How plywood and OSB stack up in the search for sustainable sheathing.
Supplied by: Scott’s Contracting, Green Builder St Louis “Renewable Energy” Missouri By:Fernando Pages Ruiz
Back when I started framing houses, subfloor and sheathing choices were limited to solid, spaced sheathing and a still relatively new building product, plywood. The latter was gaining market share, but many old-timers resisted the thin, bendable, sometimes delaminating sheets of cross-grained veneer.
But eventually not a single floor, wall, or roof had anything but plywood over the joists, studs, and rafters, and today the same could be said for oriented strand board (OSB). Just as with plywood, some builders regarded OSB suspiciously before it became well established.
Today, a new generation of sheet materials is pushing this category toward new levels of performance, with products made from more durable raw materials and healthier resins, and panels that combine features that address moisture control, air infiltration, and energy performance in addition to their structural functions.
Before we explore these alternatives, let’s review the pros and cons of the old warhorses, plywood and OSB, as the former has made a comeback among quality-conscious builders, and concerns over deforestation and indoor air quality have muddled the question of sheathing with either plies or strands.
Plywood consists of an odd number of sheets of wood, glued together with the grain of each ply in a perpendicular direction, to create a structural panel with shear strength in all directions. Plywood’s cross-grain provides strength and greater holding power for screws and nails than solid-sawn wood.
Oriented strand board (OSB) uses a similar engineering principle, but instead of creating the multidirectional structure with large sheets of wood glued together, manufacturers of OSB arrange small strands of wood (2 to 3 inches in length) into a cross-grain pattern, and then bind them into a solid, structural panel using adhesives, pressure, and heat.
From an environmental perspective, the notable difference between the two panel products comes with the natural resources required to make them. The sheets of lumber used to make plywood are peeled in thin veneers off a log with a sophisticated lathe. The logs are older and larger, and from a more limited number of tree species, than those shredded for strands of OSB. Also, the plywood peeling process leaves a spindle of wood at the center, whereas manufacturers of OSB shred the entire log.
Green building certification agencies recognize the ecological advantage of engineered lumber products, principally OSB, which is used not only to make sheathing, but also joists, rafters, and substitutes for dimensional lumber.
Nevertheless, plywood has maintained its place and has seen some gains because of its greater resistance to moisture, especially around the edges, and slight advantage in nail- and screw-holding power. Many flooring and some roofing manufacturers prefer plywood under their products because of its greater stability versus OSB in humid conditions. Miami-Dade County, Fla., prohibits the use of OSB roof sheathing, given a comparatively high failure rate once wetted during historic storms.
With the exception of Miami-Dade, all national and international building codes regard plywood and OSB as equal, and use the generic phrase “wood structural panel” to clearly denote that the code recognizes these two materials on par. The leading green certification agencies, the USGBC and the NAHB, provide points for both products.
Both products carry similar performance-based certifications, primarily from APA-The Engineered Wood Association, and the U.S. Department of Commerce Voluntary standard for Wood Based Structural Panels (PS1 and PS2) that allow consumers and inspectors to know the exposure (outdoor, indoor, or marine), strength (structural capacity), span rating over framing members (adequate for 16, 24, 32 inches on-center), surface finish quality, and, more recently, compliance with air-quality standards.
But OSB has become the clear leader in construction not so much for its ecological as economic advantages: OSB is generally several dollars per sheet cheaper.
News reports of formaldehyde concentrations in mobile homes provided to victims of Hurricane Katrina has made builders concerned about the softwood, exterior structural panels used to sheathe walls, floors, and roofs. But the moisture-resistant glues used to make exterior sheathing in the U.S. do not contain urea formaldehyde, the adhesive that has created indoor air quality concerns. According to Marilyn LeMoine, spokesperson for the APA, all of the exterior, structural panels manufactured in the U.S. today comply with or are exempt from the California Air Resources Board (CARB) Air Toxic Control Measure for Composite Wood Products, arguably one of the world’s most stringent standards regulating toxic off-gassing from building materials.
Most OSB and many plywood panels use the adhesive diphenylmethane diisocyanate (MDI) as a binder, which contains no formaldehyde and no ecological risks, says LeMoine. Some plywood and OSB contain binders made from phenol formaldehyde, which becomes stable during processing and results in such low emission levels in the finished material that these products remain exempt from all formaldehyde emission standards.
The statement “no added formaldehyde” in a wood product may sound like a hedge, but it is only because wood itself contains small measures of formaldehyde. It’s all around us, as natural as air and water. You just don’t want to breathe too much of it. How much is too much? No one knows, and hence the effort to avoid products that raise the concentrations of formaldehyde indoors beyond the background levels found naturally outside.
Some foreign-made, exterior-grade panels allegedly contain unsafe levels of formaldehyde; buying trademarked panels stamped with the U.S. Product Standard PS1 (plywood) or PS2 (OSB) ensures that you are not adding measurable risk. Panels with an APA stamp comply with the CARB standards.
Beyond the structural stamps, plywood and OSB are available with certifications that confirm the product comes from a reputable source and sustainably managed forests. Although many forests are sustainably managed, the only way to provide credible proof is through independent, third-party auditing such as from FSC or SFI. Once the wood leaves the forest, a third-party, chain-of-custody certification monitors that the wood harvested is indeed the wood received by the end-user.
USGBC’s LEED program only gives points for FSC certification, but is currently considering including others; the ANSI National Green Building Standard and many other programs provide points for either.
From a green building perspective, the most interesting developments in sheathing can be found in some new products that integrate structural features with other components, such as insulation or weather-resistive barriers.
Dow’s SIS panels, for example, combine structural lateral bracing, insulation, and a water-resistive barrier. Huber Engineered Woods’ ZIP System roof and wall sheathing offers structural panels with a proprietary coating that acts as a weather barrier.
Innovative products also are helping to stiffen floor systems and reduce squeaks. AdvanTech from Huber features advanced resins for greater water resistance than commodity OSB and plywood, according to the company, as well as greater design bending strength and stiffness.
Weyerhaeuser’s iLevel Edge and Edge Gold floor sheathing products offer similar higher performance in structural stiffness and moisture resistance.
And while roof sheathing with integral reflective radiant barriers isn’t that new, its use is growing in hot, sunny climates where solar heat absorption from roofs can really crank up cooling loads.
Various brands of fiberboard sheathing, once used as cheap filler between structural panels, now have rebranded themselves as ecological, energy-efficient, and mildly structural sheathing systems. Homasote 440, a product originally designed for sound attenuation, is being repurposed as a high-performance exterior sheathing panel made from nearly 100% post-consumer recycled cellulose fiber with a maximum shear strength of 309 pounds per square foot (compared to let-in bracing at 245 pounds).
Manufacturers are offering or exploring a number of resource-efficient sheathing alternatives, such as ERT4C’s Eco-sheet, a European plywood replacement made from a mix of recycled polymers and other recycled materials including waste electrical and electronic equipment.
And researchers at Canada’s Alberta Research Council are developing an oriented structural straw board (OSSB) product, but because of straw’s small, relatively weak fibers, this option has so far proven difficult and expensive to produce as a structural product. This group is planning to open an OSSB plant in partnership with a private manufacturer.
The sheathing category is clearly evolving quickly, driven by our expanded knowledge of building science and the technical innovations manufacturers are bringing to their products. There are some great new options for green builders these days, and I am sure we’ll see even more in the years to come.
Contact: Scott’s Contracting for your Green Building Needs