Exterior and Total Envelope Leakage of New U.S. Low-Rise Multifamily Buildings

Envelope air leakage testing of new residential buildings is becoming more common in the United States as state energy codes and energy efficiency programs add testing requirements. Leakage testing procedures and standards for single-family homes are straightforward; however, for low-rise multifamily buildings, there is little consensus on what type of envelope leakage should be applied to the standard or how leakage should be measured. The three most common testing approaches measure the following values: (1) whole building exterior leakage; (2) individual unit total leakage (i.e., to the exterior, other units, and common spaces); and (3) individual unit exterior leakage. Each method has advantages and disadvantages regarding cost and availability of qualified testing agencies, as well as construction phases in which the tests can occur and value for energy performance versus indoor air quality.
This paper provides results of U.S. Department of Energy–funded research that included all three measurements on 25 low-rise multifamily buildings in six states. The research sought to inform the development and application of testing protocols by documenting typical leakage rates, the impact of various design and construction practices, the relationships between exterior and total leakage rates for individual units, and the effects of common area leakage. The measurements showed that the exterior leakage averaged about 30% of the total for units in common-entry buildings, and there was typically more leakage to common areas than to the outside. In addition, the surface-area-normalized exterior leakage was greater for common areas than dwelling units. For units in garden-style buildings (i.e., separate entries), the exterior leakage averaged 54% of the total. All of the buildings met or exceeded the code requirement for whole building exterior leakage, but only 33% of individual units had total leakage values below the maximum.