In assessing sheathing products, we need to look first of all at the functions of sheathing. Is wall sheathing really necessary, or can its purpose be fulfilled in other, less material-intensive ways? If it is needed, what are the environmental considerations in choosing among different sheathing products?
Factors to consider when evaluating sheathing products from an environmental perspective include:
The most appropriate choice among wood-based products often depends on regional factors, so geographic information is also an important part of this assessment.
General functions of sheathing
The functional demands on a sheathing product vary with the particular application and detailing. On a roof, the sheathing provides a solid, flat surface for the roofing. On walls, sheathing is expected to provide an added layer of weather protection for the wall cavities. Other functions sometimes ascribed to sheathing include providing racking resistance for the frame and adding insulation to the wall assembly. In addition, permeability can be an important factor in sheathing performance.
Many of these functions can be provided in other ways. Diagonal bracing can provide racking resistance, for example, as can shear panels on the corners of the building. A layer of permeable housewrap can offer weather resistance, and thicker walls can add insulation value without the use of high R-value sheathing.
Where codes allow, some buildings are indeed built with no wall sheathing. From a resource efficiency perspective, this approach makes sense. It is appropriate, however, only if the durability of the structure is not compromised. Builders who continue to use some type of sheathing (and most are), should know about the environmental impacts of each type. Insulative sheathing products and their impact on the environment were discussed in the July/August issue of EBN (Rigid Foam Insulation and the Environment). Some of the environmental considerations with wood-fiber-based sheathing products are presented below.
Evaluating the status of the wood resource in North America, and determining how it should be used, is immensely complicated. Virgin, old-growth forests are rapidly disappearing from the continent. Overall, however, trees are growing faster than they are being cut—twice as fast in Canada, according to Jamie Miel, Business Economist at Forintek Canada Corporation. Making environmental choices among different wood products would depend largely on the harvesting procedures used and the condition of the forests in each region.
Using whole trees: boards, plywood, OSB
Board sheathing is still used in some regions, though boards aren’t cost-competitive with panels when purchased through building supply operations, and they are more labor-intensive to install. Boards that are sold on the national building supply market are often harvested and processed side-by-side with plywood and other products. On average, only 45% to 50% of each log milled is converted into boards, representing a relatively inefficient use the tree resource. Nevertheless, customers willing to pay a premium for minimally processed materials without chemical additives may specify board sheathing anywhere in the country.
In regions that have small, local woodlots and sawmills, softwood boards can sometimes be purchased directly from the mill. The price for local green or air-dried boards can be competitive with plywood. Although management practices among small, privately owned woodlots vary considerably, the benefits of using a local, minimal embodied energy product should not be ignored. Builders who can get local, air-dried boards and take the time to install them properly should give this option serious consideration.
In the early days of plywood production in North America (from 1905 to 1930), only large-diameter clear logs from coastal Washington state were used. The decreasing supply of premium logs since that time has caused continuing changes in the softwood plywood industry, and movement into new areas. (By the mid-1960s the search for new logs brought the first plywood plants to the Southeastern U.S. to use southern yellow pine.)
Various technological developments over the years allowed the use of differing species (the early plants used Douglas fir exclusively), and smaller-diameter knotty logs. As even the smaller logs needed for plywood veneers became harder to come by, the industry began looking to alternatives. Following experimentation with various versions of waferboard and flakeboard in the 1970s, oriented strand board (OSB) started making serious inroads into a market that had been controlled by the plywood industry. In 1981 the American Plywood Association began accepting OSB-producing mills as members.
The production of OSB, unlike that of plywood, is a fully mechanized process, requiring no direct human handling of the materials. The high-speed flaking machines that make the strands for OSB can only accept debarked, clean logs. These logs can be as small as 4" in diameter, however, and tree species that have little commercial value can be used. Aspen is the most commonly used species in the Midwest and Canada, while southern OSB plants use a mixture of softwood and hardwood thinnings from commercial tree farms.
Several sheathing products use by-products of other operations as their primary resource. Using by-products from sawmills or plywood mills is a way to stretch the resource base by getting the most out of each tree harvested. One product that uses mill waste is a hybrid of plywood and particleboard called “comply.” Currently manufactured by just one company (Oregon Strand Board, 34363 Lake Creek Drive, Brownsville, OR 97327; 503/466-5177, 800/533-3374—western states), comply has plywood veneers both on the outer faces and in the center, with layers of wood fiber compressed in between. The added fiber is primarily planer shavings from a nearby sawmill that also provide heat energy for the plant (see below). Comply is available primarily in the Northwest, where it may be a good choice for roof sheathing and subfloors because of its high strength and efficient use of the wood resource.
Fiberboard is a lightweight panel made from wood or agricultural waste (a Louisiana plant uses sugar cane) impregnated with emulsified asphalt. It is produced in a process similar to paper making, without the final pressing. Fiberboard is made by two companies in Canada and six in the U.S., mostly east of the Mississippi. It is commonly used as sheathing with plywood shear panels on the corners for racking strength.
