Plywood and oriented strand board (OSB) sheathing

Plywood and oriented strand board

Plywood — Logs are felled, debarked and soaked in hot water. Veneer sheets are peeled from the soaked logs and glued together as plies, with the grain of each ply aligned perpendicular to the adjacent ply. The plies are pressed together with heat. Each ply is typically 3 mm or % inch thick and sheets for construction are made in 3-, 4-, 5- and 6-ply thicknesses.

OSB — Oriented strand board uses flakes of wood peeled from logs that would not have been suitable for other purposes. The flakes are dried, impregnated with glue, oriented in cross-directional “mats” and then pressed under high heat and pressure to form panels.

Both materials are available in 4×8 ft sheets. In most applications, the sheets are nailed to the studs, sills and headers of a wood frame wall or to roof rafters. Joints between panels are aligned on framing members and blocking. A nailing pattern is specified around the edge and in the field of each panel.

Plywood and oriented strand board (OSB) sheathing

Tips for successful installation

  1. If sheet material is to be used as sheathing, plan to make the best use of standard sheathing dimensions. Significant waste can accumulate if many custom cuts are required.
  2. Sheet wood can be used as a primary air control layer if all joints are gasketed, caulked or taped appropriately. Typically, these products are used in conjunction with sheet-type barriers, but the barriers are not required if the sheathing itself has been sealed adequately.
  3. Plywood and OSB are not very vapor-permeable. As they are usually applied on the exterior of a wall, this dictates the use of low-permeability materials on the interior, or the use of sheet-style vapor barriers.
  4. Be sure to follow the nailing pattern recommended by codes and/or manufacturer’s instructions and use the specified type and length of nail.

ENVIRONMENTAL IMPACTS

Harvesting — Moderate to High. Logs for sheet materials mainly come from large forestry operations, which can have impacts including habitat destruction, deforestation, soil erosion and silting and contamination of surface water.

Manufacturing — Moderate to High. Making sheet materials is a multi-stage process that requires relatively high-energy inputs, in particular the heating of water baths for logs and the heat required when pressing materials into sheets. In plywood manufacturing, between 25 and 50 percent of each log is not usable for creating plies. This used to result in a lot of waste, but much of this material is now used in making OSB.

The glues used for these products are petrochemical based and impacts from their manufacture and use include habitat destruction and air and water pollution.

Transportation — Low to High. Sample building uses 1,030 kg of plywood or OSB sheathing for the exterior walls:

1.55 MJ per km by 15 ton truck 1 MJ per km by 35 ton truck

0.25 MJ per km by rail

The vast majority of sheet wood products are manufactured on the midwest coast of North America and shipped across the continent. Installation — Low.

WASTE: MODERATE TO HIGH Compostable — None.

Recyclable — Metal fasteners. Quantities are small. Landfill — Plywood and OSB offcuts. Quantities can range from small to large, depending on the number of offcuts.

ENERGY EFFICIENCY

Wood sheet materials can contribute to an airtight building enclosure, especially when gasketed, caulked or taped to the framing at all joints. The materials do not directly contribute to thermal performance.

MATERIAL COSTS: MODERATE

Sheet materials are produced on a scale that makes them a moderately priced option. Competition between producers and retailers for these high-demand products has created pricing that is not reflective of the relatively high embodied energy of the materials.

LABOR INPUT: MODERATE

Sheets cover a large amount of square footage and are sized to correspond to standard framing spacing. When plans make good use of this sizing, labor

Sheathing and cladding materials 187

Plywood and oriented strand board (OSB) sheathing

inputs can be low. The cutting of sheet materials can be time-consuming, so the more custom cuts required the higher the labor input.

Health Warnings — Sawdust contains toxins from glues; wear breathing protection when cutting.

SKILL LEVEL REQUIRED FOR THE HOMEOWNER

Preparation of substrate — Easy to Moderate.

Sheet wood applies to the studs of a frame wall. A homeowner capable of accurate framing work will create a good backing for sheets.

Installation of sheathing — Moderate. Large sheets take some practice to handle, cut and install. A homeowner capable of accurate framing work will also be able to apply sheathing successfully. Finishing of sheathing — Easy. There is typically no finishing applied to sheet wood products. In rare cases where sheet wood is the final finish, surface treatment products are rolled or brushed onto the surface.

SOURCING/AVAILABILITY: EASY

Sheet wood is widely available through building supply outlets.

