In its strictest definition, a rainscreen is not dependent on having an air barrier; just a WRB. An air barrier is a critical component of the wall system, but may or may not be part of the rainscreen system.
Rainscreens can reduce the risk of structural deterioration and improve the durability of the enclosure wall but, like all wall assemblies, they are susceptible to damage if exposed to excessive moisture at vulnerable construction details. Also, ventilation behind the cladding, water barrier/sealed penetrations, insulation placement, exterior and interior environments, and details have a large influence on wall assembly performance.
Water and vapor management
Moisture issues, especially in exterior walls, are a growing concern as building envelopes have become tighter and incorporate higher levels of thermal insulation as a result of more stringent codes and a rising demand for energy-efficient buildings.
Moisture control is fundamental to the proper functioning of any building, yet moisture problems are far too common.
Many of these problems can be traced back to poor practices in design, construction, or maintenance. A position paper by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) said several independent decisions made by different professionals can produce enough moisture accumulation in the wall cavities for a long enough period to create an issue. Rarely can one profession, acting in isolation, take all the actions that lead up to either producing or preventing a moisture problem.
Effective moisture management means designing and building wall systems that have a greater “drying potential” than “wetting potential.” Façades are not expected to keep 100 percent of rain out of the wall. In fact, Dr. John Straube suggests 10 to 20 percent of wind-driven rain ends up behind the cladding. When specifying a rainscreen, it is important consider the following:
- rainfall amount and frequency;
- wetting-drying and freeze-thaw cycles;
- wind and storm conditions;
- temperature; and
- humidity levels.
Solar-driven moisture
The famous architect Louis Kahn was onto something when he said, “The sun never knew how great it was until it hit the side of a building.” Once the plans have been envisioned, drafted, approved, and finally realized, an architect’s best friend or worst enemy is the sun, especially when it comes to moisture. This lesson is best learned before breaking ground on even the most innovative commercial architectural feat: make the sun your best friend with intelligently engineered rainscreen design.
Solar-driven moisture can penetrate many building materials and accelerate the growth of mold. However, the materials of the greatest concern are highly absorptive claddings (also known as reservoir claddings), such as conventional stucco, adhered stone veneer, and cementitious siding. Solar-driven moisture occurs when water that has soaked into the cladding is forced further into the wall by the sun’s heat. The wall looks like it dries to the outside, but it does not.
It is important to note the WRB will not protect walls from solar-driven moisture, as it is moisture-vapor permeable. The thing about moisture-vapor permeable assemblies is that they are permeable both ways, meaning without any protection, the sun-driven moisture can be pushed all the way into the wall. When moisture gets to the cooler, air-conditioned interior, one risks condensation in the wall cavity. Moisture in the wall cavity can have huge repercussions for the durability and lifespan of the building.
Solar-driven moisture can be managed with the use of drained and back-ventilated engineered rainscreens. This will allow the majority of water penetrating the cladding to drain efficiently, and through strategically designed openings at the bottom of the wall, minimize moisture accumulation. Since the rainscreen is open at both the top of the wall and the bottom, accumulated moisture can be ventilated quickly before it can contribute to rot and mold growth. This ‘bonus’ drying energy comes from the buoyancy of air through the space as the wall is heated by the sun (hot air rises) and through air-pressure differential as the wind blows against the structure.
Several types of engineered rainscreens are available today. The more familiar may be the open entangled meshes. Another is the solid-faced dual-chamber dimpled version. Each will give drainage, ventilation, and a capillary break. Only the dimple version gives the full barrier, including protection from solar-driven moisture.
