The aim of this three-part Good Building Guide is to give the best advice on achieving airtightness in new buildings. It is based on data obtained from laboratory testing and observations made while undertaking air leakage audits in buildings. This first part (8 pages) describes the common air leakage paths and sets out the principles to follow when developing energy efficient construction details. Parts 2 and 3 (available separately) provide practical guidance on airtightness techniques appropriate to most building types. Part 1