DURABILITY: MODERATE

Sheet wood is reasonably durable when kept dry. Though much work has gone into formulating glues that are water-resistant, the surface area of the plies and wood flakes result in a lot of swelling when the products get wet and the swell-shrink cycle can destroy the bond of the glue. OSB in particular is susceptible to degradation from wetting.

The most likely moisture damage to these products comes from condensation on the backside of the sheets due to air leakage from the inside of the walls. As these materials are not very permeable, moisture can accumulate and damage the sheets. This damage won’t be seen until the materials are close to or beyond the point of failure because the sheet wood is usually hidden behind cladding.

CODE COMPLIANCE

FUTURE DEVELOPMENT

The growth of third-party certification programs makes it likely that more sustainably harvested sheet wood products will become available. Effort has gone into reducing the amount and type of formaldehyde used in glues, but all exterior-grade sheet wood still contains glues with toxic chemicals. Pressure for more environmentally friendly options is driving R&D developments that may result in more benign adhesives.

The ability to use substandard logs to make OSB means that these products are likely to continue to make up a large and growing portion of the market as the quantity of available high-quality trees continues to shrink. Nontoxic glues would make the use of OSB more environmentally suitable.

Gypsum board, drywall or plasterboard sheathing

Applications for system

  • Interior wall and ceiling sheathing
  • Exterior wall sheathing (if specified for exterior purposes)

Basic materials

  • Gypsum
  • Paper coating (interior uses)
  • Fiberglass coating (exterior uses)
  • Chemical additives (dependent on type of panel)
  • Joint tape or mesh
  • Joint compound
  • Fasteners
  • Corner beads and other trim accessories

How the system works

Gypsum is obtained from quarries or from recycled flue gasses at coal-burning plants. It is dried, ground and heated to alter its chemistry. It is then mixed with water and chemical additives and set between paper or fiberglass sheets. The sheets are oven-cured in a multi-stage process of descending temperatures.

The sheets are made in a variety of thicknesses (*A, %, У2 and % inch are common) and standard dimensions of 48 and 54 inches by 96, 120 or 144 inches to suit common wall-framing dimensions. Each sheet has two tapered edges that allow for jointing compound to be applied while achieving a smooth finished surface.

The sheets are fastened to wall and ceiling framing with specialty screws, and the screw heads are covered with joint compound.

Joints are bridged with a paper or mesh tape and a gypsum-based jointing compound that is applied in several coats, with sanding to smooth between coats.

Interior gypsum board is commonly painted, but can also be a substrate for finish plasters.

Exterior gypsum products are for sheathing only, and the joints are not covered. These materials are used in conjunction with a variety of cladding options.

Tips for successful installation

  1. There are many good resources for homeowners wishing to install drywall. However, there will be a learning curve to achieving good jointing skills. It is good to practice drywall jointing in less obvious areas until results meet expectations.
  2. Interior drywall is available in many varieties, including fire code-approved, lightweight and moist-area versions. Be sure you are using the correct product for your specific requirements.
  3. The drywall industry is so well established that professional drywall installers have tools, equipment and techniques that allow them to do a job much faster and with much better results than most homeowners. Competition is ubiquitous enough that prices are surprising low, and even

a committed DIYer may find it worthwhile to hire out the job.

Pros and cons

ENVIRONMENTAL IMPACTS

Harvesting — Moderate. The bulk of the material in the panels is gypsum, a soft rock quarried from

Exterior gypsum sheathing.

surface-based pits, most of which have been long established. Impacts can include habitat destruction and surface and ground water contamination. Some drywall manufacturers use gypsum obtained from industrial chimney flues, reducing the need for virgin gypsum.

Manufacturing — Moderate. Heat is applied twice in the manufacturing process, when the raw gypsum is initially calcined and again to dry the panels once they have been formed. The temperatures for these processes are lower than other manufactured mineral and/or sheet products, but still require substantial energy input.

Manufacturing gypsum board requires a lot of water, but effluent levels are not high as the water is dried out of the panels in kilns.

A wide number of additives are mixed with the gypsum in relatively small quantities. These include paper and/or fiberglass fiber, soap-based foaming agents, plasticizers, gypsum crystal, EDTA, starch, mildew resistant agent and wax emulsion for water-resistant drywall. All the additives in drywall will have a range of impacts during their manufacturing processes.

