Presents a demonstration study showing the design and construction details necessary to achieve specified building envelope airtightness performance criteria. Follows the design and construction process and advises on modifications necessary to achieve the target specifications. Bases the study on two buildings with quite different end uses and airtightness specifications - an archival storage facility and air conditioned high-tech office building. Also summarises experience gained from testing over 300 buildings and provides generalised advice for designing-out leakage paths and eradicating those that unavoidably occur.