Understanding Air Barriers
Infrared image of 27-story high-rise undergoing whole building air leakage testing per City of Seattle Energy Code. Areas of air leakage were noted at architectural soffits, missing gaskets on operable window units and laundry venting. Despite these areas of air leakage, the building was found to comply with Energy Code and test at a level below the required 0.40cfm/sf @ 0.30″ wg. Note the thermo meter along the right shows the purple range at 41.3º F, more than 22º F cooler than the 63.4º F at the light yellow range.
Code increasingly mandates inclusion and verification testing
Because of the definitive relationships between air tightness and building envelope performance, inclusion of air barriers was mandated in the 2012 IECC for new commercial construction. In SMART|dynamics of masonry volume 2 .1, Morrison Hershfield’s Lee Durston and Jose Estrada, Senior Building Science Specialist at JRS Engineering, Seattle, formerly principle and building science engineer with Morrison Hershfield, wrote Understanding Air Barriers explaining what makes air barriers different from vapor barriers, their risks and benefits, design considerations and how to verify their effectiveness.
According to the Air Barrier Association of America (ABAA), an air barrier is a system comprised of a number of materials which are assembled together to provide a complete barrier to air leakage through the building enclosure. Air barriers must be designed as part of a complete building envelope system with considerations made for items including, but not limited to, thermal transfer, liquid moisture management and vapor diffusion characteristics. The ABAA provides a full list of requirements at airbarrier.org; similarly, the 2012 IECC provides a list of air barrier requirements in Section C402.4 and in the 2018 IECC, it is Section C402.5.1. The decision on the vapor permeability of an air barrier wall membrane for a given wall assembly depends greatly on the interior and exterior design conditions as well as on the placement of wall insulation. This is where involvement of an enclosure consultant is valuable.
Air barrier systems can be comprised of approved air-tight components that vary with architectural style and are often installed by multiple trades. In order to be effective, systems must be sealed at interfaces of different materials and roof-to-wall and wall-to-floor as well as penetrations and around openings.
Masonry is not typically considered an air barrier material, however, numerous wall profiles that include masonry materials have been successfully designed and constructed as such. The 2012 IECC, and subsequent versions, includes masonry materials and masonry wall assemblies that it deems to comply with air barrier requirements. It is important to note that the air barrier and its intended performance expectations must always be considered within the larger wall system.
Most vapor barriers are suitable air barriers, but not all air barriers are vapor barriers.
Since air barrier inclusion was introduced in energy codes, the US Army Corp of Engineers and many states have also adopted post-construction testing of buildings for air tightness. The most recent Whole Building Air Leakage Test Standard is ASTM E3158 Standard Test Method for Measuring the Air Leakage Rate of a Large or Multi-zone Building. Several methods of testing can be employed, both quantitative and qualitative. Quantitative testing provides an indication of the amount of leakage that may be occurring in an airtight building. This type of testing typically includes pressurization and depressurization in an effort to estimate the air leakage rate at a standard reference pressure point. Qualitative testing provides an indication of locations of leakage pathways and typically includes thermal imaging or use of a smoke pencil in combination with building pressurization in accordance with ASTM E1186, Standard Practices for Air Leakage Site Detection in Building Envelopes and Air Barrier Systems.
To read about specific masonry systems that comply with air barrier requirements and a short case study describing an air barrier’s value, read Durston’s full article in SMART|dynamics of masonry v2.1, pages 33-36 or online at dynamicsofmasonry.com/archives.