© 2010 ASHRAE. ABSTRACT This paper summarizes projects that deal with important requirements for achieving an airtight building. The major themes are: (1) problem areas with respect to airtightness, (2) how to find leaky areas and improve their airtightness, (3) good and airtight solutions, and (4) durability of airtight layers and joints. First, critical details for airtightness are identified and analyzed. Poor airtightness is often caused by lack of consideration of airtight details in the design phase, by poor workmanship, or by lack of understanding of airtightness during construction. The consequences of poor airtightness are described. Second, different methods of searching for air leaks are investigated and evaluated. Searching for leaks during the construc- tion phase is a good way of improving airtightness, since the airtight layers are more accessible during erection of the building than when the building is completed. Third, several buildings with very good measured airtightness are investigated in order to provide airtight solutions to design- ers and builders. A number of important details and designs are collected and presented to the building industry. INTRODUCTION Interest in airtightness has increased in Sweden over the last ten years. One reason for this is the increasing focus on energy use and passive houses 1 . Airtightness is linked to energy use in different ways, depending on whether the air passes through the building envelope or between different parts of the interior of the building. If air leaks (uncontrolled) into the building envelope, the performance of the insulation can be degraded, which can lead to cold inner surfaces. Cold surfaces, such as floors, can result in a need to increase the indoor temperature to compensate for increased discomfort due to radiational losses and, thus, to an increase in energy use. In addition, if the building is not airtight, the ventilation rate may be increased, particularly in windy or cold weather. Apart from energy, there are other reasons for having airtight buildings. If humid air is allowed into the building envelope, this can lead to high moisture conditions and, as a result, to mold, deterioration, and increased emissions. Figure 1 shows an interior roof surface with mold growth due to humid air. Another reason for having good airtightness is that it is a prerequisite for correct performance of the building's ventila- tion system. Without an airtight envelope, it is difficult to filter the air, to stop gas or particle transport inside the building and through the building envelope, or to ensure an adequate venti- lation rate in all parts of a building (particularly in windy weather). Critical parts of a building envelope are presented in Sandberg and Sikander (2005). The leakage pathways are illustrated in Figure 2. In addition to the leakage pathways 1. Passive houses are buildings, in which a comfortable temperature in winter as well as in summer can be achieved with only minimal energy consumption. Different countries have different require- ments for passive houses. Methods and Materials for an Airtight Building Paula Wahlgren, PhD Eva Sikander Paula Wahlgren is an assistant professor at the Division of Building Technology, Chalmers University of Technology, Gothenburg, Sweden. Eva Sikander is a researcher at SP Technical Research Institute of Sweden, Borås, Sweden.