By Susan McClendon, RA, CSI, CCS, and Robert Paul Dean, AIA, CSI, CCS
It's easy to take gypsum board for granted because it's used in virtually every new building project. Gypsum board is, however, a complex and sophisticated technology-driven product. It was first made at the end of the 19th century and today there are many types of board used for a variety of purposes. This article will focus on some newer types that address some key issues in construction.
These products address questions of sustainability, reducing or eliminating mold in building interiors, improving the strength and abuse resistance of finishes, improving sound isolation with reduced mass, achieving curved surfaces without losing advantages of the flat board form, and achieving better finishes.
Basic gypsum board is surprisingly strong for its mass and thickness. Compare it, for example, to gypsum plaster, a finish made of essentially the same materials and serving the same basic function. If you tried to make a ½-inch-thick "board" out of plaster, perhaps by putting a layer of plaster on a flat surface, it would be very brittle and would certainly not be durable enough to span between studs. Gypsum board, on the other hand, is durable enough to ship by truck, strong enough to mount horizontally on spaced ceiling joists without sagging, and much easier to install than plaster. Gypsum wallboard exists because of manufacturing technology that combines the basic gypsum with a reinforcing sheet on both faces and the two long edges, plus additives and reinforcing in the core. This manufacturing principle was first developed in 1894 and evolved by 1930 into the gypsum board that we know today.
With today's increased emphasis on sustainability, including the desirability of using recycled material and products manufactured within a short distance from point of use, gypsum board has taken on new significance. The gypsum used in wallboard comes from four basic sources:
Gypsum rock quarried in mines
Synthetic gypsum recovered from the flue-gas desulfurization process (FGD) used by coal-fired electric power plants (about 20 percent of total production, comprising 20 plants in 2006)
Synthetic gypsum produced as a by-product of other industries
Post-industrial recycled natural gypsum reclaimed from other industrial processes
Considerable fuel is used for all gypsum board products, both in the manufacturing process (calcining, partial dehydration) and in transportation (about 30 billion metric tonnes per year). Most of the gypsum wallboard manufacturers now provide data on location of their plants and the recycled content of their products by plant location. When the product is indicated as about 95 percent recycled, it is made of FGD gypsum. When it's about 5 percent recycled, it's made of natural gypsum. Percentages in between reflect a mix of natural and non-FGD reclaimed gypsum. A factor not often noted about FGD gypsum is that the FGD process doesn't reduce the greenhouse gases produced by burning coal — only the sulfur emissions that foster acid rain are reduced. In addition to the recycled content of the core, practically all paper used for facings is made from post-consumer recycled paper.
Because gypsum board is so heavy, shipping costs are a significant factor in pricing. The three largest gypsum companies have mines and plants that serve most of the continental United States. The other five U.S. companies are more regional in their range. In addition, most synthetic gypsum is produced in the East because that is where most of the coal-fired power plants are located. Ordinarily, it shouldn't matter where the gypsum wallboard comes from, as all of these companies produce materials that comply with the relevant ASTM reference standards. But if synthetic or recycled gypsum products are required and the plant that produces them is farther away than a plant that uses natural gypsum, the synthetic or recycled product is likely to cost more.
Many, if not most, of the new production plants built in recent years are located right next to, or at a short shipping distance from, coal-fired power plants and use the FGD gypsum from them. As mentioned previously, most of these are in the eastern half of the United States. Most of the other plants are located near the quarry their gypsum comes from, and because quarries are not uniformly distributed, production plants are also not uniformly distributed. Only a few plants use gypsum that is mined at a considerable distance — typically only if the gypsum can be shipped by sea.
Mold growth in building interiors is an area of increasing concern. One of the most significant recent technological developments for gypsum board has been the improvement of its resistance to mold growth. It has long been known that paper-faced gypsum board was vulnerable to moisture absorption. Some moisture-resistant products have been available for a long time, but the fact that paper is a food source for mold has generated a number of other solutions. Most manufacturers making a claim of mold resistance have based that claim on tests the companies have performed themselves (i.e., there has been no independent analysis). In some cases, there is comparable data to indicate performance relative to other products, but there are no standards specifying the degree of mold resistance that would be desirable. There are currently four different technical solutions. Each manufacturer has a slightly different philosophy toward the problem, resulting in differences in product offerings that make comparisons difficult.
Gypsum-less Board: Boards made of cement instead of gypsum were among the first solutions developed to address the inherent moisture vulnerability of gypsum board. As a by-product of the moisture problem, the paper facing was also eliminated. These products are typically limited to backer boards under tile or similar finishes because the surface is not suitable for painting.
Non-paper-faced Gypsum Board: Boards made with a facing other than paper were first developed for exterior sheathing, but some now are for interior use. The board is "moisture-resistant," with a water-resistant additive in the core. The facing typically is a fiber-glass fabric, usually called a "glass mat." For use as exterior sheathing, the facing is covered with a coating or a layer of water-resistant gypsum.
Mold-resistant Paper-faced Board: Lately, the "moisture-resistant" boards have been modified with an additional mold inhibitor in the core and the paper facing. These are typically referred to as "mold-resistant," but they are offered with a lot of disclaimers. These boards are "not recommended for areas which will be continuously wet or subjected to high humidity such as tub and shower enclosures behind tile, saunas, steam rooms, or gang showers."
Facing-less Gypsum Board: The latest variety is moisture- and mold-resistant without any facing at all. The composition necessary to go "facing-less" also results in a more impact- and abrasion-resistant board.
