To get air-tightness in a build, should both internal and external wraps be used, or is the external wrap enough? Is this is case of 80/20 rule, i.e. pay 20% extra (external wrap only) to get 80% of the benefit?
I'm planning a new house and aiming for energy efficiency so am looking at applying passive house principles. But I'm also trying to minimise changes to standard construction techniques to keep complexity and costs down for my builder.
Full-on certified passive houses generally have for the external walls both an external vapour permeable wrap and internal airtight vapour retarding/blocking wrap. The internal wrap is battened off with gyprock installed over the battens to create a services cavity. Plumbing and electricals are then installed in the services cavity so that penetrations through the airtight wrap can be kept to a mimimum.
That additional internal wrap and services cavity is non-standard construction so is not cheap.
I'm wondering whether a high level of air-tightness can be achieved using just the external wrap. Obviously this needs to be both air-tight and vapour permeable (e.g. Pro Clima Solitex Extasana or Enviroseal ProctorWrap) and everything needs to be taped up carefully. Presumably the wrap would then be run over the top of the walls and under the roof trusses to form a continuous barrier.
Be very interested to hear from anyone who has experience with this and with performing blower door tests on buildings having only external wrap.