One key method of decreasing the energy consumption of a building is to make it more airtight; about 13 % of the energy used in residential buildings is due to air leakage between the interior and exterior environments. This air leakage not only affects the energy consumption of a building in this direct way, it also makes the existing insulation in envelopes less effective. To understand how air infiltration affects the nominal R-value of walls, Oak Ridge National Laboratory first tested 23 homes in two climate zones using a guarded blower door method to determine the in situ leakage rate through walls of whole houses. Based on this data, an approach for determining an effective R-value that takes into account climate, air leakage rates of walls, and nominal R-value of insulation was developed. This effective R-value is a useful performance indicator to aid builders and designers in planning wall systems that maximize the thermal performance when considering all these variables.