Abuse resistance is claimed relative to other gypsum boards and not to other types of products. Both surface abrasion and impact resistance are included in the general category of abuse, and there is no truly standardized terminology for these characteristics. Each manufacturer uses the terminology in slightly different ways. Surface abrasion is addressed by means of abrasion-resistant facings while impact resistance is addressed by manufacturing a stronger board. No standards exist that define what constitutes better abuse resistance except by inference from the minimum performance prescribed for gypsum board in ASTM C 1396, the basic standard for its manufacture.
Reduced Sound Transmission
Sound transmission has long been measured on completed assemblies — studs, boards, and optional insulation — with predictable results. More mass (more board), more insulation, and more separation (doubled studs or resilient channels) have been the only means of influencing the sound transmission characteristics of gypsum board walls. But recently developed boards now can reduce transmission even more without additional mass. The trick here is the composition of the board — typically multiple layers of gypsum and/or other materials. The internal layer boundary interrupts the sound transmission. In addition, boards equivalent to several layers of standard thickness board are factory-laminated, reducing installation labor. There are many tested assemblies, some with very high STC ratings, but one example should suffice:
The Gypsum Association's publication GA-600 includes many generic wall assemblies that have been tested for fire resistance and sound transmission. GA File No. WP 1072 is a very commonly used wall, made up of 5/8-inch board on each side of 35/8-inch metal studs spaced at 24 inches on center, with 3½ inches of batt insulation. This assembly is listed as having an STC rating of 45 to 49. One of the manufacturers of acoustically improved gypsum board shows an equivalent assembly in its literature having an STC of 55 — six to 10 points difference. In addition, a further review of GA-600 shows that, to achieve an STC of over 50 in walls using standard board, additional material is always required — additional layers, staggered studs, resilient channels, or a combination of these. For an STC of 55, two layers of 5/8-inch board on each side are necessary.
Tight Radius Curved Surfaces
All the major manufacturers offer gypsum board that is thin and flexible enough to curve to fairly small radii. Typically, such board is ¼ inch thick. The actual minimum bending radius depends on whether the board is bent in the lengthwise direction, with the long edges perpendicular to the studs or other framing, or the widthwise direction. The board can also be wetted for extremely tight radii. Studs must be much closer together than normal and two or more layers are typically used for durability.
"Level 5 Finish" is intended to reduce visible defects in the finished surface of gypsum board and is defined in ASTM C 840. It can be used anywhere but is particularly useful where light will fall on large continuous surface areas, especially if the light falls at a small angle from the surface (so-called "grazing" light). Level 5 finish is the highest quality finish typically available for gypsum board, but the traditional method of achieving a Level 5 finish on gypsum board is labor-intensive and sensitive to labor quality. The joints must be carefully treated and a skim coat of joint compound or other material applied over the entire surface of the board. Two newer types of products claim to make a Level 5 finish more affordable and easier to achieve. One type is a high-build spray coating functioning as both primer and finish coating; the other is a complete system incorporating a factory-applied skim coat.
Basic gypsum board is a product that is familiar to every architect. However, there is a plethora of new products that extend the applications for the material and address a variety of problems and improve its versatility. Recycled board, mold-resistant board, abuse-resistant board, and sound-resistant board are all relatively new types that should become part of the vocabulary of all design professionals.
Susan McClendon (firstname.lastname@example.org), a registered architect, is executive vice president of BSD. She is project manager for BSD SpecLink®, which now includes CSI-DBIA's PerSpective®. She manages the ongoing data development and updating of BSD SpecLink and also manages the maintenance of BSD's website. Robert Dean (email@example.com) is an architect and president and chief operating officer of BSD. He is responsible for business administration and new business development and is also one of the most experienced and knowledgeable experts on automated specification systems in the country.
Standards for Gypsum Board
ASTM C 1396/C 1396M, Standard Specification for Gypsum Board: This is the industry standard prescribing the minimum properties of gypsum board in the United States. This covers all the traditional types of board, based on board with paper faces. Non-paper-faced boards and facer-less boards can also be evaluated to these criteria but will not, strictly speaking, comply with this standard. The current edition date is 2006a. ASTM C 1396 replaced multiple standards (ASTM C 36, C 37, C 79, C 442, C 588, C 630, C 931, C 960, C 1395), which began to be phased out in 1998. Referenced by ICC International Building Code.
ASTM C 840, Standard Specification for Application and Finishing of Gypsum Board: This standard covers minimum framing spacing for different thicknesses of boards, fastening to framing, control joint spacing and details, some other details for special conditions, and finishing, including Level 5 finish. The finishing portion of this standard is essentially identical to Gypsum Association GA-214, Recommended Levels of Gypsum Board Finish. The current edition of ASTM C 840 is dated 2007. Referenced by ICC International Building Code.
ASTM D 3273, Standard Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings in an Environmental Chamber: This test method is the one most commonly cited by manufacturers who claim "mold-resistant" products. It is intended only for comparative rating under controlled laboratory conditions. The results are rated from 0 to 10, with 10 being the "best" result - no mold growth. The current edition is dated 2000 (Reapproved 2005).
: The Gypsum Association represents all the U.S. manufacturers of gypsum board (all eight of them).
GA-214, Recommended Levels of Gypsum Board Finish, 2007
GA-216, Application and Finishing of Gypsum Panel Products, 2007
GA-226, Application of Gypsum Board to Form Curved Surfaces, 1996
GA-600, Fire Resistance Design Manual, 2006
Responsible Solutions to Mold Coalition: "The Responsible Solutions to Mold Coalition comprises 13 companies, associations, and government and academic organizations that are dedicated to identifying and communicating accurate, science-based information on mold prevention and control." The website and publications are sponsored by USG and National Gypsum.
U.S. Department of the Interior, U.S. Geological Survey, publications.
USGS 2006 Minerals Yearbook, Gypsum
. Data on production of gypsum for all purposes. Eighty-five percent is used for gypsum board and similar products.