Vapour control and vapour barrier installation

It is well known that using an AVCL (Air and Vapour Control Layer) in a building envelope will help to gain optimum energy efficiency and thus promote building longevity. This, however, can only be realistically achieved if the standard of installation is high.

Over the course of a normal winter’s day up to 0.7kg of water vapour can pass through a 1mm gap over 100mm building component thickness, across 1m length at 10Pa pressure difference. The following article covers some best practice experience sharing by the DuPont™ Tyvek® Technical team on how to ensure a successful installation.

Continuous sealing

To achieve a satisfactory level of airtightness, vapour control and vapour barrier must be sealed at all laps, junctions and penetrations. To reduce the risk of inadequate sealing, it is recommended to keep penetrations to a minimum and consider a services void for electrical fittings, e.g. sockets, light fittings, etc. A services void can be created by spacing off the internal lining (plasterboard) from the vapour control and vapour barrier. Timber battens, minimum 25mm may be used for this.

Not all penetrations are avoidable

For penetrations, such as windows, doors or pipework that cannot be addressed with a services void, careful attention must be paid to sealing. Those penetrations can be sealed with standard acrylic tape (Tyvek® single sided) or flexible tape (DuPont Flexwrap™ made of an elastic grade Tyvek® with a butyl adhesive). The advantages of flexible tape compared to conventional tape include: Faster installation, as it requires less tape strips to be cut and applied. Safer installation due to its flexibility, the tape easily adapts to shapes.

To ensure a good bond and tight sealing, it is crucial to press the tape firmly onto the membrane and to the item being sealed.

Repair all damage

Any accidental rips, tears or perforations made in the membrane should be immediately made good to avoid air leakage after installation. The damaged area may be repaired with tape, but extensive damage should be covered with an extra piece of membrane laid over the damage and sealed with tape. The new ‘patch’ should extend at least 100mm around the damage in all directions.