Water Penetration Control

  • Cladding (wood siding, brick, vinyl siding, stone, stucco, etc.) forms a first plane of protection and is detailed to limit the amount of water that gets past it.
  • The sheathing membrane is a second plane of protection that prevents water from entering the building envelope and allows water vapour to drain and diffuse out of a wall assembly.
  • Several materials can be used as sheathing membrane including asphalt-impregnated paper (tar paper), spun-bonded polyolefin (housewrap) and self-adhering or liquid waterproof membranes.
  • A rainscreen (a drained and vented air space acting as a capillary break between the cladding and sheathing membrane) is required in wet climates, to allow water that gets past the cladding to drain to the exterior and to allow the space to dry. A rainscreen design is recommended for all walls that will frequently be wet.

All walls are required to have a primary and a secondary line of defence against rain penetration. The first plane of protection is typically the cladding (for example, siding, masonry or stucco). It includes accessories such as trim or caps that are part of the cladding system. The second plane of protection ordinarily consists of a sheathing membrane or insulating sheathing, flashing, sealants and other materials, and is intended to:

  • intercept any water that has penetrated the first plane; and
  • drain those relatively small quantities of incidental water to the exterior.

Both the first and second planes of protection must provide adequate protection of the wall assembly where penetrations have been made for windows, doors, ventilation ducts, piping, wiring and electrical outlets; and at the interface with other wall assemblies. These penetrations and interfaces at openings are where most wall assemblies develop problems over their lifetime.

Deflection: components and features of the building envelope (such as overhangs) that limit the exposure of assemblies and details to water running along surfaces of the building envelope and divert it from the assembly (shingle lapping materials and drip edges and flashings).

Drainage: surfaces (such as the back side of cladding, or the sheathing membrane) within assemblies that redirect any water that enters the envelope downwards and out to the exterior.

Drying: features that encourage vapour diffusion and air movement to transport water vapour, and evaporate moisture from surfaces. These features include vapour permeable materials (which allow water to diffuse to the surface of materials), and in some cases, vented or ventilated air cavities located on the interior side of the cladding.

Source : Canada Mortgage and Housing Corporation (CMHC)