Interior Finish Systems: Judging a Building by Its Inside Cover
Imagine a building product coming from a raw material that is naturally occurring, abundant, healthy, noncombustible, and that can readily be dehydrated and rehydrated to become a hard, durable, paintable surface. Then imagine that after four thousand years of use worldwide with little change—and with plenty of examples of applications still in place and sound after centuries or even millennia of use—the material is almost entirely phased out over a span of just twenty years. Finally, imagine that in just twenty more years the replacement product’s primary advantages—speed and ease of installation—become its most widely cited weaknesses. No need to pinch yourself—welcome to the real world of plaster and gypsum board.
There are few building components so dominated by a single product as interior wall finish. While there are alternatives—three-coat plaster and lath, skim-coat plaster over lathboard, and a relatively new gypsum-fiber composite board—conventional drywall currently holds more than a 90% market share of interior finish materials. Its relative economy, ease of installation and modification, and noncombustibility have made drywall the material of choice for interior walls since the 1950s. But how does it stack up from an environmental standpoint? Which interior finish material is the greenest, and why? This article reviews environmental considerations with gypsum-based finish materials and offers suggestions on minimizing environmental impacts with their use.
Gypsum is a mineral composed of 75% to more than 90% calcium sulfate. Calcium sulfate is one of many salts dissolved in sea water, and gypsum is one of many minerals geologically layered in deposits by evaporation of retreating seas. Calcium sulfate has two common crystalline forms: calcium sulfate dihydrate (CaSO4·2H2O) or gypsum, and calcium sulfate hemihydrate (CaSO4·1⁄2H20) or plaster of Paris (the gypsum industry uses the term
stucco for the hemihydrate form). Transformation from one crystalline form to the other occurs readily and reversibly—with the addition of heat (180°F) in a process called
calcining (gypsum to plaster of Paris) and by
hydration (plaster of Paris to gypsum). During hydration, there is the opportunity to give gypsum any desired shape with little to no shrinkage, a phenomenon that builders and artists have taken advantage of throughout history. In building materials, gypsum has the added benefit of containing water molecules in its crystal structure—during a fire, driving off this water takes energy and time.
Gypsum is abundant worldwide with only 20% of the total 107 million tons (97 million tonnes) mined annually involved in international trade. The United States is a net importer primarily from neighbors Mexico and Canada. Only in western nations, most notably the U.S., is gypsum used primarily for wallboard production. In developing countries, most of the gypsum is used in cement production.
Table 1 characterizes interior wall surfacing in the United States. While there are a few other approaches for interior building surfaces—for example, solid-sawn and panelized wood products and structure-as-finish systems (see
Vol. 9, No. 3)—gypsum-based systems account for the vast majority of interior wall surfaces.
Plaster and lath
Plaster is a mixture of lime and gypsum (or Portland cement), sand, and water, with some specialty plasters containing additives such as epoxies or other resins for superior resistance to shrinkage or very alkaline environments. Hair or fiber was added to plaster for bonding to increase strength but is less needed today with better substrates. There are many types of plaster, but gypsum-based products are almost always used on interiors and the Portland cement-based products are used on exteriors (often broadly referred to as stucco—not to be confused with the use of this term in gypsum production). While plaster can be installed in a variety of ways, the most common is the three-coat system—a scratch coat, a brown coat, and a hard finish. The proportion of ingredients varies widely with three-coat plaster, but the basic composition does not. Lath—originally wood strips, then metal mesh, and finally gypsum board—is required as backing when spanning wall studs or on a solid wall that experiences minor shifting over time.
The time-consuming installation required for three-coat plaster resulted in the development in the 1930s of simpler, thinner, one- or two-coat plaster systems that could be applied over a paper-faced gypsum board. The gypsum board used today is commonly called
blueboard because the highly absorbent front paper face required for a proper bond with the plaster skim coat is tinted light blue. Skim coats require high purity plaster, but the end product offers a very hard and abrasion-resistant finish—much like traditional plaster.
After developing a gypsum board lath product to speed up plaster work, someone got the bright idea of doing away with plastering altogether by making the front paper face ready to accept paint itself. (You will often see the front paper on gypsum board described as “sized,” which in today’s world simply means a higher grade of paper on the front than the back face.) The trick is to make a joint compound capable of joining and hiding all board margins. It is difficult and may require several coats of paint to achieve the same sheen and brightness at the margins as well as face of boards. Having a finished paper surface rather than a plaster surface comes with other compromises—paper is not as hard and hence not as durable as plaster, it is much more permeable, and it can support mold growth. More on this later.
