Sheathing

Topics: About Plywoods; Mitigating Outgassing; Sheathing Alternatives (MgO Board)

Plywoods: It is standard procedure to build homes with a sheathing layer, which significantly increases the shear strength of the wall. Sheathing is typically done with plywoods that contain formaldehyde adhesive. See How Plywood is Made. Formaldehyde comes in many synthetic resin mixtures, such as phenol-formaldehyde (PF), malamine-formaldehyde (MF) and urea-formaldehyde (UF).

Formaldehyde has been classified as a known carcinogen by the State of California (Prop 65) and the World Health Organization (International Agency for Research on Cancer). According to the American Lung Association, symptoms such as bronchial irritation, asthma, etc, are often seen at levels ranging from 0.4 to 3 parts per million (ppm) and can affect sensitive individuals at concentrations as low as 0.037 ppm.

Although plywood with formaldehyde (as the glue) is not a healthy choice, "exterior grade" is preferable over "interior grade." The phenol formaldehyde binders of exterior grade are waterproof and more stable than the urea formaldehyde binders of interior grade, which are only water resistant. Urea formaldehyde readily volatilizes at room temperature.

The most widely used formaldehyde-free alternative resins are MDI (methylene diphenyl isocyanate) and PVA (polyvinyl acetate). Both of these resins are very toxic. Some plywoods that use MDI as a binder are marketed as "non-toxic." Unfortunately, MDI is a dangerous chemical, even at relatively low air concentrations of .005ppm.1 Kirei Wheatboard, shown right, is marketed at "non-toxic," but uses MDI as an adhesive

Mitigating Outgassing: In general, outgassing will take much longer if the temperature is low, if the humidity is high, and/or if there is inadequate fresh air exchange. The best method for decreasing outgassing time is the following: using large fans, or better yet, an industrial machine called a “negative air machine" to pull in as much fresh air as possible, and exhaust the air out a window or door. The idea is to move as much air as possible, and to achieve as much air exchange as possible. Unless the humidity levels are high outdoors, this will greatly speed up outgassing and cure the offending products faster. You can rent negative air machines, also called air movers, from reputable mold remediation companies that are certified by the Indoor Air Quality Association.

There is a product called Safe Seal by AFM SafeCoat that is designed to reduce toxic outgassing on particle boards and plywoods. It is an "aqueous acrylate-styrene copolymer dispersion". Styrene is also known as vinyl benzene or phenyl ethene, which is regulated as a "hazardous chemical." I don't recommend using this product. Why attempt to block outgassing by using another product itself which is toxic? I've had good results using pure acrylic to seal outgassing materials. Earthpaint carries a few products with pure acrylic.

Alternatives to Formaldehyde Plywood


Columbia Forest Products makes a "cabinetry grade" board called PureBond with a soy-based glue. They market their technology as "formaldehyde free," but take a close look at their MSDS and its footnotes, which shows that their products still contain either phenol formaldehyde adhesive or MDI

The footnotes of the MSDS claims that through the manufacturing process the phenol formaldehyde is bound into the product and unable to be released into the air. In their own words,
These adhesives, phenol formaldehyde and diphenylmethane diisocyanate (MDI), unlike urea formaldehyde adhesives, are chemically reacted into stable bonds during pressing which do not have the potential to hydrolyze and re emit formaldehyde back into the atmosphere. Once the phenol formaldehyde and MDI platform adhesive systems are reacted, the final panel products have such low formaldehyde emission levels (background or “de minimis” levels of 30 ppb or less) that they easily meet or, in certain cases, are exempted from the world’s leading formaldehyde emission and building standards.
I've talked to the Columbia Forest Product representatives and have had trouble getting a full understanding about all this. PureBond is manufactured for "interior, decorative use only." This product is carried in Home Depot.


MgO Board, or Magnesium Oxide Board, is a non-insulating sheathing board that can be substituted for drywall, backerboard and underlayments, but with much improved characteristics, such as fire resistance, weatherability and resistance to mold and mildew. It is made of Magnesium Oxide, a type of mineral cement.

MgO board is not as strong as plywood, but has been used for exterior sheathing in place of plywood. I use MgO board for my interior walls and also for the exterior sheathing, as long as there is sufficient cross-bracing in the framing.

There are a number of suppliers of MgO board. You can find a list of them here.




1 http://oehha.ca.gov/air/chronic_rels/pdf/101688.pdf