Once the leakage rate for a building has been measured, it is useful to estimate the cumulative size (in square inches) of all leaks or holes in the building's air barrier. The estimated leakage area provides us with a way to visualize the physical size of the measured holes in the building. This can be particularly important when explaining the results of a test to a building owner. Leakage area calculations are also used in infiltration models to estimate the building's natural air change rate (i.e. the air change rate under natural weather conditions).
TEC's airtightness test analysis software calculates two separate leakage areas, based on differing assumptions about the physical shape of the hole. These leakage area calculations are compatible with the two most commonly used infiltration models. Energy analysis or rating software that require the user to input airtightness test results typically specify one of these two leakage areas.
The Equivalent Leakage Area (EqLA) is defined by Canadian researchers at the Canadian National Research Council as the area of a sharp edged orifice (a sharp round hole cut in a thin plate) that would leak the same amount of air as the building does at a pressure of 10 Pascals. The EqLA is used in the AIM infiltration model.
Effective Leakage Area (ELA) was developed by Lawrence Berkeley Laboratory (LBL) and is used in their infiltration model. The Effective Leakage Area is defined as the area of a special nozzle-shaped hole (similar to the inlet of your blower door fan) that would leak the same amount of air as the building does at a pressure of 4 Pascals.
Importantly, when using leakage area calculations to demonstrate physical changes in building airtightness, we recommend using the Canadian EqLA measurement. Typically, EqLA more closely approximates physical changes in building airtightness. For example, if you performed a blower door test, and then opened a window to create a 25 square inch hole and repeated the test, the estimated EqLA for the building will have increased by approximately 25 square inches from the initial test result. The EqLA is also easier to measure, especially in windy weather, because the measurement is taken at a higher building pressure than the ELA.