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We know that different components in the building envelope control water, air, moisture and heat flow. Although these components are referred to as control layers, it's worth knowing that the National Building Code of Canada refers to these layers as "barrier systems".

When dealing with these systems, it's helpful to remember what each layer is meant to control:

• The Water Control layer:
  • sheds bulk water away from the building, using materials to create a three dimensional drainage plane that is also called the rainwater boundary. The water control layer includes materials like flashing, membranes, rain screens, foundation drainage, house wrap, eavestrough and sealants.
• The Thermal Control Layer:
  • controls the flow of heat through an assembly - insulation materials are the main component of this layer.
• The Air Control Layer:
  • controls the flow of air that moves because of pressure differences. This layer includes air barrier fabrics, membranes, sealants and caulking;
• The Vapour Control Layer:
  • controls the movement of water vapour that is transferred by diffusion. The most commonly known vapour barrier is polyethylene but there are other materials that do a similar job, such as vapour retarder paints and smart vapour barriers of polyamide nylon.