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Air Barrier Systems

Design, Testing, and Impacts on Building Energy Use

By Sean M. O'Brien, PE, LEED AP, and Michael B. Waite, LEED AP -- Building Design & Construction, 10/1/2009 12:00:00 AM

The basic procedure for testing air barrier systems is to generate a pressure differential across the specimen, either through the use of large fans or blowers (above) or the building HVAC system. Photo: Simpson Gumpertz & Heger

Building owners, designers, and contractors are becoming increasingly aware of the effect of the building enclosure on energy use. With rare exception, however, green building rating systems, energy standards like ASHRAE 90.1, and building energy codes in the U.S. all but ignore air leakage. Despite growing evidence of the benefits of air barriers, the AEC industry has been slow to adopt air leakage control as a priority. This results in buildings with admirable intentions but inadequate performance.

Design: More than choosing a product

Air barrier system design requires much more than specifying an air barrier product. The air barrier system straddles multiple specification sections, including air barrier materials, roofing, windows, doors, and curtain walls. As a result, specification of effective air barrier systems is still a major stumbling block for many designers and specifiers.

Continuity is arguably the most essential element of an air barrier system. Airtight roofs, walls, and windows provide little benefit if the transition areas are of poor quality. Air leakage at the interface between envelope assemblies is a major issue for most buildings. Designers must provide large-scale detail drawings for critical transition areas if they expect a properly constructed air barrier. Attempting to address these areas in the field is a common cause of inadequate air barrier continuity.

For testing of localized components and whole buildings, it is useful to perform qualitative testing using infrared thermography to locate air leakage sites.

Another challenge comes in the form of specifying and verifying an acceptable level of performance. The table at right lists common performance requirements for air barrier systems and components to give designers a general guide for system performance. (Note: For fenestration, specific requirements may only apply to certain classes of products.) Unfortunately, until recently there has been relatively little data on the air tightness of typical buildings, and the data that is available is often misleading. Building air tightness levels, often categorized as “tight,” “loose,” or “leaky,” stem from the ASHRAE Handbook of Fundamentals and were derived from a 1976 study whose findings were not deemed conclusive. Until more comprehensive data is available, designers' specification of building air tightness performance criteria will continue to be somewhat speculative.

Testing: Measuring the leakage

The basic procedure for testing air barrier systems is to generate a pressure differential across the specimen, either through the use of large fans or blowers or the building HVAC system itself and to measure or observe leakage through the air barrier. For localized quantitative tests, it is necessary to measure both leakage through the test chamber and leakage through the chamber and specimen, using a removable seal on the specimen to differentiate between the two. While this is relatively simple for windows or free-standing mockups, quantitative testing of installed air barrier systems can be very difficult or impractical due to extraneous leakage through the test chamber. Exterior chambers with operable access panels (for removal of the initial specimen seal) are often necessary, but may be difficult to construct reliably.

In situ quantitative testing is typically used for “proof” testing of fenestration or measurement of whole building air leakage. This data can be very useful for quality control and inclusion in building energy simulations. For localized components and even whole buildings, it is also useful to perform qualitative testing using tracer smoke detection liquids, or infrared thermography to locate and repair actual air leakage sites rather than just quantifying leakage without identifying sources.

Energy analysis: Seeking high performance

Whole building energy analysis or energy modeling is becoming more prevalent in the building design community, particularly in high-performance building design. Energy modeling allows users to include many more aspects of building design and operation than traditional HVAC sizing approaches. The complexity and interdependence of building systems require that engineers constructing energy models understand how the systems interact and how the inputs for individual systems relate to other systems and affect the predicted building performance as a whole. The accuracy of models in predicting energy absolute use is a source of considerable debate in the industry. Our focus here is on their primary function as a tool to compare design options.

The assumed level of air leakage affects not only the predicted energy performance of a building, but also the relative performance (i.e., the percentage reduction in energy use) of some energy-efficiency measures, even if the assumptions are consistent across all analyses. At lower leakage rates, the heating and cooling requirements are generally lower. In most cases, the incremental change in energy use associated with a specific energy-efficiency measure is not affected by the amount of air leakage.

Consequently, the relative change in energy performance, when measured as a percentage of the total, will tend to be higher at low air leakage rates, even though the actual absolute change is the same as it would be for higher leakage rates. This can result in overstated energy savings in models that underestimate actual leakage rates, which can affect the economic analyses for a project and, more generally, the designer's understanding of the extent to which key design considerations can affect a building.

On existing building projects, designers are often asked to evaluate and predict the reduction in energy use associated with “tightening” the building enclosure. In these cases, improvements in thermal (or solar heat gain) performance are coupled to reductions in air leakage in the same energy-efficiency measure. The best approach is to evaluate the envelope modifications with multiple air leakage rates to gauge the sensitivity of the building's predicted performance to the assumed air leakage. The testing techniques described above are useful in evaluating the performance of an existing building and effect of envelope improvements by establishing a baseline and incremental targets for improved air tightness.

HVAC operation: A breath of fresh air

Despite the potential problems associated with inconsistent ventilation, most building codes still allow operable windows as the primary means of providing fresh air to occupants of single-family or multifamily residential buildings. The primary disadvantage of this approach is that occupants are unlikely to open windows during the winter in a heating climate or during the summer in a cooling climate. However, most single-family homes in the U.S. are leaky enough that incidental air leakage can provide most, if not all, of the ventilation air necessary to maintain reasonable interior air quality and moisture levels.

Installing a continuous air barrier will greatly reduce the level of incidental air leakage and may create problems for ventilation strategies that rely solely on operable windows. Without sufficient ventilation, interior moisture (from occupants' breathing and physical activity) can accumulate and lead to high relative humidity levels and moisture-related problems. Without ventilation or air infiltration, RH levels are extremely high during the winter, when condensation risk on and within the enclosure is highest. The potential for damaging interior RH levels demonstrates the need to carefully consider ventilation options when designing airtight building enclosures.

The authors offer an in-depth discussion of these topics in a five-part series on air barrier systems at: www.BDCnetwork.com/info/ca6699627.html.

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