There have been several products introduced over the last decade or so —for example, Gypsonite™ and Fiberbond™ (EBN
Vol. 2, No. 3)—that are made of a more-or-less homogeneous mixture consisting primarily of gypsum and cellulose fiber. The latest incarnation is called Fiberock™ (EBN Vol. 9, No. 2). VHI Abuse-Resistant Fiberock, the interior wall product, has a perlite core to reduce the panel’s weight.
A Look at LCA
The total environmental footprint of a building material or system is determined in an environmental life-cycle analysis (LCA). An LCA can be broken down into three stages: manufacture, use, and final disposition. Manufacturing involves resource extraction, production, and transportation of raw materials and finished products. Use involves maintenance and the service life of the system (as well as any impacts on energy consumption). Final disposition deals with whether and how materials are discarded, reused, or recycled.
LCA work with most building materials, including interior wall systems, has been “cradle-to-gate” or limited to the first stage of the life cycle. One such environmental analysis of interior wall systems was carried out for the AIA’s
Environmental Resource Guide in 1993 (first reviewed in
Vol. 1, No. 3). The state of the science at that point in time yielded very detailed but often qualitative information. Two years later, the Athena™ Institute conducted its own, more detailed environmental analysis, “Life Cycle Analysis of Gypsum Board and Associated Finishing Products.” While both environmental analyses reveal the manufacture of gypsum systems to be relatively benign overall, there are a number of environmental considerations of note.
Gypsum can be mined underground in shafts or above ground in open pits, with the latter accounting for the vast majority. In the United States, only 8 of 61 mines are underground. In general, the surface disturbance associated with open-pit mining represents the greatest environmental impact, particularly to local ecosystems. About 40% of the total material excavated is gypsum; the rest is overburden simply stockpiled on-site.
Gypsum can also be obtained as a by-product of other chemical processes—in which case it is commonly called
synthetic gypsum. For over 30 years, two East Coast gypsum wallboard manufacturing plants have used titanogypsum, a synthetic gypsum formed as a by-product of titanium dioxide production, a material used extensively in paints (see EBN
Vol. 8, No. 2). An enormous source of synthetic gypsum—approximately 30 million tons (27.2 million tonnes) annually—is phosphogypsum, a by-product of fertilizer production from phosphate deposits. In the U.S., however, phosphate deposits, primarily in Florida, contain radium and heavy metals, which makes the phosphogypsum largely unusable.
Within the last ten years, a growing number of gypsum manufacturing plants have begun using flue-gas desulfurization (FGD) gypsum (see
Vol. 2, No. 3). The exhaust gases from coal-fired power plants are high in sulfur dioxide, a primary agent in acid rain formation. By bubbling flue gases through limewater, the sulfur dioxide reacts with calcium carbonate in the limewater to produce calcium sulfate (gypsum), which can be recovered for wallboard manufacturing. Flue-gas desulfurization can result in very high quality gypsum, but processing is required to deal with impurities, superfine particle size, and high moisture content. Use of FGD has been increasing sharply, with 8 of the 10 wallboard plants to come on line in the recent past or near future using 100% FGD gypsum. The
1998 Mineral Yearbook reports synthetic gypsum use of more than 3 million metric tons per year, or nearly 10% of all crude gypsum utilized in the U.S. Both demand for, and supply of, synthetic gypsum have been bolstered by a strong economy and clean air requirements.
According to Wayne Trusty of the Athena Sustainable Materials Institute in Ottawa, FGD gypsum’s environmental advantages fall into four areas: avoided mining, reduced transportation of raw material, lower processing energy of raw materials, and avoided landfill disposal of the utilities’ synthetic gypsum. However, notes Trusty, this must be balanced against the fact that “only 20% of the total embodied energy for wallboard production comes from the raw material portion of the picture; the lion’s share comes from the actual forming and drying of the board products.”
100% recycled-content paper is used for all gypsum board products. A low clay content in the paper is important in terms of bonding characteristics between the paper and gypsum core.
EBN obtained conflicting reports from manufacturers on the bleaching of the “ivory” paper stock—the paper used for the front face. Over time, the industry has permitted the ivory stock to gray—it’s concern that the market would associate graying with board quality proved unfounded. According to Rik Master, manager of architectural systems at USG Corporation, USG has never used bleached paper on their board. “Some companies are still using bleached paper,” stated Master. To figure out whether bleached paper is used, he suggested checking out the big retailers shortly after holidays, especially Christmas and Valentine’s Day, and looking for the pinkish tinge to some companies’ products and not others. Because of the enormous amount of paper involved in board production and the environmental impact of conventional bleaching processes, whether or not facings are bleached is an important environmental consideration.
