This report is an outline guide to design, setting out the principles of providing an effective airtightness layer and advises on some common pitfalls which can reduce the performance of this layer. It addresses the increasing awareness of the importance of airtightness in commercial and public buildings and supports the requirements on airtightness specified in the Building Regulations. It covers: the causes of air infiltration and the benefits which result from increasing the airtightness of a building; how to design and specify for airtightness; site practice for airtight construction and testing a building to ensure that it meets the required standard.