According to the recent estimates, construction accounts for 28% of global CO2 emissions. A report by the United Nations Intergovernmental Panel on Climate Change shows that reducing the energy consumption of buildings in turn reduces CO2 emissions. One of the best ways to do this is to improve the airtightness of building envelopes, which vary greatly in terms of performance. A study by the United States Department of energy shows that air leaks consume about 6% of the total energy by commercial buildings in the United States. In particular, about 15% of the energy used by commercial building envelopes was due to air leaks. How can construction professionals better design, specify and execute projects that reduce this transfer?
An air barrier reduces airflow between indoor and outdoor environments. It is one of the components of a complete set of weather barriers, along with a heat (thermal) barrier and a vapor retarder. According to the definition of the Air Barrier Association of America, a weather barrier is a “set of assemblies designed to withstand the loads imposed by all weather elements, including the sun, wind, airborne debris, heat, overflowing, liquid water and water vapor-usually referred to as building enclosure.”
Many materials can perform the function of an air barrier. Because these layers control the flow through a building envelope, materials can ultimately perform multiple functions; for example, a membrane applied with a liquid can serve as a barrier against water, steam and air.
The air barriers were one of the aspects of an impressive environmental strategy undertaken during the realization of a new building for Credit Human in San Antonio. Designed by architect Don B. McDonald with Kirksey Architecture and built by Joeris, the classic style center near the bustling Pearl Brewery stacks eight floors of office space on four levels of parking. The project “uses biophilic principles, high indoor air quality and transparency of materials to create an environment that radiates well-being”” according to Kirksey.
Steve Hennigan, president and CEO of Credit Human, wanted to create a healthy indoor environment for the company’s employees. This vision, for which Hennigan has set ambitious climate targets, has expanded to include air leaks. While the U.S. Army Corps of Engineers air density standard is 0.25 cfm / sf @ 75 pa, the project managers have set a target rate of 0.1 cfm/sf @ 75 pa. To ensure this goal was achieved, Joeris conducted extensive on-site testing and then documented it in an in-depth matter study. (The complete Book is available online.)
To begin with, the curtain wall units were tested with workshop water before installation. The building, which used Carlisle air and vapor barrier as well as other sealants, contained 311 envelope penetrations and 9,184 brick fasteners, each of which were sealed with Barritech VP to prevent possible air leaks. The flashing and sealant have been inspected to prevent “fish mouths” or creases. The basement UGC and the central walls were sealed, and waterproofing and an air barrier were installed between the garage levels and the upper interior floors. The cold joints on the underside of the roof were also sealed with a heatrelieving product.
The detailed inspection of the waterproofing included water tests and a selected demolition to observe the movement of the water under the finished surfaces. TSI Energy Solutions has conducted tests using a PosiTest air leak Tester in addition to wind tunnel and water tests conducted by others. The results showed that the top and bottom of the inner walls were the weakest points for air transmission, and especially the floors, because dust and dirt were not cleaned before installing an acoustic sealant, causing a leak, in addition to the screw heads that were not sealed and floating or covered with an air barrier. Other small gaps were inspected and filled after installation, for example on the exterior thresholds where a fire-resistant sealant was used.
How has the Human credit building behaved in general? The results of the last tests showed an extremely low level of air leakage, which corresponds to the purpose of Hennigan. This has been combined with other sustainability features, including geothermal wells, rooftop solar installations and rainwater harvesting. As a result, the LEED Platinum certified facility uses 96% less electricity from the grid and 97% less urban water than a typical building of its size. The careful installation of an air barrier reduces the operating costs of a building during its lifetime. It may be a small saving compared to the global numbers, but every little bit helps.