Water Control Layer

I have written previously of the importance of understanding a building’s control layers. Click here to revisit that article.

A brief reminder. The building envelope control layers, in order of importance, are:

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

One important thing to consider is that the external cladding is generally not one of these cotrol layers. The function of the external cladding is to:

  • Make the building look nice,
  • to physically protect the control layers, and
  • to protect the building from exposure to Ultra Violet light.

If we rely on the cladding as our Water control layer we are asking for perfection in its physical properties and perfection in its application. Any caulking around windows, doors and intersections must be perfect and remain perfect for the lifetime of the building. How likely is that to happen?

Cladding leaks. Yes, the cladding keeps most of the water out but it also lets some in. This is why in our typical timber framed construction the Water control layer is the wall wrap. This is also why attention should be paid to adequate installation of the wall wrap. Overlaps in the wrap should allow water to drain away. Joins and penetrations should be flashed and taped.


Here are a couple of photos taken on a building site after some rain. The cladding was not yet installed but it is a good example of the failure of the wall wrap as a Water control layer when not installed properly. The wall wrap was not taped and did not extend past the bottom plate.