Air Leakage: Difficulties in Measurement, Quantification and Energy Simulation

Air leakage through the building enclosure can have a significant impact on heating and cooling loads. Unfortunately, the measurement of air leakage through enclosure systems and components can be very difficult in practice. Inaccurate quantification of air leakage can affect building performance and may lead to incorrectly-sized mechanical systems and unreliable results from whole building energy simulations used during design. The relative impact of air leakage on energy use can negate the benefits of other enclosure systems (e.g. thermal resistance and glazing solar heat gain) predicted by an energy model. Therefore, for high performance buildings, the accurate prediction and measurement of air leakage is more important than for traditional construction. This paper examines the difficulties of measuring air leakage through building enclosures on components, assemblies, systems, and whole-buildings. The authors discuss commonly specified air leakage criteria and testing requirements from a practical standpoint based on both theoretical and field testing experience. Energy simulation software is used to demonstrate the effect of air leakage on building energy use. The impact of inaccurate air leakage input values are compared to the predicted performance improvement attributed to common energy efficiency measures associated with the building enclosure (e.g. increased thermal insulation and glazing systems with reduced solar transmission). Readers will gain an understanding of both the need for, and difficulty of, air leakage testing in buildings. This understanding is fundamental to more accurately predicting building energy usage, evaluating material and component options, and designing more effective building enclosure systems for both new and remedial construction projects. 1 Michael B. Waite, P.E., LEED AP, Simpson Gumpertz & Heger Inc., New York, NY 2 Sean M. O’Brien, P.E., LEED AP, Simpson Gumpertz & Heger Inc., New York, NY