The air-tightness of a home is often indicated by air-change rate; the number of times the home's air changes over with outside air. For example, if the amount of air that enters and exits in one hour equals the total volume of the heated part of the house, the house is said to undergo one air change per hour.
Air change rate is very difficult to pin down because it depends significantly on how the house is used, as well as the wind and temperature differentials it experiences during the year. Even if the rate were determined with some precision, which is established with a blower-door test, there is no assurance that value would apply under other conditions. For this reason, rough estimates are generally used when referring to a home's air tightness.
The national average of air change rates, for existing homes, is between one and two per hour, and is dropping with tighter building practices and more stringent building codes. Standard homes built today usually have air change rates from .5 to 1.0. Extremely tight new construction can achieve air change rates of .35 or less. Most homes with such low air change rates have some form of mechanical ventilation to bring in fresh outside air and exchange heat between the two air streams.
To get an idea of what your home's air change rate might be, consider that a tight, well sealed newly constructed home usually achieves .6 air changes per hour or less. A reasonably tight, well constructed older home typically has an air change rate of about 1 per hour. A somewhat loose older home with no storm windows and caulk missing in spots has an air change rate of about 2. A fairly loose, drafty house with no caulk or weather stripping and entrances used might have an air change rate as high as 4, and a very drafty, dilapidated house might have an air change rate of as high as 8.