Joint compound is made with varying proportions of gypsum or dolomite or limestone; talc, mica, certain types of clay, and a synthetic latex binder of polyvinyl acetate or vinyl alcohol polymer. (Neither substance contains any chlorine, so they do not carry the same environmental risks as polyvinyl chloride, PVC.) There are actually three types of joint compound: setting compounds, powder drying compounds, and ready-mix drying compounds. Industry representatives report that no setting-type joint compounds contain any “anti-fouling” agents or fungicides; drying-type mixes sold in a dry (powdered) form normally do not contain them either. All ready-mix compounds—those with water already added that come in the familiar plastic buckets—do contain fungicides. Mercury-containing fungicides were removed from joint compounds in the late 1980s and replaced by a variety of other common fungicides; these are used in very small quantities.
The specific formulations of joint compound are all proprietary, and the amount of anti-fouling agents used is low enough that they are not included in the material safety data sheets. At least one firm, Murco Wall Products, makes an all-purpose joint compound specifically marketed for its lack of fungicides or preservatives. The material is only available as a dry powder and cannot be stored after it is mixed without spoiling. American Formulating and Manufacturing, a firm focused on the production of low-toxic finishing materials, at one time produced a joint compound with a nonsynthetic binder and without preservatives, but they dropped the product several years ago due to lack of demand.
Although gypsum fiberboard shares many of the same inputs as conventional paper-faced wallboard, its production is quite different. Far less water is added during board formation, and the board is
pressed as opposed to being formed between paper skins. This would result in the fiberboard having lower embodied energy, except that—with the Nova Scotia plant, for which the Athena Institute collected data—the perlite used in the core comes from Greece. According to George Venta, a longtime consultant to the gypsum industry and principal investigator for the Athena report, “Great care must be taken in comparing conventional drywall and gypsum fiberboard. Paper-faced board manufacturing processes are all very similar, while the several manufacturing processes for gypsum fiberboard are quite different.”
The usage phase of long-lived materials, including building products, can contribute significantly to their overall environmental footprint or LCA. Generally, building components involved in the thermal performance of the building—insulation, windows, and HVAC equipment, for example—have use-phase impacts that can dominate their LCA. While this is clearly not the case for interior wall finishes, durability, maintenance requirements, and impacts on indoor air quality are aspects of the use phase worth noting. With conventional drywall, all three of these considerations are linked to the paper surface. Rik Master of USG states, “The switch from plaster to a paper board finish did not come without effect—the finish of drywall is by no means as good a performer as plaster.” The hard dense surface of plaster is inherently more durable and less permeable to moisture than conventional drywall. In a high-moisture environment, the already-shorter service life of drywall (compared to plaster) can be further shortened. The paper surface can also serve as a medium for mold growth and a sink for volatile organic compounds, either of which can compromise indoor air quality.
Joe Lstiburek of Building Science Corporation in Westford, Massachusetts has recently been involved with three large production builders on the problem of sulfurous odors emanating from interior wall surfaces. The odors may result from interactions of the interior paint, drywall, and joint compound. Lstiburek told
EBN that “no one—not the paint, the wallboard, or the joint compound manufacturers—can say exactly what is in their product or what the interaction with the other materials might be. This is leading to some serious performance issues.” The situation has led to one builder buying back several homes and becoming embroiled in litigation on others. Lstiburek went on to add, ”We now know how to clean up the problem when it occurs—wiping the walls down with a weak vinegar solution and repainting—but we are still working on the cause. It doesn’t help any that both paint and joint compound manufacturers alter their formulations by region and season.” The absorbent and organic nature of the paper finish on drywall supports both chemical interaction and biological activity; clearly more study is needed on this issue.
Gypsum fiberboard shares plaster’s use-phase advantages over conventional drywall. Fiberboard’s more durable finish, lower moisture permeability, and lack of paper surfaces give the material significant environmental advantages but also contribute to its higher cost and more difficult installation. An additional advantage unique to plaster systems is the ability to incorporate color into the plaster, eliminating the need for paint.
