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Fenestration Forum: June 2014

Dry eyes


June 4, 2014
By Brian Burton

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A new North-American trade association was formed early this year, and
it immediately attracted my attention because of its focus on
controlling the impact of moisture on buildings.

A new North-American trade association was formed early this year, and it immediately attracted my attention because of its focus on controlling the impact of moisture on buildings. The Building Envelope Moisture Management Institute (bemmi.org), with offices in Winnipeg and Washington D.C., was organized by a group of charter members to promote rainscreen products and to share knowledge about moisture management for buildings in general. Engineered rainscreen materials are designed to manage water within wall systems and help minimize or prevent the potential damage it may cause.

Controlling moisture, whether in the form of vapour or liquid, is vital to mitigating the risk of premature wall system failure and avoiding any moisture-based threat to occupants. In practice, because many wall cladding systems and the fenestration components within them may not always be entirely watertight under all conditions, managing moisture is important to optimizing performance and ensuring durability. Given that there are a number of forces which drive moisture into wall systems, it is not surprising that water often manages to find a way to penetrate wall assemblies. These forces include kinetic energy, gravity, capillary action, surface tension and wind pressure. Over 50 per cent of reported construction deficiencies involve moisture penetration or retention.

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The concept behind rainscreening focuses on designing walls to prevent wind-driven water penetration – the concept is by no means new. In fact, Scandinavian nations employed rainscreen concepts quite successfully when they pioneered ventilated cladding on some of their timber buildings. They were the first to begin formal research on the topic. In the 1960s, Canadian building scientists at the National Research Council began researching rainscreen wall systems and recognized that this design approach substantially reduce the risk of uncontrolled rain penetration. We have been studying the topic for over 50 years and know from experience that so-called “face sealed” wall systems don’t perform very well in Canada’s four climate zones.

In our severe climate, almost all of the components in the building envelope are expanding and/or contracting in response to temperature extremes, solar radiation and a number of other factors. As a result, reliance on sealants to prevent moisture penetration can be problematic. Temperature gradients promote both heat and moisture flow through the building envelope in the direction of decreasing temperature and create differential contraction which can result in distortion and increased loading on building components. Building scientists recognize that if moisture is trapped, either as a liquid or as a vapor, it is very likely to cause performance or environmental problems. The goal is to minimize the amount of moisture penetrating into the wall system, providing a means of free drainage and making provisions for ventilation so that any moisture that is present can flow out or evaporate. This ventilation is important because it provides natural air conditioning and prevents heat build-up. It also protects the thermal insulation and vents any penetrating humidity in colder climates.

To effectively drain water, there needs to be a capillary break. Two layers of building paper do not always provide a sufficient capillary break that will perform adequately under all conditions. Used behind wood, fibre cement and other claddings, rainscreen drainage mats can reduce the risk of mold and structural decay. There have been numerous advancements in the development of drainage and ventilation mats over the last decade. Some drainage mats come in the form of an entangled matrix with or without a bonded filter fabric. Others are designed to offer dual ventilation and drainage. All of these are high-quality products. However, when an assembly varies from the conventional rainscreen, assess the situation carefully to make sure the drainage mat will provide the performance needed for the conditions to which it will be exposed.


Brian Burton is involved with an innovative, multidisciplinary firm that specializes in technical business writing, Award Bid Management Services (award-bid-management-services.com). The firm assists companies interested in selling goods and services to governments and institutions. He can be reached at burton@award-bid-management.com.


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