This paper explores how air leakage through the envelope can have a significant impact on the energy use of a building, particularly in heating dominated climates. However, infiltration is not often given an appropriate amount of attention by most projects during the design stage of a development. Possible reasons for this include the difficulty of determining what infiltration rate to apply to an energy model, and the fact that common energy codes in Canada do not allow savings to be directly claimed for technologies and strategies that reduce infiltration. Infiltration rates used in energy models often just become a ‘best guess’, which in turn leads to a reduced understanding of the project’s expected performance. The overarching intent of this document is to inform this ‘best guess’, and to encourage a more consistent representation of infiltration across projects that go through the SBD program.