For a resource-efficient sheathing where high strength is not important, Thermo‑ply® may be a good choice. Only about
8" thick, Thermo-ply (Simplex Products, P.O. Box 10, Adrian, MI 49221 (517/263-8881) is manufactured from 100% recycled kraft paper with foil or polyethylene facings. A similar product, Energy Brace®, is made in Virginia by Fibrelam (P.O. Box 2002, Doswell, VA 23047 (804/876‑3135). The Fibrelam product uses only about 25% recycled paper, however, and only one side is faced with foil. Both products are made in several color-coded strength grades. If installed with the foil facing next to an air gap, it can add to the wall’s insulating value. Try to get the 100% recycled Thermo-ply instead of the 25% recycled Energy Brace. If you’re counting on it for racking resistance, use a grade better than is required by code, and follow manufacturer’s recommendations for fastener spacing.
All sheathing products except plain boards have additives to bind the materials together or provide moisture resistance. While none of these additives are things you’d want your baby chewing on, they may be tolerable on the outside of your walls. None out-gas as much as the urea-formaldehyde resins used in most interior panel products.
Almost all exterior grade plywood and 90% of OSB panels, and com-ply use phenol formaldehyde as their binder. Phenol formaldehyde (PF) resins are synthesized from petroleum (phenol) and natural gas (formaldehyde). Plywood typically contains 1
2% PF resin by weight, and OSB 2%. PF is a relatively stable, heat-cured resin, used in exterior grade products that require moisture resistance. Out-gassing is most severe when the product is new, so it is a good idea to ventilate buildings well during construction. A tight vapor barrier in the wall should effectively seal off formaldehyde gas from reaching the building’s occupants.
OSB products that claim to be “formaldehyde free,” such as Louisiana-Pacific’s Inner Seal® products, use an isocyanate resin known in the industry as MDI. MDI is wholly derived from natural gas, and can be highly toxic until it is cured. After curing, it is considered very stable and safe.
Water resisting additives: fiberboard, Thermo-ply
Fiberboard is impregnated with emulsified asphalt to stiffen it, and wax for moisture resistance. Both the asphalt and wax are derived from petroleum. Thermo-ply is also made with wax. In addition, the layers of kraft paper inside Thermo-ply are glued together with polyvinyl alcohol (PVOH), which is the main ingredient in many hot-melt glues.
Environmental problems in the wood products industry have come under heavy scrutiny by federal and state environmental officials in the last two decades, especially at large facilities. Assessing the relative damage from pollution of one type or another can be extremely complicated. Governmental controls, however, are bringing the major pollution problems associated with wood products into line with the requirements of the Clean Air and Clean Water Acts.
The most visible form of pollution from plywood manufacturing is the “blue haze” that once enveloped whole areas surrounding the plants. As veneer comes off the lathe, it is dried very quickly at high temperatures. The extracted water vapor draws with it resins from within the wood. It is these volatile organic compounds (VOCs), that comprise the bulk of the blue haze. The lighter of these resins (turpenes) are the same as those released naturally by growing trees. The American Plywood Association has estimated that a mid-sized plywood plant emits as much of these turpenes as 100 acres of growing forest.
The stresses in a veneer dryer also draw out heavier forms of the resins (resin acids and sesquiturpenes), which are not released to the atmosphere under natural conditions. These heavier VOCs condense into droplets in the air. It is these droplets, less than one micron (1 millionth on a meter) in size, that make the blue haze most visible. Aside from contributing directly to local smog, these VOCs react with nitrous oxide (NO
x) from fossil fuel emissions to form ozone. The industry has invested large amounts of money in various technologies for controlling the dryer emissions, with varying degrees of success.
OSB production causes a similar problem, as the strands are dried in a tumble dryer down to 3% or 4% moisture content. Many of the wood species used for OSB are not resinous as is the pine or Douglas fir used in plywood, so VOCs as such are less of a problem. Instead, the OSB industry has fatty acids from hardwood species to control, as well as tiny wood particles from the drying chips, and even smoke released as the high dryer temperatures singe some of the fiber.
Other environmental issues at panel plants include wastewater discharge and flue gas emissions. Once a serious problem, wastewater is now recycled in all plants, and discharge problems are rare. Flue gas emissions are strictly controlled under the Clean Air Act, though even legal emissions are not benign. Higher energy use in the manufacture of a product almost always means more emissions, though the major plants all have scrubbers to trap most of the pollution.
People in the forest products industry are quick to point out that wood products are not particularly energy intensive to produce because the trees have already done most of the producing, using solar energy. Much of the energy that is required, at least for some products, can be produced from waste wood fibers. Still, significant amounts of fossil fuels and electricity are required for harvesting and processing, so it makes sense to look into choosing products with less embodied energy. This assessment varies from region to region due to the importance of transportation and local resource availability.
