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 construction 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 designers and builders. A number of important details and designs are collected and presented to the building industry.