Transportation — Low to High. Sample building uses 1,007 – 1,473 kg of drywall for interior wall sheathing: 1.5 – 2.2 MJ per km by 15 ton truck 0.95 – 1.4 MJ per km by 35 ton truck 0.25 – 0.37 MJ per km by rail 0.16 – 0.24 MJ per km by ocean freight

Gypsum board production is centralized and manufacturing plants exist close to most population centers in North America. However, complexities in the supply chain does not guarantee that drywall purchased in a region was produced in that region. Installation — Moderate to High. The installation of gypsum board creates a high amount of dust on

the building site, especially during the sanding of joints. The dust is extremely fine and can become dispersed throughout the home (including heating and ventilation ductwork) and in the environment around the home. Many joint compounds contain anti-fungal agents and other toxic chemicals that installers will not be able to contain fully.

Compostable — None.

Recyclable — Drywall offcuts. Drywall recycling programs are starting to become more common. If such programs exist, all scrap will be able to be diverted. Quantities tend to be high.

Landfill — Drywall offcuts. If no recycling program is available, offcuts will go to landfill.

Gypsum board products may be used as the primary air control layer on the interior and/or exterior if all seams, penetrations and intersections are properly

WASTE: MODERATE TO HIGH

ENERGY EFFICIENCY

Gypsum board, drywall or plasterboard sheathing

gasketed and/or caulked. Gypsum board does not contribute to thermal control.

MATERIAL COSTS: Low TO MODERATE

Scale of production and price competition for this widely-used material have led to economies of scale that result in pricing that is lower than embodied energy inputs would typically require.

LABOR INPUT: HIGH

Interior gypsum board is a multiple-stage material to install. Sheets must be cut to size and mounted, joints taped/meshed, and joint compound applied in two or three coats with drying time and sanding required between each. Professional tools can greatly reduce labor input.

Health Warnings — Dust from drywall and in particular joint compounds is toxic and proper breathing protection must be worn.

SKILL LEVEL REQUIRED FOR THE HOMEOWNER

Preparation of substrate — Easy. Basic framing and carpentry skills.

Installation of sheathing — Easy to Difficult. Large sheets are easy to install, level of difficulty increases with quantity and complexity of cuts, penetrations and intersections.

Finishing of sheathing — Difficult. Mudding and sanding joints, corner beads and intersections to a good degree of finish requires experience.

CODE COMPLIANCE

Acceptable solution in all codes. Particular types of drywall may be required for specific purposes (fire code, moisture resistance).

INDOOR AIR QUALITY: Low TO HIGH

There can be minor to serious issues with gypsum board and IAQ. Synthetic gypsum (which represents a growing percentage of all available gypsum products) is reclaimed from the chimneys of coal-burning installations and may contain contaminants, including sulfur and heavy metals. There are lawsuits currently pending in the United States regarding sulfur gas contamination ofhomes from drywall sourced from Chinese and American synthetic gypsum. While the number of claims is small in comparison

to the amount of drywall installed, the precautionary principle suggests avoiding synthetic gypsum products.

Non-synthetic gypsum board for interior use is typically faced with paper that is adhered using starch-based glues. This combination is an excellent food source for mold, and drywall in humid or moist conditions can easily become moldy, with dramatic impacts on IAQ.

Pre-mixed joint compound will contain fungi – cides/biocides that are persistent in the environment. Most wet and dry joint compounds will contain formaldehyde, ethylene vinyl acetate latex and other additives. These will be listed on the product’s MSDS sheet.

Complicating the issue is the level of dust created when sanding joint compound. The resulting dust is very fine and pervasive, carrying traces of all the chemical additives as well as silica throughout the home. Heating and ventilation ducts are particularly vulnerable to being coated in this dust.

There are a few brands ofjoint compound that do not contain any chemical additives and are considered hypoallergenic. Note that the dust from these compounds is still high in silica and should not be inhaled.

FUTURE DEVELOPMENT

The amount of potentially dangerous synthetic gypsum being used in the industry is an issue that is currently controversial, and the next decade will see developments that either lead to safer synthetic gypsum or proof that the material is not as dangerous as some currently claim.

Less toxic joint compound will likely make inroads into the industry, with R&D directed toward eliminating toxic chemicals from the material.

Drywall is likely to remain the dominant type of interior wall sheathing as its market acceptance, convenience and aesthetic are key to the majority of construction in North America.

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