On the Origin of Leakage-Infiltration Ratios Previously Hidden By Means of Natural Obfuscation

Infiltration is the ingress of ambient air under normal operating conditions through adventitious openings located in the façade of a building. The importance of reducing infiltration to save energy is highlighted by standards and building codes in many countries. The mean heating season, or other typical, infiltration rate is often inferred from a measurement of an air leakage rate at a pressure differential of 50 Pascals from a whole building pressurization test, often made using a blower door. A simple linear relationship between the air leakage rate and the infiltration rate (infiltration equals air leakage divided by 20) is sometimes assumed to exist and is known by many terms, such as the rule-of-20, leakage-infiltration ratio, Sherman’s ratio, or the Persily-Kronvall rule. The value of 20 is not always fixed, and can be adjusted according to a number of factors, such as building height, shielding, air leakage path size, and climate. The origins of this relationship have previously been unclear, which is problematic if a ratio is to be used with any confidence. This paper investigates the origins of leakage-infiltration ratios (LIRs) and shows that they emerged in the early 1980s from various studies (some unpublished) that measured airtightness and air change rates in a range of single-family dwellings and established empirical relationships between them. It is shown that there is no technical basis for LIRs, yet they are applied by building codes around the world and used to make policy decisions. There widespread use is likely a function of their simplicity, yet they have significant limitations. Accordingly, limitations their use are recommended and more complex alternative models are presented that can be used when more rigorous predictions of infiltration rates are required.