Sunday, December 11, 2005
Rainscreen Technology
Vancouver, BC is the largest city in the province of British Columbia and the third largest city in Canada. It is surrounded by water on three sides and is nestled alongside the Coast Mountain Range. In addition to a spectacular natural scenery and a bustling metropolitan core as well as boasting one of the mildest climates in Canada, Vancouver records one of the heaviest rainfalls in the world. Typical rainfall patterns average 169 mm in November and December and 135 mm in February and March. In Vancouver anything can happen to you, but dying of thirst is not one such thing. Vancouver, furthermore, holds one other record – although this one is very seldom publicized on tourism websites: it is the leaky condo capital of Canada.
To be fair to my hometown, Vancouver is not by any means the only place in the world to have experienced this kind of problem. Toronto and Winnipeg have suffered of the same calamity, and so have Chicago and London, England. The problem here, however, is that a lot has been said and very little done to correct building envelopes imperfections common to so many high-rises, until the aggregate estimated cost of repairs to leaky condos has exceeded CAD $1 billion. Engineering, now, has come to the rescue changing things finally for the better with a new advance in building envelope construction and repairs. It is called Rainscreen Technology.
Most of the high-rise towers have been built using face-sealed outer wall assemblies. Developers refer to ‘cladding’ as the material or component of the wall assembly that forms the outer surface of the wall which is, thus, exposed to the full force of the environment. Face-sealed outer wall assemblies have invariably relied on a strategy for rain penetration control based on the elimination of holes through the cladding. As face-sealed walls are designed and engineered to deal with exterior moisture, in the form of rain, by sealing the exterior of the wall and preventing water from penetrating past the face of the cladding, if water does penetrate past the cladding it cannot readily drain out of the wall and remains, therefore, stagnant within the assembly. Here, water can damage moisture sensitive material and components. It is, therefore, essential to insure that no water penetrates the outer cladding.
The water management strategy created by face-sealed outer wall assemblies can work in certain conditions where the wall is in a protected location and receives little exposure to wetting. However, as we have so disgracefully learned first-hand here in Greater Vancouver, in most situations face-sealed walls do not perform well. This is so because it is extremely difficult to fully seal the exterior cladding and ensure that no water will enter. The increased exposure experienced by high-rise buildings further limits performance expectations for this type of wall.
In contrast, rainscreen walls manage water in a different fashion. The exterior cladding is still intended to deflect most of the water that contacts the wall. However, a cavity is provided behind the cladding. If water does penetrate the cladding it reaches the cavity and cannot move further into the wall assembly. Instead water in the cavity will drain down on the inside face of the cladding or on the waterproof membrane at the other side of the cavity, and it will be deflected out of the assembly at a cross-cavity flashing. In practice the cavity acts as a capillary break to remove the capability of water to stagnate inside walls. The end and most important result of all this is that with rainscreen walls it is not essential that the outer cladding be completely sealed. Imperfections are acceptable, and this will surely make a lot of developers happy – and a great many condo owners even happier.
Luigi Frascati
Real Estate Chronicle
