Sheathing

  • Use locally or regionally manufactured wood products to reduce transportation and increase community economic benefits. Exterior oriented strand board and fiberboard sheathing are manufactured within 500 miles of the Minneapolis/St. Paul metro area (closer to outstate areas).
  • Use materials that maximize rapidly renewable resources. Fiberboard and exterior oriented strand board use fast-growing aspen and waste products. Also, use FSC certified materials to minimize plantation-grown wood and ensure sustainable forestry practices. To date, OSB production has led to an increase in forest monoculture and plantations in many areas, contributing to dramatic forest composition and ecological shifts in Minnesota. Fiberboard sheathing can be made with significant amounts of recycled fiber.
  • When sheathing could be exposed to the elements for up to six months, choose fiberglass-faced gypsum sheathing for durability. Select products with synthetic gypsum to reduce overall material resource use.

Options and Analysis

alternatives cost/sf energy (R-value) warranty global warming potential (lbs of CO2/sf) additional LCA measures
(per sf of building footprint)
IAQ practice
exterior plywood $0.49 R=0.5 - 0.62 for ½" Generally not available 0.32 energy consumption (Btu) 5,491 no significant effects standard
solid waste (lbs) 0.153
air pollution index 0.028
water pollution index 0.002
exterior OSB $0.36 (based on PATH, 12/07) R=0.5 - 0.62 for ½" Generally not available 1.09 energy consumption (Btu) 14,872 no significant effects standard
solid waste (lbs) 0.285
air pollution index 0.565
water pollution index 0.001
exterior fiberglass faced gypsum* $0.65 R=0.5 for ½" 5-10 years, 1 yr weather exposure 0.9 energy consumption (Btu) 9,667 no significant effects standard
solid waste (lbs) 1.530
air pollution index 0.144
water pollution index 0.000
fiberboard (asphalt impregnated) $0.29 R=2 for ¾" None available NA energy consumption (Btu) NA no significant effects standard
solid waste (lbs) NA
air pollution index NA
water pollution index NA

Costs based on Means CostWorks 2007, unless otherwise noted. LCA measures and global warming potential modeled using Athena EIE 3.03
* Moisture-resistant gypsum used in lieu of fiberglass-faced gypsum for calculation of global warming potential and LCA measures.

Cost
Fiberboard and OSB sheathing are typically the least expensive option for exterior sheathing. Plywood and gypsum sheathing may be twice as expensive.

Durability
With adequate moisture protection, sheathing products should last the lifetime of the structure. Moisture protection includes a continuous interior vapor retarder, an air barrier, and a cladding system with a continuous drainage plane that includes properly executed flashing and details around all penetrations. However, some moisture intrusion is inevitable. With moisture intrusion, fiberglass-faced gypsum confers the greatest resistance to deterioration and will not foster mold growth. Plywood offers some resistance, but ordinary OSB panels can foster mold growth. The difference between OSB and plywood is largely due to the tree species used in production and the size of the wood pieces composing the panel. Generally, the smaller the wood chips, the greater the susceptibility to mold. With moisture intrusion, fiberboard and OSB are also susceptible to swelling at the edges, which can be an aesthetic problem. Plywood, because it can delaminate, may be the most susceptible to structural damage from moisture exposure at all stages of its use. Fiberboard's high degree of breathability may reduce problems with excess moisture build-up in walls as long as the siding is also breathable (i.e. not EIFS or stucco). However, breathable sheathing and siding cannot substitute for a properly installed interior vapor retarder and air barrier.1

Life Cycle Analysis (measures are given per square foot of sheathing)

Energy Consumption
OSB requires large amounts of energy for processing, pressing, and drying. Plywood requires several times less energy than OBS. Gypsum products require significant amounts of energy through every phase of their production.

Pollutants Generated in Production
Plywood generates the least amount of pollutants during harvesting and production, while OSB generates the most. Since large amounts of chemical binders are used in the production of OSB, workers in OSB manufacturing facilities are exposed to considerable levels of indoor air pollutants.

Indoor Air Quality
Because these are exterior products and will be enclosed by other materials, their effects on the indoor environment are minimal.

Future Recycling
Plywood has the most direct reuse potential, primarily because it can be downcycled one more step before being landfilled or used for fuel. However, in locations where gypsum board can be recycled into a soil amendment, it has the least long-term detrimental effects. Use of finishes, sealers, and coatings can increase negative impacts when sheathings are recycled or composted. Fiberboard sheathing is typically heavily treated with flame retardants and impregnated with asphalt. This eliminates the potential for composting and limits recycling options.

Practice
All of the sheathing products discussed above follow standard practice. The most significant changes related to sheathing involve alternative construction methods that preclude the use of sheathing or allow for less structurally significant products, such as rigid insulation board used in place of traditional sheathing.

1 D.M. Burch and A. Tenwolde. "A Computer Analysis of Moisture Accumulation in the Walls of Manufactured Housing." ASHRAE Transactions, v99, 1993.