From the perspective of energy-efficient buildings, the airtightness of the building envelope plays a significant role. Presently, the requirements set out in the Canadian National Energy Code for Buildings (NECB) for estimating energy use through simulation consider the effect of airtightness of buildings to be modeled as a fixed value. Determining the air leakage during and after the building construction stage through air leakage tests is a standard energy performance method that could be used to increase the accuracy of the model. A simplified airtightness modeling methodology is desired because it would enable the industry to account for this phenomenon numerically during the design stage before construction. This paper describes a multiyear project being undertaken at the National Research Council Canada (NRC) to develop and propose such a methodology for modeling the airtightness of buildings. The basis for the methodology lies in completing air leakage tests of buildings. We tested four commercial buildings for airtightness. We focused on stand-alone commercial retail buildings to complement existing data sets. The measured air leakage characteristics of these retail buildings in terms of normalized flow rates ranged from 0.8 to 1.7 L/ (s·m²) at a pressure difference of 75 Pa. For modeling purposes, the Specific Leakage Area (SLA) ranged from 0.57 to1.2 cm²/m². We calculated SLA utilizing the Effective Leakage Area (ELA) with a pressure difference of 4 Pa. We then normalized the ELA value using the whole envelope area (walls and roof), including the on-grade floor area.