{"id":7293,"date":"2015-02-27T10:50:46","date_gmt":"2015-02-27T15:50:46","guid":{"rendered":"http:\/\/www.constructionspecifier.com\/?p=7293"},"modified":"2016-12-22T14:44:41","modified_gmt":"2016-12-22T19:44:41","slug":"draining-the-rain-advancements-in-engineered-rainscreen-walls","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/draining-the-rain-advancements-in-engineered-rainscreen-walls\/","title":{"rendered":"Draining the Rain: Advancements in engineered rainscreen walls"},"content":{"rendered":"
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Images courtesy Advanced Building Products<\/strong><\/figcaption><\/figure>\n

by Keith Lolley, CSI
\n<\/strong>Over the years, there has been a tremendous amount of innovation in the building industry\u2014still, the ability to achieve a waterproof wall system still eludes design\/construction professionals. If the wall is not properly designed, this moisture will remain trapped causing numerous issues, such
\nas the corrosion of structural reinforcing, and the proliferation of rot and mold.<\/p>\n

Trapped moisture will decrease the effectiveness of certain insulations and decrease the overall life span of the building. Anecdotally speaking, roughly 90 percent of all wall failures are the result of moisture-related issues. Moisture intrusion is a concern architects and contractors need to give strong consideration to during the designing stage and especially the building process of projects.<\/p>\n

Cavity wall systems
\n<\/strong>The cavity wall system is designed to properly \u2018drain the rain.\u2019 These systems are typically designed with a backup wall, airspace, and outer veneer. Moisture management components\u2014such as through-wall flashings, mortar deflections, and drainage devices\u2014at flashing locations are used to divert moisture entering the wall back to the outside. Figure 1 is a typical cavity wall detail.<\/p>\n

There are a few key factors to a successful cavity wall. First, the through-wall flashing specified must last the life of the building. It costs roughly $275\/sf to replace failed flashing, so the economic value to doing it right the first time is obvious.<\/p>\n

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Figure 1:<\/strong> Typical cavity wall design.<\/figcaption><\/figure>\n

Another important aspect to a successful cavity wall is a clear cavity. Without a clear air space, moisture will not drain effectively. A 50-mm (2-in.) cavity is the industry standard for commercial cavity wall construction. However, new energy standards and codes\u2014such as American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings<\/em>, and the 2009 International Energy Conservation Code<\/em> (IECC<\/em>)\u2014mean a call for increased insulation. This increase in wall insulation is causing the cavities to get larger and actually making the air space smaller. This air space reduction is causing wall performance concerns. What if there was a way to minimize the cavity space without jeopardizing the overall effectiveness of the wall?<\/p>\n

Finding a better way<\/strong>
\nEngineered rainscreen wall systems have been around for quite some time, but there remains confusion as to the difference between a pressure-equalized rainscreen wall and a cavity wall. First, one must modify the terminology slightly. Pressure equalization is a lofty goal and one that is difficult to truly achieve. In reality, the goal is to create a pressure-moderated wall system. These systems are known as ventilated fa\u00e7ades or modified rainscreen walls.<\/p>\n

Differential air pressures between the inner wall and outside environment will draw moisture into the wall system\u2019s inner structure. A cavity wall system does nothing to prevent this from happening; a pressure-moderated rainscreen wall, on the other hand, will cut down on the differential air pressure that draws moisture into the building by allowing air into the wall system to partially neutralize the air pressure behind the cladding to the air pressure outside of the wall system (Figure 2).<\/p>\n

As air is introduced into the cavity, it works its way up the wall and out through vents installed at the top of the wall. The presence of the air\/moisture\/vapor (AMV) barrier allows the introduced air to circulate in a convective fashion. This convective airflow removes excess moisture vapor while drying any residual moisture within the cavity at the same time (Figure 3).<\/p>\n

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Figure 2:<\/strong> These figures demonstrate pressure neutralization. Images courtesy International Masonry Institute<\/strong><\/figcaption><\/figure>\n
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Figure 3:<\/strong> The correct path of drainage and ventilation can be made possible by utilizing an engineered rainscreen drainage mat.<\/figcaption><\/figure>\n

Pressure-moderated wall systems consist of:<\/p>\n