Let's look at some reasons why this is true:
1) 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 when it rains.
2) A weather barrier is installed on the exterior face of the back-up wall to further protect against any moisture that bypasses both cladding and air cavity. This airtight wall buffers the remaining differential air pressure.
3) Compartment seals should be installed at building corners and parapets. Adhesive-backed, oil-coated, expanding foam seals are used for this purpose.
4) Cladding that does not stand off from the wall sheathing to create a cavity is not considered to be a rainscreen. However, a masonry veneer can be called a rainscreen wall if it is ventilated.
5) In some cases a rainscreen wall is called a pressure-equalized rainscreen wall where the ventilation openings are large enough for air pressure to nearly equalize on both sides of the cladding. This term has been criticized as being redundant and only useful to scientists and engineers.
6) Numerous terms have been applied to rain screen walls including basic, open, conventional, pressure-equalized, pressure-moderated systems or assemblies. These have caused confusion as to what a rain screen really is, but all reflect the rainscreen principle of a first and second line of defense. One technical difference is between a plane, a gap of 3/8 inch or less; and a channel, a gap of more than 3/8 inch.
If you have questions about Rainscreens or would like to learn more, please visit our facades page at www.WesternFacades.com.