Many of our most common construction products are manufactured with cellulose(1)-based materials that, given certain environmental conditions, can be extremely vulnerable to moisture damage, and can hold and feed mold. Of these products, arguably the most important in North America are gypsum board panels. These products are used in both building interiors as well as exterior sheathing. Typically manufactured with a gypsum core and covered or ‘faced’ with a cellulose-based paper, these versatile and cost-efficient products have saved countless lives due to their excellent fire-resistive properties.

Though these products are durable, it is important to follow proper handling, installation and storage guidelines in order to maintain the integrity of this material and combat potential mold and moisture problems from the outset. The well-respected Gypsum Association publishes several relevant standards regarding the design, storage and handling of exterior gypsum sheathing, including the following guidelines:

“Gypsum panel products shall not be stored in areas of excessive humidity nor shall they be stored beneath overhead equipment which may have a tendency to drip grease, oil, or water.

“Exposure of gypsum panel products to rain and other high moisture levels may result in water stains, discoloration, mold, paper delamination, and sag. This sensitivity of most gypsum panel products to adverse moisture conditions requires that gypsum panel products NOT be stored outdoors without complete protection from the weather.”(2)

With regard to design, the Gypsum Association recommends:

“Gypsum sheathing used in building construction shall not be less than 8 inches from the finish grade in fully weather- and water-protected siding systems…”(3)

These important recommendations are designed to help prevent mold and moisture damage throughout the service life of the product. Often times, failure by designers and/or contractors to follow these guidelines can result in irretrievably damaged sheathing, before, during and after installation.

Occasionally, local building codes may dictate methods that are at odds with the Gypsum Associations’ guidelines. For instance, codes may require exterior sheathing panels that extend to the bottom of framed walls, which often terminate less than 8” above the finished grade. In these cases, it is the responsibility of the designer to specify a sheathing/cladding assembly that fulfills code requirements while still protecting the sheathing from mold and moisture damage. This can be achieved by closing off the base of the gypsum, thus encapsulating the base and eliminating the minimum distance to grade.

Prior to construction, the contractor should review project documents to identify and evaluate the designer’s instructions; means and methods intended for the project should be considered. If the specifications are not consistent with accepted industry standards (such as those provided by the Gypsum Association referenced previously), then further information should be requested in order to clarify how the contractor should proceed. This directive should be in written form (RFI). This additional step can protect the contractor against future litigation.

In order to satisfy code, structural or situational requirements, the designer might specify the use of an alternative sheathing material. Though more expensive than paper-faced exterior gypsum sheathing, these materials provide an alternative to paper-faced gypsum. These products have greater water-resistive characteristics and enhanced protection against mold and moisture damage. Alternative products include DensGlass Gold (gypsum core with a glass mat facing layer), Fiberock (uniform gypsum/cellulose core with no facing layer) and GlasRoc (gypsum core with embedded glass matting and a protective acrylic facing layer). Additional alternate products will become available in the future.

One advantage of exterior-grade gypsum is its’ relatively high vapor permeability and open joint installation configuration that allows high levels of vapor transmission out of the exterior wall cavity during colder months. Higher vapor transmission is important because trapped moisture can become free moisture at cooler temperatures, providing a needed element on a cellulose-based material to promote mold growth. Comparatively, standard 5/8” exterior-grade gypsum sheathing publishes significantly higher vapor permeability(4) than the previously mentioned alternative sheathing materials. It is 7% more permeable than 5/8” Fiberock sheathing, 30% more permeable than 5/8” GlasRoc Type X sheathing, and 44% more permeable than 5/8” DensGlass Gold. Moreover, non-standard gypsum sheathing products are more often utilized in closed joint configurations, further reducing their ability to transmit vapor from the building envelope. Care should be taken when modeling and analyzing exterior wall assemblies to ensure the correct sheathing product and sealing method is used in the modeled wall assembly.

Though these alternative sheathing products do have some limitations, all three publish the highest score of ‘10’(5) regarding resistance to the growth of mold, a clear advantage in wet climates or when there is a potential for moisture build-up in the wall cavity.

Simple and basic preventative solutions to mold and moisture damage can be achieved by utilizing well thought out storage, design and installation practices that keep materials dry during construction and maximize drying of the exterior wall cavity wherever there is the potential for free water at the gypsum layer. While alternatives to paper-faced gypsum sheathing are available, one should thoroughly examine all situational factors and product characteristics prior to selecting one for use on a project.

November 2007
Issue Eleven, Volume Two

This article written by Colin Murphy.

Colin Murphy
Murphy is a founder and managing partner of Trinity | ERD.

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(1) An organic compound comprised of linked sugar molecules that form plant cell walls and give wood its strength
(2) From Gypsum Association guideline GA-801
(3) From Gypsum Association guideline GA-253
(4) Based on testing completed in compliance with ASTM E96 (dry cup method)
(5) Based on testing completed in compliance with ASTM D3273-00 “Standard Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings in an Environmental Chamber”