IAQ & ADHESIVES
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.
Fernando Pagés Ruiz develops ecologically minded affordable housing in the Midwest and mountain states. His two books, Building an Affordable House and Affordable Remodel, are available from The Taunton Press.
READING THE LABEL
The certification agency stamp will show the following information for panel identification.
The certification agency stamp will show the following information for panel identification:
8-APA’s performance-rated panel standard
12-Panel grade, Canadian standard
13-Panel mark—Rating and end-use designation per the Canadian standard
14-Canadian performance rated panel standard
15-Panel face orientation indicator
Source: APA-The Engineered Wood Association
1 Panel grade
2 Span rating
4 Bond classification
5 Product standard
7 Mill number
8 APA’s performance-rated panel standard
11 HUD recognition
12 Panel grade, Canadian standard
13 Panel mark—Rating and end-use designation per the Canadian standard
14 Canadian performance rated panel standard
15 Panel face orientation indicator
Source: APA-The Engineered Wood Association