It is safe to say that although you lose heat and energy through gaps in windows and doors, the biggest culprit of energy loss in the home is through the basement or the attic.  Basically, cold air enters in your basement, rises up through the home, causing drafts and exits through the attic, otherwise known as the chimney effect.  When repairing energy loss in the attic or basement, it is sometimes more beneficial to call in the professionals to do the job, rather than attempting to do it yourself.  However, there are some things to look for and fix immediately.

One cause of heat and energy loss in the attic is through the attic pull down stairs, door or access panel.  A quick fix is to attach fire rated insulation to the backside and install a weather stripping around the sides of the door.  Ensuring that the door has a tight fit will prevent air from flowing up into the attic.  Recessed lights are another perpetrator of air leaks.  To right the problem, the lights either need to be replaced, or consider having a professional properly seal the top side of the box with the appropriate insulation.  Chimneys and vent stacks may have gaps around them; these can be filled with foam sealant or caulk.

By taking care of these problems, the chimney effect has been minimized.  However, there are still leaks in the basement, and it is wise to take care of them to prevent any more heat or energy loss.  The most common source of leakage in the basement is through cracks or gaps in the rim joist.  The rim joist is the perimeter framing of the flooring.  These gaps can sufficiently be filled with caulk or spray foam.  In addition, be sure to seal other gaps around dryer vents or ductwork, electrical passages or any other utilities that require access to the outside.  Now that the major causes of leaks have been taken care of, fixing gaps in windows and doors will make more of a difference.

http://www.energycircle.com/learn/air-sealing/plan-your-attack

Stack effect is the movement of air into and out of buildings through chimneys, or flue gas stacks.  The stack effect is also referred to as the “chimney effect”, and it helps drive natural ventilation and infiltration through buoyancy.  Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The result is either a positive or negative buoyancy force. The greater the temperature differences from inside the building and the outside temperature, combined with the height of the stack structure, the greater the buoyancy force that is created.  This buoyancy is called the stack effect.

It is important to have stack effect as it helps to balance the air flow through your home.

The stack effect will always bring air infiltration, since buildings are never completely sealed.  During the heating season, warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or leakage. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through either open doors, windows, or other openings and leakage. During the cooling season, the stack effect is reversed, but is typically weaker due to lower temperature differences.

High rise buildings, that have a well-sealed building envelope, must account for significant air pressure differences due to the height of the building.  There must be a well planned mechanical ventilation system.  Stairwells, shafts, elevators, and the like, tend to contribute to the stack effect, whereas interior partitions, floors, and fire separations can mitigate it.  In case of fire, the stack effect needs to be well controlled to prevent the spread of smoke and fire.