All Things Gypsum: Gypsum Sheathing and Exterior Coverings
Gypsum sheathing functions well as a thermal barrier and it has long been used as a substrate for EIFS systems. Most, if not all, EIFS manufacturers maintain evaluation reports that list gypsum sheathing as an acceptable substrate for EIFS. Gypsum sheathing also works well as a substrate for non-combustible claddings such as brick or stone, especially in installations that require a fire-rated wall underneath the cladding.
The long historical relationship between gypsum sheathing and exterior plaster is also well documented. Early iterations of gypsum sheathing were primarily developed to function as a substrate for exterior plaster systems. Even as model codes migrate from prescriptive text toward an increased reliance on reference standards, sections of Chapter 25 of the IBC still spell out in specific terms the requirements for the installation of gypsum sheathing when it is used in conjunction with Portland cement plaster.
In recent years, the U.S. residential exterior cladding market has been dominated by the use of combustible siding materials. While vinyl siding is not the only combustible exterior cladding available, it has become the predominant exterior residential cladding. In 2006, according to the Vinyl Siding Institute, approximately one-third of all the new single-family homes constructed in the U.S. were clad with vinyl siding.
Like gypsum, most combustible sidings are easy to install and adapt well to a variety of conditions. Vinyl siding, for example, when coupled with gypsum sheathing, is usable in nearly every geographic area of the U.S. except for extremely high-wind-speed coastal locations. Combustible sidings, however, present some design challenges, most all of which relate to fire and exposure to heat. Vinyl, for example, will not ignite unless exposed to flame, but it can melt when exposed to significant heat.
This is of particular concern when structures are erected with limited separation. A house that sits in the middle of an open field has little to fear from a fire in an adjacent structure; however, a house that is sited immediately adjacent to another dwelling can be negatively impacted by heat or flame exuding from the neighboring structure.
The use of noncombustible substrates can mitigate the heat-sensitive characteristics of combustible claddings when they are installed in close proximity dwellings or structures. A recent code development in Canada clearly points this out.
In July 2007, an intense fire in the MacEwan Green neighborhood in Edmonton, Alberta destroyed or damaged over 80 dwellings. Whipped by high winds, the fire destroyed almost 20 duplex dwellings in little over two hours, creating a moonscape of charred timbers in a high-density residential area.
In an ironic twist, the fire occurred almost simultaneously with work commenced by a committee charged with studying large-loss building fires in Alberta. In June 2007, following a seven year period of catastrophic fires throughout the province, the Government of Alberta established The High-Intensity Residential Fires Working Group and gave it a “mandate to review the factors surrounding high-intensity residential fires in Alberta and develop recommendations...” On October 31, 2007, the group completed its work and issued its final report.
The Working Group collected fire reports from seven municipalities in Alberta, including both Edmonton and Calgary, and identified 86 fires that it classified as a “High-Intensity Residential Fire” (HIRF) that occurred between 2000 and 2007. Such fires characteristically displayed “rapid heat release and fire spread beyond the point of origin that usually involve(d) adjacent buildings.”
It also reviewed recommendations proposed to the model National Building Code of Canada by the Canadian Commission on Building and Fire Codes (CCBFC) Task Group on Spatial Separations. That task group had been appointed in 2006 to study issues relating to spatial separation between buildings of combustible construction.
Based on its examination of the available information, the group made 22 recommendations. These addressed a myriad of topics including public education and awareness, suggested changes in building construction and materials, the creation of contractor education programs, and municipal organization policy.
Included in the report was a recommendation to adopt, in Alberta, the proposed recommendations of the CCBFC, including a requirement to mandate that all “walls or parts of walls of houses having a limiting distance of less than 1.5m (slightly less than 5 feet) that are constructed with a combustible cladding are to incorporate a sheathing membrane that is noncombustible, such as exterior gypsum wallboard, or the cladding is to have a fire-resistant coating.” According to a May 23, 2008, news release from the Alberta government, that recommendation will be submitted to Alberta’s code development and adoption process for action and implementation.
So what does the action mean? Specifically, when formally adopted, it will require any dwelling constructed in Alberta that is sited within 1.5m of a property line and that is finished with a combustible exterior cladding to have noncombustible sheathing installed as the substrate beneath the siding. Gypsum sheathing is an example of such a cladding.
It is also interesting to note that the Working Group also recommended that garages have gypsum wallboard, or “a similar performing membrane” installed on the interior wall and ceiling surfaces of all attached garages. Such a requirement has long been a part of U.S. codes, but is not currently a part of the Alberta code.
As codes evolve, the inherent noncombustible characteristic of gypsum sheathing is slowly being recognized as a viable means of limiting the spread of fire between dwellings. The Alberta response to catastrophic fire is one early step in expanding the use of the material in a manner that recognizes and utilizes a natural benefit of gypsum sheathing. W&C
Author’s Note: Unattributed quotations included in the article are taken from the “High-Intensity Residential Fires Working Group Final Report, Dated October 31, 2007.” The report is available from the Ministry of Municipal Affairs; Province of Alberta Web site: www.gov.ab.ca/.
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