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DEPARTMENTS INSIDE CSI * HORIZONS * FAILURES HORIZONS Introducing ‘Breathability’ to Curtain Walls Raymond Ting, PhD, PE, is the president of Ting Wall, an exterior wall system engineering and design company. An expert in exterior wall and composite foam panel technologies, he has been involved in building product/system research and development since 1969. Ting is the inventor of more than 30 patents worldwide. He can be reached at The Horizons column examines emerging technologies and the possibilities of new ways to design buildings. The opinions expressed are based on the author’s experiences, and do not necessarily reflect those of CSI or that of The Construction Specifier. The building codes, including the International Energy Conservation Code (IECC), have much to say about air barriers, infiltration, and the importance of airtightness, however, some design engineers are wondering if this is the right direction. In this author’s experiences, new thinking is needed regarding ventilation, curtain wall and HVAC design, and the ways in which we test laboratory mockups. For those buildings designed for human occupancy, the most important performance parameter should be good indoor air quality (IAQ). The factors affecting this criterion include carbon dioxide (CO 2 ) levels, interior mold growth, and interior air temperature. In the first case, air exchanges between interior and exterior are needed. To control the second, water leakage through the curtain wall, and indoor condensation, must be eliminated. Finally, to maintain a comfortable interior air temperature, the exchanged air from the exterior must be heated or cooled. In traditional building design practice, the air exchange and air heating/cooling are in the hands of the HVAC engineer, while the water leakage and interior water condensation problems are handled by the curtain wall designer. The common design practice in the industry includes the following two steps: 1. The curtain wall designer provides a system with test/calculated data on water leakage and water condensation resistance, thermal insulation value, and air leakage rate. 2. The mechanical engineer designs the HVAC system using the thermal and air leakage rate data given by the curtain wall designer. In this process, however, it can be argued there is no concerted effort or communication between the curtain wall designer and the HVAC engineer to improve the overall building performance by adjusting the curtain wall performance parameters. The concept of ‘breathable walls’ can help forge a collaborative effort between these two realms, improving building performance and IAQ. The breathable wall concept Due to awareness of the need for energy efficiency, curtain wall designers have been competing for less air leakage in recent years, constructing ever-closer to airtightness. However, the importance of air exchange has come to light with recent reports of increasing asthma attacks due to difficulty encountered in the air exchange mechanism. A typical HVAC system consists of: • air inlet ducts for delivering pre-heated or pre-cooled air into the occupiable spaces; • return air ducts for sucking a portion of the air from these spaces and returning the air to the air distribution center; • air intake ducts with a pre-heating or pre- cooling device and air blowers to draw the fresh outside air into the air distribution center; • air exhaust ducts and device to expel the desired amount of return air to the outside as the amount of air to be exchanged; and • air distribution center with air blowers connecting to all ductworks mentioned. In a high-rise building, registers spaced far apart for the return air ducts are most commonly located near the ceiling area of the occupiable space. Due to the fact the ‘dirty’ interior air with higher levels of carbon dioxide is heavier, it is more concentrated at the floor level. As a result, it is not effective for exchanging the air from the return air. The ‘breathable wall’ concept recognizes the air leakage through the curtain wall is a natural behavior of effective air exchange. It is effective since all curtain wall joints are equivalent to a uniformly distributed giant return air register 10 the construction specifier | august 2013 CS_August2013.indd 10 7/17/13 2:58:13 PM