the work.
Even in newer homes, though they are generally tighter than legacy structures, these smaller air leakage points were likely not addressed during construction. Since 10-30% of residential energy costs are due to air leaks, air sealing your home will go a long way toward improving its performance and lowering your utility bills.