Section R703 of the International Residential Code and Chapter 14 of the International Building Code address exterior coverings and claddings for exterior walls including, but not limited to, EIFS, stucco (plaster) systems, and combustible and non-combustible siding products.
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
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
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
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
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:
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All Things Gypsum: Gypsum Sheathing and Exterior Coverings
July 28, 2008