Vapour Control Layer

The third most important building envelope control layer is the Vapour control layer. Here’s the list of control layers again as a reminder:

  1. Water
  2. Air
  3. Vapour
  4. Thermal

If Vapour gets trapped in a structural assembly it can condense on an internal surface. Again, this MOISTURE causes health issues and structural damage.

It’s worth knowing that 97% of the airborne moisture problems in the structural assembly is due to air infiltration. Once air is dealt with, most of the problem disappears. The vapour issue, although smaller than the air issue, is worth considering particularly because it is also related to the water and air issues.

Remember Rule 7 of Building Performance – Everything gets wet, let it dry. A typical house has plasterboard or timber lining on the inside face, which is vapour permeable. This allows vapour to travel through. Recent changes to our building code now require the wall wrap to also be vapour permeable. This is a good thing, but it is vitally important that vapour ‘permeability’ is not confused with ‘breathability‘. House wrap membranes with tiny holes are classed as breathable but they are not vapour permeable. It is the pereability that allows the drying. Wrap with tiny holes actually allows air infiltration which is something that we don’t want.

If Moisture is present in the structural assembly due to liquid water infiltration, air infiltration or vapour diffusion, the vapour permeability of the structural assembly becomes very important. The trapped moisture mostly needs to dry through evaporation and this can only happen if there is a vapour permeable material to allow the drying to occur in the direction of the vapour drive. (Rule 2 – Moisture moves from warm to cold. Rule 3 – Moisture moves from more to less).

It is safe to assume that our structural assemblies are most often going to fall short of perfection. This is why allowing for a drying capacity such as vapour permeability will increase the durability of the structure and be beneficial for the health and wellbeing of the occupants.

Click the links below for further information:


A photo of some of the membranes that I tested for vapor permeability. The links above will provide more information.

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