Rain Screen Wall Systems
A rain screen wall design will have two distinct components: an outer leaf and an Inner Leaf. The outer leaf or cladding element sheds and controls most of the rainwater. Joint designs in the outer leaf have openings through which allow the cavity to pressure equalize with the exterior pressure. This pressure equalization thus eliminates the water from entering the cavity that would normally occur due to a pressure difference across the outer leaf. The more joint openings, the easier it is to achieve pressure equalization. However, the more openings in the outer leaf also will allow a greater chance of rainwater entry through means other than pressure difference across the outer leaf. The Inner leaf performs multiple functions. It is 1) a water barrier, 2) an air barrier 3) a thermal barrier and 4) possibly, a vapour barrier as well as 5) the wall structural element.
In a PERS system, the allowable amount of water entering the cavity and contacting the air barrier is strictly limited and be achieved through compartmentalization, smaller and protected joint openings in the outer leaf.
In a D/BV system, the joints are more open and the water entering the cavity and contacting the air and water barrier is unlimited. Hence in this system, we likely have much more water in the cavity than a PERS system and it would be mandatory for the designer and detailer to understand the volume of water that must be managed and drained from the D/BV system.
Specifying these systems is often confusing. Since the outer leaf and inner leaf are often by different suppliers in different specification section, the system performance criteria are frequently incorrectly applied to the open outer leaf by itself since the elements have no air or water tightness alone. Specified performance features need to be for the system and not the separate components.
According to the specified systems explained above, Aluminum Composite Panel (ACM) Installation, as one of the most common materials in cladding, classified into two main types of distinct applications: “Dry” and “wet” systems.
“wet” systems utilize caulking at panel joints to prevent water infiltration. Although fabricated material costs are typically cheaper with wet systems, labour costs can increase due to a large amount of caulking that is required at panel joints.
Maintenance costs should also be considered as caulking can fail over time. A detail of a wet system could be seen in detail No.1
NO.1 Wet system
“Dry” type ACM provides a watertight rain screen using engineered extrusion systems that conceal drainage through the use of baffles and weep holes. Dry systems are usually categorized as pressure equalization or rear ventilated systems. Both provide capillary action and eliminate water penetration through the building envelope by the use or specially engineered gasket and extrusion systems.
Fabricated Dry systems are slightly more expensive. However, because they can be more efficient to assemble, and because they require significantly less caulking, labour costs can offset material costs. A detail of a dry system could be seen in detail No.2