After our blower door test the question was how does that relate to ACH. Some proposed it is just ACH50 divided by 20. The testers had a sheet that chose values of 15 to 18 for the denominator depending on the the number of stories.
I've found onearticle, from 1986. They have been references to it in other papers as recently as 2009.
The paper recognized three issues:
It uses adjustment factors based on:Stack effect. Rising warm air induces a pressure difference, or "stack effect," that causes exfiltration through the ceiling and infiltration at (or below) ground level. The stack effect depends on both the outside temperature and the height of the building. A colder outside temperature will cause a stronger stack effect. Thus, given two identically tall buildings, the one located in a cold climate will have more stack-induced infiltration. A taller building will also have a larger stack effect. Even though outside temperature and building height affect average infiltration rates, neither is measured by the pressure test. During the summer, stack effects disappear because the inside air is usually cooler (especially when the air conditioner is operating). Wind-induced pressure therefore becomes the dominant infiltration path.
Windiness and wind shielding. Wind is usually the major driving force in infiltration, so it is only reasonable to expect higher infiltration rates in windy areas. Thus, given two identical buildings, the one located in a windy location will have more wind-induced infiltration. Nevertheless, a correlation such as ACH50/20 does not include any adjustment for windiness at the house's location.Trees, shrubs, neighboring houses, and other materials also shield a house from the wind's full force. Since a brisk wind can easily develop 10 Pascals on a windward wall, the extent of shielding can significantly influence total infiltration. A pressurization test does not directly measure the extent of shielding (although a house with good shielding may yield more accurate measurements since it is less affected by wind).
Type of leaks. The leakage behavior of a hole in the building envelope varies with the shape of the hole. A long thin crack, for example, responds less to variations in air pressure than a round hole does. The pressure/air change curve (determined with a calibrated blower door) often gives clues to the types of leaks in a house.
Is there anything more current?
Infiltration: Just ACH50 Divided by 20?
Building Tightness Guidelines: When Is a House Too Tight?