The building code in British Columbia finally evolved to requiring the “rain-screen” wall system in areas of the province subjected to prolonged and extreme wet weather exposure. There has been an evolution of wall assemblies and detailing over the past 25 years but the underlying goals have remained constant. When looking at any exterior wall with weather exposure in the Lower Mainland or Vancouver Island one must consider the “4 – D’s”. Or, specifically, DEFLECTION, DRAINAGE, DRYING, and DURABILITY.
DEFLECTION involves the visible components of the wall facade of the building that are the primary line of defense against inclement weather. These are the cladding (brick, stucco, siding, paneling), windows, doors, and roofs. And, the sealants and flashing interfaces in between these components. The primary goal is to shed and repel the wind driven rain at the first point of contact. Done well almost 100% of the rain could be managed with deflection. However with dozens, if not hundreds, of material interfaces on a building elevation there is a lot of opportunity for some minor moisture penetration to occur. This moisture, over time, can accumulate and damage structural and thermal materials behind the cladding.
The rain-screen wall introduces a small gap behind the wall cladding. This gap allows moisture that penetrates the exterior cladding to DRAIN behind the cladding. The wall cladding is generally segmented between each floor with a flashing that allows any water penetrating a wall area to exit the wall at each floor line. Done correctly you can achieve the effect of a pressure equalized cavity. Wind may push rain into the wall cladding but, once inside the cavity, gravity takes over and allows the moisture to drain within the cavity and not be pushed further into the building.
Further, moisture that is present behind the cladding can DRY as the cavity can vent bottom to top when the cladding warms up and causes a convective airflow of rising warm air within the rain-screen that harmlessly vents out at the floor line above.
And, last, rain-screen components are made of very DURABLE materials that can tolerate some moisture exposure. Rain-screens use cedar or preservative treated wood components or galvanized sheet metal or steel components. Durable waterproof membranes are used at interfaces with windows, doors, and railings and at transitions in plane (corners, saddle details) where moisture may collect.
A rain-screen wall that adheres to the 4-D concept will not rely on any visible sealants to maintain weatherproof performance!