The rainscreen principle, time-tested throughout Europe for two centuries, produces healthier, more energy-efficient and longer lasting walls. Furthermore, it liberates aesthetic considerations of a façade from the important functions of a weather barrier wall.
However, the typical rainscreen system retains certain shortcomings, which are now eliminated by integrated rainscreen design. Before considering this innovation, we first should examine typical face-sealed wall design concepts used extensively in the United States.
Problems Associated with Face-sealed Wall Design
With face-sealed wall construction, the outermost cladding material attempts to serve multiple functions, including aesthetics, air barrier, water barrier and/or wind barrier. These types of designs are problematic for a number of reasons, primarily because they are functionally overburdened and denied the benefit of the “belt and suspenders” approach offered by an integrated rainscreen system.
To begin with, traditional face-sealed design relies on the air and water tightness of hundreds of panel-to-panel joints and other panel transitions including jamb, head and sill conditions, and interfaces between various cladding materials. In many face-sealed wall systems, caulking is commonly used to create these seals, requiring perfect installation. In addition, no caulking is permanent. It degrades over time, usually as soon as five to 10 years. Caulking can stain, collect dirt and is costly to maintain.
When joints or other interfaces fail, large amounts of moisture will penetrate the wall. Moisture-laden air is drawn into any breach in the joint because of air pressure differences inside the wall versus outside the wall. When liquid water is present in the wall cavity, traditional weather barrier wall design has no method of effectively draining or drying it. The infiltrating water damages insulation (rendering it much less effective), corrodes any metal components and creates a desirable condition for mold propagation.
Typical Rainscreen Wall Construction
Rainscreen design acknowledges that face sealing exterior walls is an exercise in futility. With a rainscreen assembly, the burden of weather/climate management moves from the outer wall to the inner wall. The outer cladding’s only role, beyond aesthetics and long-term durability, is to serve as the initial defense against rain and, in some applications, to handle heavier wind load.
The inner wall now does the heavy lifting: it serves as a drainage plane/water barrier, air barrier and thermal barrier.
The open cavity is essential to promote air flow via the chimney effect to rapidly dry out the cavity. The preferred practice is for an integrated ventilation system to be engineered into the wall assembly. The air and thermal barriers are continuous and outboard of the framing studs to eliminate gaps and minimize thermal bridging via the floor slab or steel framing elements.
Even with a well-designed high-performance rainscreen system, certain obstacles still exist to delivering the system as intended. These include product substitutions that not only compromise the purpose of the design and fail to consider an individual component’s role as part of a unified system, but create a climate of ambiguity regarding who (which supplier or installer) is ultimately responsible for system performance.
Integrated Rainscreen Wall Design
Problems can be managed when they are anticipated. An integrated rainscreen system, such as the IntelliClad Complete Rainscreen System from IMETCO, Norcross, Ga., is designed with the understanding that preventing all vapor and water penetration is nearly impossible. By anticipating that some water and vapor will penetrate the exterior cladding, a properly designed integrated system can manage it quickly and effectively within the cavity.
A properly designed integrated rainscreen leaps over both traditional face-sealed walls (reliant on imperfect joints and seals) and air barrier systems that are vapor impermeable
(providing no exit strategy for when, inevitably, vapor penetrates inside) by making sure that all components work together to create a unified system.
If we use the IMETCO system as an example, an integrated rainscreen assembly includes:
• A variety of exterior cladding options, each concentrated on three priorities-shedding rain, material durability and design latitude.
• A 1-inch air cavity, created in this case by perforated hat channels that eliminate the need for vertical subgirts, and require minimal fasteners to attach to the studs resulting in negligible thermal bridging.
• High-density continuous insulation board (the IMETCO system uses a stone wool product that is unaffected by water and provides outstanding thermal insulation).
• A continuous vapor-permeable air barrier system with an effective transitional interface between the walls and the roof to maintain its continuity.
An integrated rainscreen system is a sustainable solution suitable for any building application, especially when an owner is concerned with life cycle costs and long-term, reliable energy efficiency.
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Amy Stokes is the marketing manager at IMETCO, Norcross, Ga. For more information about IMETCO and its IntelliClad Complete Rainscreen System, visit imetco.com/IntelliClad.php.
