In 2019 the National Construction Code introduced the requirement for vapour permeable, pliable building membranes for climate zones 6, 7 and 8.

This requirement was introduced to mitigate the problem of moisture accumulation in wall assemblies. Moisture can enter the wall assembly in either liquid form, (rain, flood, ground water or leaks) or in vapour form, (air leakage = 97%, Vapour diffusion = 3%). The vapour form of moisture ingress leads to condensation within the wall assembly.

A pliable building membrane has two jobs to do.

  1. Keep water out.
  2. Let water out if it gets in.

Keep Water Out: The wall wrap is a weather resistant barrier, therefore it should be adequately fixed to the structure and taped and flashed around openings to prevent the ingress of liquid water. A percentage of water always gets passed the external cladding so it is important for the wall wrap to protect the building. The water that gets passed the external cladding should be impeded by the wall wrap and have the capacity to drain down and out of the wall assembly.

Let water out if it gets in: The integrity of the wall wrap is not going to be perfect all the time. Breaches in the house wrap can be via non-taped joints, tears in the membrane, and even the staple holes. The percentage of water that passes through the wall cladding can enter the wall assembly through these holes. A vapour permeable membrane will allow this water to eventually dry out. If the membrane is not vapour permeable the water remains in the wall assembly and can cause problems.

So why is vapour permeability important? Because wall cavities do get wet, roofs leak, condensation occurs, plumbing leaks, construction materials are installed wet and internal moisture loads can be very high. However it happens, walls get wet and require a way to dry out. When a wall can’t dry out, it becomes vulnerable to moisture-induced damage including mould and rot.

I have recently done a backyard building science experiment with pliable membranes to test their permeability. More details in my next blog. Click here to see my previous post about vapour permeability.


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