Installing the right membranes, right

Choosing the right building wrap, with the necessary performance properties and compliance with national standards (AS/NZS 4200.1 and AS/NZS 4859.1), is only the first step. Installing it incorrectly can have near- and long-term impacts on the function of the building, including a reduced R-value, leaks, draftiness, and condensation. Different manufacturers will provide different advice in their literature. Ametalin provides comprehensive installation guides, and further details can be found in AS 4200.2 Pliable building membranes and underlays, Part 2: Installation. Important instructions include:

  • Membranes should be stored in a dry place out of the weather prior to installation. Once installed on the building frame they are exposed to the force of wind and rain, and the effects of UV radiation in sunlight. The remaining components of the wall should be completed as soon as possible to avoid damage which may require repairs.
  • The functions of vapour and air control require a continuous seal at all overlaps, with carefully applied high quality pressure sensitive tape such as Ametalin Reinforced Insulation/Ducting Tape.
  • Sarking acts as a water control membrane (as classified according to AS/NZS 4200.1) within roofs. Under metal sheets it can be installed in either orientation, and overlapped by 150 mm or by 50 mm with taped joins. As tile underlay it is installed horizontally and only requires 150 mm overlaps (this also applies in walls when intended as a weather barrier). No more than 40 mm of sag between rafters is required to ensure adequate liquid water drainage.
  • When a reflective membrane is chosen for thermal control, the system R-Value needs to be calculated as per AS/NZS 4859.2, and the reflective face must face an air gap. Under metal roof sheets, extra care should be taken to avoid condensation on the underside of sarking due to rapid cooling of the roof at night lowering sarking below the dew point where it contacts the sheets.
  • When a reflective membrane is chosen for thermal control, the system R-Value needs to be calculated as per AS/NZS 4859.2, and the reflective face must face an air gap. Under metal roof sheets, extra care should be taken to avoid condensation on the underside of sarking due to rapid cooling of the roof at night lowering sarking below the dew point where it contacts the sheets.
  • Major penetrations in the building envelope, such as windows, require systematic overlap and sealing of the membrane around the frame to facilitate drainage and preserve the functions of the membrane (e.g. thorough taping around cuts to restrict air exchange when installed as reflective insulation).

Building wraps are incorporated in building envelopes for the whole life of the building, so a little care in selection and installation will invariably pay off down the road. The relevant standards are there to help Australian designers and builders to get it right.