The Green Thumb: Cover Story (Part Three)
HISTORICAL PERSPECTIVEIn the early part of the 20th century (roughly between 1905 and 1930), plywood veneers were derived from old growth, virgin forests in the Pacific Northwest. In particular, coastal Washington State provided high quality large diameter logs ideal for making plywood. However, it was not until the '40s that the demand for plywood would grow exponentially.
Inevitably, by the '60s, this growing demand would cause a decline in the availability of large diameter logs. Prior to the 1940s, 1-foot-by-8-foot boards were used as sheathing. Though boards have the advantage of being minimally processed and free of chemical additives, they are labor intensive and not cost-competitive with plywood and/or OSB panels. However, softwood boards may be purchased directly from small, local mills whereby the cost for green or air-dried boards may be cost-competitive with plywood/OSB.
From an environmental perspective, the upside for board sheathing is the lack of chemical additives, such as formaldehyde. The downside is the inefficient use of the raw material: Only about 45 to 50 percent of a milled log can be turned into boards. By the late '40s, when the post-World War II building boom was in high gear, architects increasingly began specifying 4-foot-by-8-foot plywood panels to save time and reduce labor costs.
SUPPLY AND DEMANDWith the demand increasing and the large diameter log supply decreasing, the plywood industry was, by the mid-'60s, searching for an alternate softwood for the Douglas fir of the Pacific Northwest which had been used exclusively by the industry for plywood production in the preceding decades. Southern Yellow Pine from the Southeastern U.S. would become the standard for softwood plywood panel production and dimensional lumber from that time forward.
The 1960s also saw the introduction of paper-faced gypsum sheathing. By the '70s, even small-diameter logs were getting harder to obtain. It was then that the industry began experimenting with wafer and flake-board. By 1981, OSB had made significant inroads into the sheathing market and the American Plywood Association began to accept OSB producers as members.
Unlike plywood production, the manufacture of OSB is a fully mechanized process absent of any direct human handling. Logs as small as 4-inch diameter from tree species with little commercial value, such as aspen (in the Midwest and California) and mixtures of soft and hardwood trimmings from commercial tree farms, are used in OSB mills in the Southeastern U.S. High-speed flaking machines make the "strands" from clean, debarked logs.
After the 1973 oil embargo, energy efficiency was at the top of everyone's list. It was then that rigid foam insulating sheathing was first introduced as a means to increase the built environment's energy efficiency. However, those early foam-sheathing products contained chlorofluoro and hydrochlorofluoro carbons, which destroy the atmospheric ozone. The ozone layer provides essential protection from harmful solar radiation.
By the '80s, Georgia-Pacific introduced a new category of gypsum-based sheathing-glass-mat faced gypsum sheathing with DensGlass Gold. A fourth class of sheathing was also introduced in the 1980s: cement-based.
Sheathing products can be categorized into four basic types:
A fifth "hybrid" group uses wood or agricultural waste to produce fiberboard. For example, a plant in Louisiana uses sugar cane as the waste Ag-fiber in its panels. From an environmental perspective, by using byproducts and waste as the primary source of raw material, efficiency of the raw material is maximized by "reuse" (one of the four "R's": recycle, reduce, reuse, restore) of the green building movement.
Fiberboard is often used in conjunction with plywood or OSB, whereby the building's corners use plywood or OSB for strength and racking-resistance while the "field" utilizes fiberboard for economy. Though it uses raw materials efficiently, fiberboard uses large amounts of natural gas, therefore it has a high embodied energy. Wood pulp or Ag-fibers are mixed into a slurry, then shaped and dried in 300-foot long ovens. It is impregnated with emulsified asphalt for stiffening and, for moisture resistance, wax. Both wax and asphalt are petroleum-based and will off-gas petroleum odors. With the exception being the pressing process, the manufacturing process for fiberboard is very similar to that of paper.
DEALER'S CHOICEWhen selecting the type of sheathing product to use, it's important to consider the performance criteria of each type. Beyond providing a surface for the application of exterior claddings and enclosure of the building envelope, building code requirements must also be taken into consideration. In general, performance criteria typically includes:
- Racking/shear resistance
- Moisture/water resistance
- Dimensional stability
- Fastener holding strength (aka "pull-out" resistance)
- Installation difficulties/concerns
- Contractor acceptance
Another important consideration when choosing a sheathing product is climate. For exterior finishes, such as brick veneer, EIFS, cement plaster, etc., the best choice for wet climates (20-plus inches of rainfall per year) would be cement-based. For moderate moisture climates, gypsum/cellulose works well and for dry, moderate wind-speed climates either wood or gypsum-based is appropriate. Heat and/or moisture can/will cause sheathing panels to expand and/or contract thus, the more stable the sheathing under these conditions the lesser the potential for cracking and buckling of the exterior finish. Wood-based sheathing, when exposed to prolonged temperature extremes and/or moisture exposure, can warp and shrink but under normal atmospheric temperature and humidity conditions, remains stable. As such, cement- and gypsum-based products are considered more stable under variable temperature and moisture conditions.
For shear resistance, the ability of the sheathing to maintain its integrity (not collapse or move out-of-square) under positive (inward) and negative (outward) wind loads and/or a seismic (earthquake) event, plywood/OSB is the best choice. However, as mentioned, it is combustible therefore only suitable for residential construction. With more choices in commercial (non-combustible) construction, gypsum/cellulose comes out on top. With a flatter, smoother panel surface, gypsum/cellulose can resist wind loads of 61 psf with fasteners spaced 8 inches o.c. over 16 inches o.c. framing (holding strength of 163 pounds per fastener). Core-reinforced gypsum/cellulose panels have twice the shear resistance of surface-reinforced gypsum-based panels and outperform even cement-based sheathing panels in this regard.
Lastly, three factors should be considered concerning installation when choosing a sheathing material:
- Handling and distribution
- Field cutting
- Attachment to framing
The easier sheathing is to handle, cut and attach the more likely it will be installed correctly and utilize labor most efficiently. As such, gypsum-based sheathing is preferable since it is relatively lightweight and can be scored and snapped with a utility knife negating the need for power tools to cut it in the field. Standard self-piercing and/or self-tapping screws in a pre-determined edge/field pattern provide a positive, mechanical attachment of the sheathing allowing for quick, easy installation. Of course, cost of material and environmental impacts are major concerns but they should be measured against long-term performance and the overall benefit-to-burden ratio.
As we shall see in upcoming articles in this series, each type of sheathing product has its pros and cons. In part four, we'll begin a discussion focusing on one of the most popular non-combustible sheathing types: gypsum-based.
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