Air leakage of defects in the vapour barrier of compact roofs

The harsh Norwegian climate requires buildings designed according to high standards. The airtightness of the building envelope is crucial to attain an energy efficient building and to avoid moisture problems. A considerable part of building defects registered in the SINTEF Building defects archive are related to compact roofs.
Flat compact roofs with insulation of EPS or high-density mineral wool between the vapour barrier and the membrane roofing are a common solution particularly in large buildings. In order to retain the membrane roofing from wind forces, the membrane roofing is fastened to the load bearing construction beneath the insulation by use of long screws that penetrate the vapour barrier. Increased requirements related to the airtightness of buildings in the Norwegian building design codes has drawn the focus on airtightness also for compact roof constructions. In the recent years, it has become common to use adhesive tapes to ensure increased airtightness of joints and penetrations in the vapour barrier. However, practitioners have questioned the use of adhesive tape to ensure airtightness, as the vapour barrier at the same time is penetrated by screws. The presented study is trying to provide some answers to the matter.
A laboratory investigation using a special test rig has been conducted in order to measure the air leakage of screw holes and overlapping joints in the PE foil. Different screws and load bearing materials were investigated. Additionally, the air leakage of an overlapping joint was tested for different overlapping widths.
The results of the measurements are used to calculate the air leakage through compacts roofs of two model buildings of different size. The calculations show that air leakages through the screw holes and overlapping joints in the vapour barrier had a minor effect on the airtightness (n50) of the model buildings.
The results from the study imply that a given a vapour barrier clamped between two plane materials it is unnecessary to use adhesive tapes in overlapping joints of the vapour barrier. However, the application of adhesive tape is advisable in order to avoid air leakages between the vapour barrier and other building components, e.g. transition between roof and cornice or penetrations in the vapour barrier such as ventilation channels.