ASHRAE Journal - April 2009 - 58

Building Science Perspective By Lee Durston The U.S. Army Corps of Engineers has adopted an airtightness level of performance into its requirements for new building and renovation work to reduce energy use and provide for an all-around better building enclosure. They are requiring building commissioning to include air barrier testing that show building envelope leakage rates of no more than 0.25 cfm/ft2 of building envelope, when tested at 75 Pa. Unfortunately, failure rates of air barriers systems are high when actual performance testing is carried out, and systems evaluated. It was quickly realized that the typical design and construction methods, implemented prior to this requirement, must include the design, installation and testing of an effective continuous air barrier system. To prevent failure, it is necessary to implement into the design/build process an education program for the design and construction teams, advising them of the pitfalls of the continuous air barrier component in the design/build process. If continuous air barrier requirements are not addressed in the design phase, and properly tested in the commissioning phase, the success rates of building envelopes meeting the air leakage specifications are low. With the complex nature of building types being developed, a knowledgeable consultant team must provide predesign, design, preconstruction consulting, as well as observation during construction, pretest visits, airtightness testing, air leakage investigation, and consulting on effective remedial works should the building fail to achieve the required standard. Consultants should have experience that includes a comprehensive database of “best practice” guidance documents, including details and specifications that provide practical advice for builders, architects, developers and building owners. Peer review of the proposed construction methods and materials should be undertaken by analyzing drawings and specifications and providing feedback where warranted. Site visits should be undertaken to observe construction quality and detailing, and to confirm that an effective airtight barrier is being provided. Airtightness tests can be preceded by the issuing of a pretest inspection list. In the event of a failed performance verification test, a forensic building science consultant should be hired to use infrared thermography, smoke testing, and invasive measures to best understand the failed airflow pathways before moving towards remedial actions. Lee Durston is the director of the Building Science Group for BCRA Inc., in Tacoma, Wash. with additional door fans to isolate one section of envelope for analysis. A subjective estimation of the air leakage reduction can be made, along with a subjective analysis of the effects that reduction may have on ventilation rates. Airtightness for a new building starts with a good design. Leakage rates of wall sections, windows and doors are important, but most problems occur where walls or floors meet. This detailing is most important. When the air leakage target is set, the ventilation system can be designed to match the load. Consider that the envelope may not supply much accidental ventilation or exhaust, and if possible, specify a balanced ventilation system for each unit. Unless the building is fitted with windows that cannot be opened, assume that windows will be open, and place more emphasis on the internal barriers to airflow: the partition walls, hallway walls, floor slabs, elevator shafts, elevator hoist rooms, elevator lobbies, stairwells, chases, and garbage chutes. Many of these features can be door-fan tested separately to discover what parts of the building are responsible for the largest portions of air leakage. Timely measurement and sealing is essential while the building is under construction to ensure that airtightness detailing is accomplished at the right stages. Methods may be altered early in the process to ensure that problems are not repeated. This may be done best by companies or community action agencies with air leakage testing and air sealing experience. Witnessing 58 ASHRAE Journal compliance with the air leakage specification is the responsibility of the mechanical engineer. Builders can constantly monitor results for any completed section with easy to use door fan equipment that gives them the results they need. Sealing Air Leakage Sites Spray foam and a sprayable water-based elastomeric fire-rated rubber coating are the two most effective ways to seal the vast expanses that must be made tight in high-rise buildings. Several makes of rubber provide a fire rating but must be sprayed over a backing of rock-wool or foam. The advantage of both is that they move with the building and adhere well to dusty, irregular, surfaces. An essential element of air sealing is using a door-fan air leakage measurement device that provides instantaneous feedback for installers as work progresses. In some cases, effective air sealing can only be accomplished when the schedules of construction are taken into account. It is not possible to come in after drywall installers are finished to attempt to air seal. The air sealing must be done as they proceed since successive steps often cover up the opportunity to create effective seals in key locations. Conclusion It is essential to begin fixing existing buildings and designing high performance new buildings now. It is unnecessary to ashrae.org April 2009

ASHRAE Journal - April 2009

Table of Contents for the Digital Edition of ASHRAE Journal - April 2009