Rainscreen Exterior Wall System
By Robert G. Wilkin, P.E.
The Massachusetts College of Pharmacy and Health Sciences is expanding their Worcester facility with a new five-story building utilizing a rainscreen exterior wall system. With conventional exterior walls, the intent is to limit the amount of water allowed to pass through the outside face of the cladding, be it brick, wood, EIFS or some other material. Wind driven water penetrates the conventional wall materials through small and unanticipated holes in the materials. There is, or should be, a waterproofing membrane and through wall flashing system built into the wall system to collect and direct water penetrating the face of the wall back to the exterior.
The principle of a rainscreen is to provide an open wall face over a waterproof barrier. The open face allows water and wind to pass through the face of the wall. However, because of the design, when the wind blows, the air pressure behind the wall is equalized so that only a limited amount of the wind driven rain trickles down behind the face of the wall. The waterproofing system then directs the water out.
Obviously, the waterproofing system in all wall cladding should be considered the primary water barrier whether on the face of the wall with metal panels, with even older EIFS systems, or behind the face of the wall such as brick veneer. With rainscreen walls, it is the only waterproofing system since the wall face is open to the weather.
The rainscreen on this building was designed with a waterproof system of a self adhering air vapor barrier [AVB] material. The locations of penetrations through the wall (for clips and fasteners, for example) and changes in the plane of the wall are always the most sensitive in terms of exposure and careful attention to the flashing details and installation are very important.
The face of the cladding on this building is a polymer wall panel screwed to aluminum framing. The panel joints are open about 1/2”. The AVB was turned into all openings, such as at the windows, where an aluminum flashing was applied to cover the AVB and provide a frame around the windows for perimeter sealant. The window perimeters were sealed on the interior with AVB membrane to complete the air barrier and to drain any leakage around or through the windows to a pan flashing.
The importance of the AVB membrane in a rainscreen is paramount. The seams must be lapped in the direction of water flow and each seam must be tight. All seams on this project were sealed with a bead of compatible sealant for added protection. The membrane must be UV stable for long term viability even though it is not directly exposed to sunlight. The membrane is the primary waterproofing system for building walls and, as such, must last for the design life of the building. UV rays can bounce around and affect the materials that are out of direct sun light causing deterioration to occur.
For a successful rainscreen wall system the details are all important, as well as the experience/expertise of the installer to provide a water tight installation. It is important to follow the expected water paths to provide seals and flashings that will deflect the water away from the interior for the expected life of the cladding system.