With few exceptions, interior wall surfaces of gypsum-based products are landfilled at the end of their useful life. The degree of commingling with other building materials and the fact that most such products have been painted makes recovery for reuse or recycling technically and economically very difficult. Gypsum board’s relatively benign nature makes its disposal in properly operated C&D landfills uneventful. However, the potential for hydrogen disulfide generation in any landfill lacking proper drainage—but particularly MSW landfills inherently containing lots of organic matter—can represent a significant environmental hazard. During construction, however, there are real opportunities with gypsum-based interior finish materials to reduce impacts of disposal. (Technically, this is the very first step in the usage phase, not product end-of-life, but because it is disposal-related, it will be addressed here.) Of the three R’s—reduction, reuse, and recycling—the biggest environmental benefits are provided by waste reduction. Any panel system generates inherently more waste in the form of cut-off product than a site-mixed system like plaster. But there are significant opportunities to reduce cut-off waste and reduce the amount of building materials used to support the interior surface. Modular room and building dimensions—designing to even-increment wall heights and lengths—can result in significant waste reduction. The ability of most wallboard manufacturers to run board widths of both 48” (1.2 m) and 54” (1.4 m) means reduced cut-off waste in rooms where 9-foot (2.7 m) ceilings are required functionally or aesthetically. Techniques such as drywall clips or ladder blocking don’t reduce board waste but do improve framing and energy efficiency with no loss in performance of the interior surface. Perhaps the most ingenious waste reduction strategy for interior walls is a technique called “back-blocking,” first developed by USG but refined by professional drywaller Myron R. Ferguson (see constsruction detail, page 12). Developed for appearance reasons, back-blocking can also significantly reduce cut-off waste.
There are limited opportunities for reuse of cut-off waste wallboard. If waste reduction strategies are employed, the total amount and dimensions of cut-off waste will be greatly reduced, so use in less critical areas—such as closets and utility rooms—will be less of a possibility. But conventional drywall cut-off waste can be recycled both on the job site and at the manufacturing facility.
Research conducted by the USDA Agricultural Research Service in Beltsville, Maryland and funded in part by the Gypsum Association showed that uncontaminated wallboard waste could be effectively used as a soil amendment, particularly in high-clay-content or sodic soils. Follow-up field research conducted by the NAHB Research Center with builders in Indianapolis demonstrated that a mobile grinder can be a cost-effective way to handle both drywall and wood waste on the job site, particularly for production builders. (For more information, see resources at article’s end.)
Wallboard manufacturing plants have long possessed the ability to recycle their defective board—generally about 1–3%—back into new board product. Only a few of the plants or those in areas with really low disposal costs still landfill their culled, defective board. A handful of plants are producing new board with up to 20% recycled-content, utilizing processed cut-off waste from recyclers or accepting board scrap back from job sites and modular building facilities. Unpainted gypsum board can be recycled either by separating the paper and gypsum for recycling each material individually, or by recycling the entire product as a paper-gypsum blend. Each approach has its challenges. Markets for paper containing even small amounts of gypsum have been hard to identify, and blending paper homogeneously with the gypsum core is technically difficult.
No statistics on gypsum recycling are available from either USGS or the Gypsum Association. “The manufacturers consider this to be proprietary information and a part of their competitive advantage,” says Bob Wessel, technical director of the Gypsum Association. “We know that quite a bit of board is being recycled and [that] the quantities are growing; we just cannot state in what amounts or by whom.” Some manufacturers have back-haul arrangements with large, single-source accounts, such as modular home plants—flatbed trailers delivering new wallboard return to the wallboard plant loaded with pallets of cut-off waste for recycling. But such arrangements are considered a special customer service, and the economics don’t currently support broad-scale job-site recycling into new wallboard.
Gypsum systems for interior surfaces have significant practical and environmental advantages—the raw materials employed are abundant and largely benign, and the systems are economical and versatile. When selecting and working with interior finish systems, use the following principles and practices to guide your decision-making:
• Plaster systems are the most durable, generate the least waste, and have the lowest potential to generate indoor air quality problems. Use this system when the economics support these life-cycle advantages. Consider integral-color systems to further reduce the environmental impact of plaster.
• Gypsum fiberboard systems have many of the environmental and performance advantages of plaster and have the additional advantage of lower unit cost. This option is particularly well suited to buildings or areas of buildings where durability and moisture resistance are required or desired.
• Flue-gas desulfurization gypsum has a smaller environmental footprint than mined gypsum, but it’s not easy to determine which board products contain FGD gypsum. Additionally, it would not take more than 100–200 additional miles of transport of the finished product to offset the lower production energy of the FGD gypsum relative to mined gypsum.
• Use powder-form setting or drying joint compound. The packaging waste is less, transportation impacts are less (about half the weight of ready mix is water), and potential problems with preservatives are reduced or eliminated.
• At every opportunity, employ the waste reduction techniques in design and construction as suggested in this article. In typical construction, about one pound of wallboard cut-off waste is generated per square foot of building floor area (4.9 kg/m
2)—literally tons of opportunity to reduce construction waste!
• Using the resources listed below, determine the feasibility of recycling gypsum board products on-site or recycling cut-off waste at a wallboard manufacturing facility.