Form follows functionSheathing is used for both wall and roof applications providing a solid, flat monolithic surface for finish materials to be attached. In residential (combustible) construction, wood-based sheathing, such as plywood and/or oriented strand board provides racking resistance for the framing to resist shear forces generated by ground movement in a seismic (earthquake) event and/or the effects of air pressure differential (the wind). As well, sheathing provides protection of wall/roof cavities and adds insulating value to the overall assembly (U-factor).
In light-gauge metal-framed structures, diagonal "X-bracing" can often substitute and/or supplement the racking resistance of an exterior wall assembly. Typically, "flat-stock" material is used to form the bracing and it is either screwed or welded to the framing members.
Nowadays, a permeable housewrap, such as Tyvek, is used to provide weather resistance (air penetration). Typically placed over wood-based sheathing (plywood, OSB) in residential construction, it helps to maintain the value of the cavity insulation by preventing air penetration through joints between sheathing panels and/or other "flanking paths" in the building envelope. It is also permeable, allowing water vapor to pass through it by means of vapor diffusion, thus preventing water vapor from condensing into liquid water in a wall cavity.
Code permitting, sheathing may be omitted for certain wall assemblies but it's never a good idea. For example, certain cement plaster stucco wall assemblies do not include wood sheathing-the metal lath is attached directly to the framing and then the three-coats (scratch, brown and finish) are applied. Known as an "open assembly" (often referred to as a "Florida wall"), organizations, such as the Texas Bureau for Lathing & Plastering (funded by the TLPCA) do not recommend such assemblies. Excessive cracking can/may result from the lack of a solid substrate-there is no substitute for the solid substrate wood sheathing provides. Because liquid water and wood sheathing are enemies, it's typically required to place a moisture barrier, such as building paper, over wood sheathing to prevent any water that gets behind the finish material from making contact with the wood.
Because the rule of thumb for vapor diffusion states whatever gets into a wall must be able to get out of the wall, permeable materials must compliment one another. In a previous article called "If I Only Had a Brain" (W&C, June 2004), I talked about the problems with 6-mil polyethylene vapor barriers-it is always impermeable. Also, the fact that plywood is 100 times more vapor permeable than OSB makes it the better choice for solid wood sheathing. In effect, OSB, like polyethylene, is an impermeable vapor barrier.
Environmental concernsAs we found in the Déjà Vu x 2 series, when used as a binder for wood panel products, formaldehyde can have serious health consequences since it is a known human carcinogen. Fortunately, where wood sheathing is concerned, the lesser of two evils insoluble phenol formaldehyde is used as the standard binder for wood based exterior sheathing products: plywood and OSB. Unlike interior grade, water-soluble urea formaldehyde, formaldehyde off-gassing is much less of a concern with a PF binder. PF resins are synthesized from petroleum (phenol) and natural gas (formaldehyde) providing a stable, heat-cured resin for a strong, permanent bond to the wood fiber. It is ideal for exterior grade products where moisture resistance is required.
By weight, plywood contains 1.5 percent PF, whereas OSB contains 2 percent. As well, plywood and OSB panels (unless they are non-added formaldehyde) use 100 percent and 90 percent PF as a binder, respectively. Off-gassing of formaldehyde is more acute when the plywood/OSB panels are new so there should be good ventilation when installed. However, since sheathing is installed on the exterior side of an exterior wall typically, off-gassing is not a problem since mother nature provides the ventilation. Still, for a particularly sensitive person, it might be a problem.
Plywood/OSB with a PF resin binder off-gases about 0.04 parts per million under normal circumstances-about the same level contained in outdoor air. An effective vapor barrier, such as polyethylene or foil, will prevent any minor off-gassing into the indoor environment. For cold climates, the vapor barrier is typically placed on the "warm-in-winter" or room side of the exterior wall. In warm, humid climates it is placed on the exterior side of the exterior wall assembly.
Blue hazeProducing plywood and/or OSB underwent a catharsis when the Federal Clean Air & Water Act was passed in 1990. Nowadays, flu-gas emissions are strictly controlled by industrial scrubbers trapping pollutants before they become airborne in the atmosphere. Even so, these "legal" emissions are far from being benign. Once a serious problem, wastewater discharge is now recycled and reused in all manufacturing plants. Before the clean air/water act, a "Blue Haze" would envelope entire areas around plywood manufacturing plants. The veneer used in plywood panels dries very quickly under high temperatures when it comes off the lathe in the milling process. Then, extracted water vapor draws with it resins from within the wood releasing volatile organic compounds in the form of a blue haze.
Turpenes, the same as those that are naturally released by pine trees, are the lighter of these resins. However, the veneer dryer also draws out the heavier forms of resins:
- Resin acids
These heavier VOCs condense into droplets in the air making blue haze visible. Less than one micron (one millionth of a meter in size), they react with nitrous oxide from fossil fuel emissions and form ozone-an unstable form of oxygen harmful to the earth's atmosphere. Under natural conditions, these heavier VOCs would not be released into the atmosphere. The American Plywood Association estimated that a mid-size plywood plant emitted the same volume of turpenes into the atmosphere as 100 acres of forest. Under the clean air/water act, the industry invested millions of dollars in technologies that help to control the emissions from veneer dryers effectively eliminating the blue haze phenomenon.
Though producing OSB had similar problems, there were many differences-mainly due to the use of "strands" rather than veneers. Typically Douglas fir or pine is used for plywood production. These species tend to be much more resinous than the aspen-a commercially unviable species, typically used to make OSB. Containing about 3 to 4 percent moisture, the strands are tumble-dried. Though off-gassing VOCs are not a major problem, there are other obstacles for OSB:
- Fatty acids from hardwood species
- Minute wood particles from drying strands
- Smoke released from singed fiber from the high-temperature tumble dryer
- OSB requires more energy to produce than plywood:
- More energy required for chipping (vs. peeling veneer of log for plywood)
- Tumble-drying strands uses more energy than a veneer dryer
- OSB panels are pressed together at three times the pressure of plywood
In part two, we'll continue to look at the environmental impacts of wood sheathing from a broad perspective.
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