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Defense Against Water


What is the rainscreen concept for an exterior wall?



The simple definition is that the concept provides a secondary line of defense against water, in both its liquid and vapor state, from penetrating the exterior wall cavity and eventually gaining entrance into the building interior. The concept has been around for years but has recently come into the spotlight as it is being codified in model codes. There is no single rainscreen product, rather it’s a rainscreen system or an assemblage of building materials designed and manufactured for this singular purpose. It is an assembly that effectively separates the exterior cladding from a water-resistive barrier and ultimately water sensitive building materials and the structural elements that resist wind and gravity loads. The Rainscreen Association of North America adheres to the following definition: “Assembly applied to an exterior wall which consists of, at minimum, an outer layer, an inner layer, and a cavity between them sufficient for the passive removal of liquid water and water vapor.”

    

While the concept of a rainscreen has been around for decades, it evolved over the years with the development of specialty products and systems specifically designed to facilitate a rainscreen. One significant change is the reduction in the size of the cavity or gap. The original size of the gap was 3/4 of an inch, but with ongoing testing and research, that gap has been drastically reduced. The recently codified gap in the International Building Code is 3/16 of an inch.

    

Using the RAiNA definition, the outer layer will consist of the exterior cladding. This can range from an aluminum composite material panel system to a more traditional cladding such as cement plaster where the gap is now required. The system can also work with conventional residential exterior finish material. This cladding’s purpose is to impede the flow of water, in its liquid state, through the wall. The assumption in a rainscreen system is that the cladding will stop the bulk of the water, but some water will make it through to the water-resistive barrier, which will stop its inward progress. This assumption is valid for there are many factors that lead to the breaching of the exterior cladding. The cladding, by design, will shed water down the face of the exterior surface. The cavity between the outer layer and the inner layer is vertical in orientation. RAiNA calls this a rainscreen or drainage cavity. The purpose of this cavity is for a “passive means of directing liquid water to the exterior.”

    

The “inner layer” would start with a water-resistive barrier and then potentially sheathing material and the structural elements. The drainage cavity achieves its purpose by two methods. First, it provides an open space where water in its liquid state can be shed off the water-resistive barrier and by gravitational forces be directed down the wall. At some point to complete the system, and this is vitally important, there must be a flashing mechanism to redirect the water out and away from the building structure. The other method is by venting the cavity, which allows for the movement of air to help the drying process in the cavity.

    

The drainage cavity can be obtained with the use of furring strips. These strips could be made of wood or steel and would be installed over the water-resistive barrier. The exterior cladding would then be fastened to the outer face of the furring strips. To facilitate the vertical orientation of the drainage cavity, the strips should be installed with the long dimension running vertically, and fasteners should extend into the underlying framing or structural elements.

    

There is an option within the model code to fill the drainage cavity with a proprietary material called a drainage mat. This mat is specially designed to allow for the free passage of liquid water. One of the benefits of the mat is that it maintains a constant gap along the exterior wall. These mats come in various materials, profiles and depths. Some even have an integrated water-resistive barrier.

    

There is the very real possibility that contractors who are not currently familiar with rainscreens will soon find that they must not only understand the concept, but also master its impact on subsequent bids. There is an increase in the installed cost of the rainscreen system, and it should be included in any bid. Construction techniques will have to be adjusted as new materials are added to the exterior wall. This may in turn alter the sequencing of the overall construction. The evolving code process may soon require the use of a rainscreen system with cement plaster over all sheathing material. The first code change was cement plaster over wood-based sheathing. Most likely there will be confusion, compliance issues and cost penalties as this transition period evolves.



Robert Grupe is AWCI’s director of technical services. Send your questions to [email protected].

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