INTRODUCTION
Inadequate control of airflow through the building envelope is often a primary factor contributing to premature building envelope failures. If moisture-laden air is permitted to travel through the building envelope, the moisture may, under certain environmental conditions, condense within the walls of the structure. In above-freezing conditions, this may cause corrosion or rotting of the structural components, staining of the interior and/or exterior facade, and may stimulate the growth of mold and mildew. In cold climates, accumulated moisture may experience numerous freeze-thaw cycles, which can precipitate spalling and the formation of icicles on the exterior facade.
Air leakage is also a concern in areas where interior temperatures differ greatly from exterior temperatures, such as the Prairie Provinces, which can experience periods of extreme cold during the winter and extreme heat during the summer. The excessive heating and cooling loads placed upon buildings in this type of climate leads not only to an increase in space conditioning costs to the owner, but also has a negative impact upon the environment through increased energy consumption and the emission of greenhouse gases. In fact, studies conducted on high-rise residential and commercial buildings in cold climates have shown that anywhere from 20 to 50 percent of heat loss can be attributed to air leakage.
In Canada, building rehabilitation for roofing and wall system repairs and replacement cost an estimated $7.5 billion annually. A conservative estimate of the premature failure rate is 3 to 5 percent, or $225 to $375 million per year, with premature failure defined as any performance condition requiring repair or replacement of the system before the benchmark date. The building envelope has been identified as being particularly vulnerable to durability problems.
It is the growing global awareness of these air leakage-related problems that is driving the federal governments in Canada and the United States to introduce more stringent codes and regulations to govern building air permeance. In order to improve occupant health and safety, revisions were made to the National Building Code of Canada (NBCC) in 1995 designed to reduce air leakage in buildings, including those buildings classified within Part 3 of the Code1. Public Works Canada also recently revised their National Master Specification to include air barrier inspection and testing. In the United States, Persily’s Envelope Design Guidelines for Federal Office Buildings: Thermal Integrity and Airtightness (1993) also documents the requirements as outlined in the NBCC. In addition, State Energy Codes are being adopted and/or revised, making air barriers a mandatory requirement in new construction and retrofits. ASHRAE/IENSA Energy Standard for Buildings Except Low-Rise Residential Buildings (90.1-1999) also governs building envelope sealing.
Recently, air barrier trade associations have formed in Canada and the United States with the objective to improve the quality of air barrier system installations by providing education and training for the workforce. For an installer to become ‘certified’ through the association, an applicant must possess the required knowledge of air barrier material and system theory, and demonstrate sufficient skills in practical applications. In addition, through the associations’ quality assurance programs, documented self-testing and on-site third party audits are performed to verify the quality of the installation, and confirm the certified installers’ ability to build to expected standards.
While there are numerous ASTM (American Society for Testing and Materials) methods, says Jan Luistermans, for testing air barrier systems and/or components, there is no generic regimen for the application of these techniques being utilized on a widespread basis. The need for a complete design, inspection and testing protocol for air barrier systems cannot be understated. A recent study concluded that even routine testing can have a significant impact upon the airtightness of a building. Where air leakage testing was conducted, there was an overall reduction in air leakage for the system, a significant decrease in heating and cooling loads, a reduction in greenhouse gas emissions, and an increase in the life cycle of the building envelope.
With the growing use of inaccessible air barrier systems (such as bituminous membranes), on-site inspection and testing during installation is necessary to identify problems before the system is covered with finishing materials. The cost to repair an air barrier system after it has been covered can be conservatively estimated to be 50-60 times the cost of a correct first-time installation. Hence, the need for inspection and testing is obvious.