What is a Rainscreen?
The term rainscreen (rain screen) can be defined as the first interruption between conditions that exist on the outside of a walled building and conditions that are required on the inside of a walled building. In order to be effective at managing moisture, the rainscreen needs to stand off from the moisture resistant surface of the structural back up wall. There must be a permanent, predictable void between the backside of the rainscreen and the exterior facing surface of the moisture resistant material (WRB) on the surface of the structural backup wall. This void is the rainscreen drainage plane.
What is Rainscreen Drainage Plane?
The exterior surface (veneer) of the rainscreen building envelope is not expected to be waterproof, but it will block most of the moisture. The rainscreen drainage plane behind the veneer should deal with any moisture that penetrates into the building envelope, either from the outside or the inside. An effective rainscreen drainage plane will be both a predictable rainscreen drainage plane and a predictable pressure equalization plane.
According to Toolbase.org, “Rainscreens effectively ‘drain the rain.’ They control powerful building wetting forces-gravity, capillary action, and wind pressure differences.” That’s the beauty of a rainscreen building envelope, and probably the reason that so many new structures feature them.
Unfortunately, much of the material that is passed off as rainscreen drainage plane violates some of the necessary criteria to be effective. Remember, an effective rainscreen drainage plane must be predictable. It shouldn’t collapse under pressure. When cladding is applied to furring strips or mesh drainage plane, there is a tendency for waviness and nail drive-through. Most experts agree that at the very minimum, a 1/8” predictable void is needed for effective drainage. Is a patterned WRB providing this kind of space? Additionally, moisture that is forced to wander around a maze of interwoven fiber certainly isn’t exiting the building envelope very quickly! Even a small amount of moisture left for any period of time in the building envelope will cause problems!
One other issue related to effective rainscreen moisture management is the weep at the bottom of the wall. The best rainscreen drainage plane will be rendered impotent if the water can’t escape the building envelope when it reaches the bottom of the wall. Effective weeps have three critical characteristics: they are plentiful; they are at the lowest point of the wall; they allow water to flow quickly out of the building envelope.
Finally, when it comes time to select rainscreen drainage plane for your project, are you dealing with a company that specializes in rainscreen drainage, or are they just moving product? Will they work with you to design an effective rainscreen drainage “system” for that project? Can they provide all the material, and have these materials been designed to work together?
These are all important issues that are too often not considered when designing a building with rainscreen technology.