Rainscreens in BC

Here's a link to an August 2008 newsletter about rainscreens up in British Columbia. Lots of great info about rainscreens and ventilation behind exterior cladding that is pertinent everywhere, not just in Canada. We recommend reading the whole thing, but here's a particularly salient passage:

Rainscreen Walls Benefit From Cladding Ventilation

The function of the airspace between the cladding and the back-up construction of a rainscreen assembly is to provide drainage, ventilation, and a capillary break. Drainage removes any bulk water that penetrates past the cladding. However, drainage alone cannot remove small droplets of bound water or water absorbed into the sheathing, strapping or cladding.

Ventilation is shown to have a significant impact on this aspect of performance in rainscreen walls. A 3/8? to 3/4? continuous open gap behind the cladding, which is common for most residential construction, is generally sufficient. This airspace is typically created using vertical strips of treated wood strapping/furring or metal girts. Proprietary products are also available to create this gap.

Some proprietary drain-mat products are impermeable to vapour, therefore ventilation is critical and extra care must be taken not to block any of the vent openings with these systems. Cladding ventilation reduces relative humidity within the rainscreen cavity and serves to help dissipate the small amount of water that may get past the cladding and through penetration details from time to time.

Absorptive claddings such as brick, stucco, or cement board can also introduce vapour into the cladding cavity and ventilation can remove this moisture as well. Where ventilation rates are insufficient, an inward vapour drive can result in increasing moisture within the wall structure to the interior. To ensure adequate ventilation, large and unrestricted vent openings should be provided through the cladding.

Continuous cross-cavity strip vent openings are ideal. Discrete vent openings (such as the ones used in brick veneer) are also effective but they can reduce the ventilation flow and result in moisture ingress if designed or constructed incorrectly. Venting of the wall cavity at both the top and bottom of each wall region is the most effective–and allows for continuous natural ventilation.

Cor-A-Vent Siding Vent is the perfect product for a rainscreen - a continuous cross-cavity strip vent that allows moisture to drain and air to flow through. And we'll be introducing a new addition to the Siding Vent family in 2009, so check back here soon for all the details.