Evaluating the amount of energy embodied in a product is extremely complex, and any absolute figures can easily be contested. For comparing products, however, this kind of analysis can be useful. In the wood products industry, the embodied energy assessment must include the energy consumed in the process of cutting trees, transporting them to the mill, processing them into products, and distributing the finished products to their end use. Any added components, such as binders, also have to be included. While an exhaustive embodied energy analysis is beyond the scope of this article, some conclusions can be drawn from existing work on the subject and a general look at the industry.
Harvesting raw material & transporting to mill
The energy used to harvest trees and transport the logs to mills is primarily in the form of diesel fuel for vehicles. The amount of energy used depends on how the wood is harvested (clear-cutting is probably more efficient in the short term) and the distance to the mill. For products that use whole logs, the energy costs of getting them to the mill are usually similar. In fact, many large companies have sawmills, plywood mills, and other production facilities all at the same site. Every delivery of logs is sorted according to the best use of each log.
Sheathing products using wood waste or other industrial by-products require no energy for harvesting the resource. Transporting the material requires some energy, but the distances involved are usually small. Other potential uses of these by-products are important, however, and would need to factored into a comprehensive energy analysis. Competition for sawmill waste among various uses makes it an increasingly valuable commodity.
Energy use for processing the logs and manufacturing sheathing products is divided between heat energy, usually generated on site, and mechanical energy, usually from electricity. Because the amount of mechanical energy used is small compared to the amount of heat energy used to dry wood products, producing kiln-dried lumber takes almost five times as much energy as producing air-dried lumber.
The mechanical energy uses for plywood are similar to those for lumber. The thermal energy requirements are greater, however. In addition to drying the veneers, heat is used to precondition the logs before peeling and to cure the binders in the pressing stage.
OSB production consumes more energy than plywood. More energy is required for chipping than for peeling veneers, drying the strands uses more energy, and the panels are pressed at three times the pressure of plywood. Fiberboard also uses large amounts of energy as the wood pulp is mixed into a slurry, then shaped and dried in 300 foot-long ovens. Figures for Thermo-ply are unavailable.
The binders used in panel products also contribute significantly to their embodied energy. Energy is required for extracting and transporting the petroleum and natural gas raw materials, as well as for processing these raw materials into phenolic resin (phenol formaldehyde), MDI resin, or asphalt. Some steps in the process, however, also release energy, which is recycled into the operation and can be credited against the energy demand.
Heat for drying and curing wood products is the biggest consumer of energy in the production process, but in some cases this heat is obtained by burning by-products. OSB and comply, which are relatively new and therefore have more efficient plants, are typically 100% self sufficient in this way. Some plants even generate a portion of their mechanical energy needs from waste wood combustion.
Plywood plants and some sawmills also generate some of their own heat, although many of the by-products of plywood production and lumber milling are sold for particleboard, pellets, or other commercial uses. Whether they are reclaimed within the plant or sold as an energy source or raw material for another product, the energy value of these by-products can be credited against the energy demand of the process. Researchers disagree, however, over how much credit to apply. By any calculation, boards that are installed green or partially air-dried consume a fraction of the energy cost of plywood or kiln-dried boards.
Fiberboard production consumes large amounts of fossil fuel, typically natural gas. Very little wood fiber by-product is generated in that industry because most of the wood fiber the plants receive goes into the boards. The vast majority of fiberboard sold for wall applications is non-structural, used only to provide an added layer of weather protection and a little insulation. Even though it is made from by-products, given the large amounts of energy required to make fiberboard, it makes sense to look into less energy-intensive ways to meet these needs.
The final component of the embodied energy equation is the energy required to get the finished products from the mill to the building site. While it’s impossible to track each individual board or panel, some general patterns are clear. Transportation is expensive, especially in the highly competitive building products market. Whenever possible, specify products that are manufactured in your region (see maps). Doing so will not only save the energy and pollution associated with shipping, but also will get you better value because the cost of the product will reflect more of the investment in producing it and less of the cost of transportation.
There are no real “bad guys” in the wood-fiber sheathing field. There are better and worse choices, however. Plywood plants and large-scale commercial sawmills rely on a dwindling resource—large logs. Many mills in the Pacific Northwest depend on timber from old-growth forests that is rapidly disappearing. Comply uses less of this valuable wood resource, while OSB is made from newer growth in areas where the original forests disappeared long ago.
As the price of plywood continues to climb, perhaps it’s time we move either back to basics, with locally milled boards, or into the high-tech world of composite panels such as OSB or comply. When the sheathing isn’t needed to brace the building, no sheathing at all or a light-weight product like Thermo-ply is worth considering. Taking a good look at the functions sheathing has to fulfill in your design, the resource and energy requirements of each material, and the local resource supply, you can make choices that make sense on all counts.