Rain Screen Specialists
A successful method for deterring rainwater intrusion into walls is the rain screen approach. You have probably seen them before-even a rain fly over a tent is a simple example of rain screen. Rain screens shed most of the rain and manage the rest, preventing moisture intrusion and the resulting premature decay in homes. Rather than attacking the symptoms of moisture intrusion, rain screens tackle the source-the forces that drive water into the building shell. By neutralizing these forces, rain screens can withstand extreme environments. They appear to be effective in any climate and handle any weather condition short of a disaster.
They control powerful building wetting forces-gravity, capillary action, and wind pressure differences. The exterior cladding deters surface raindrop momentum. It is typically porous with several air bypasses. An airspace separates the cladding from the support wall. The airspace decouples most of the cladding from the support wall, thereby reducing splash and capillary moisture transfer. Large, protected openings (e.g., vents, or weep holes) positioned at the top and bottom of the wall promote convective airflow, allowing moisture to quickly drain or evaporate from the air cavity. The exterior face of the support wall is protected with a drainage layer to further protect against any moisture that bypasses both cladding and air cavity. The wall airtightness (i.e., sealed assembly) buffers the remaining differential air pressure force. Simple rain screens rely on the airspace next to the drainage plane to quickly and freely remove water from the wall. In a brick veneer wall, for example, rain water passing through cracks can trickle down the back face of the veneer to either leak out of weep holes, evaporate, or be reabsorbed into the masonry.
There are four basic approaches to water penetration control in buildings
traditional, solid assemblies that shed most surface water, effectively absorb the remainder, and subsequently release absorbed moisture as a vapour. Examples include solid concrete, masonry, and timber structures. This approach has variable effectiveness in most climates.
Surface designed to completely shed surface water with no moisture penetration. Examples include barrier-type exterior insulation finish systems (EIFS) and stucco or clapboard walls built without a drainage plane (e.g., house wrap, building paper). These are effective in climates with less than 30 inches of precipitation annually.
A drainage plane or moisture barrier located between the exterior cladding and the supporting wall that provides redundancy of moisture resistance. Examples include typical stucco and clapboard walls built with a drainage plane. These are effective in climates with an annual precipitation of less than 50 inches.
A moisture-management system incorporating cladding, air cavity, drainage plane, and airtight support wall to offer multiple moisture-shedding pathways. Rain screens diminish the forces attempting to drive moisture into the wall. There are two types of rain screens: simple rain screens and pressure-equalized rain screens (PER). Examples include brick veneer cavity walls, furred-out clapboard walls, and drainable EIFS. Simple rain screens are effective in climates with an annual precipitation of less than 60 inches; PERs are effective in climates with an annual precipitation of 60 inches or more.