Describes the results of air tightness tests of various service penetrations through a polythene vapour barrier and associated insulation. The service penetrations studied were insulated and uninsulated ventilation ducts, plastics, pipes, electrical boxes and electrical conduits. Various methods of sealing the penetration were tested for each case. Recommendations are made for choice of sealing method based on performance and case of installation. Examples are given of the effect of leakage through service penetrations on total building leakage.