Sustainability 87: Fire-Rated Oriented Strand Board

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Oriented strand board (OSB) is an engineered wood sheathing panel that over the past several decades has seen increasing use across the United States in residential construction. While there are still a minority of code jurisdictions that prohibit the use of OSB, it has been approved for use in far more others throughout the country.

In the manufacture of OSB, rectangular ‘strands’ of wood — most often manufacturing scrap or even recycled material — are aligned at varying orientations in a cross-directional arrangement. Once fused under heat and pressure with water-resistant adhesives and wax, the panel is ‘thermo-set’ becoming dense, stiff, and durable. OSB may outperform plywood, as it resists splitting, cupping, warping and delamination. As it is comprised of small scrap strands, rather than full veneers in thin layering, it is also typically cheaper than plywood.

Dimensionally stable OSB is also excellent at holding construction fasteners. OSB can be obtained with specifically engineered characteristics optimized for roof sheathing, subfloor construction and wall sheathing.

Within the last twenty years, a new variant of OSB has been developed: fire-rated cementitious coated OSB, or (FRCC OSB). This product has been introduced to address a specific construction concern: the degradation over time of fire-rated plywood used (primarily) in roof and wall sheathings. As a fire-rated plywood roof or wall is exposed to the heat of its normal operating temperature ranges or the increased heat of a fire, the specific fire retardant chemicals used to treat the plywood can, through heat-induced chemical change, actually degrade and weaken the plywood.

FRCC OSB is an OSB sheathing that has received an additional application of a non-toxic and non-combustible coating of a fiberglass-reinforced magnesium oxide cementitious product on one or both surfaces. The composite panel can then be employed in a variety of fire-rated roof, wall, floor, and interior finish assemblies. As the FRCC OSB panel also provides structural strength, stiffness and resistance to shear, it can be a beneficial component of wood-frame construction of a wide variety of building types. A single panel of FRCC OSB may replace a double-layer sheathing of fire-rated gypsum board over plywood, resulting in less cost, speedier construction and thinner walls. FRCC OSB offers yet one more advantage over fire-treated plywood: it can better withstand the periodic wetting during construction that arises from occasional on-site rain.

Under the intense heat of fire conditions, water that is otherwise chemically ‘locked into’ the cementitious coating is released, cooling and damping the progress of the fire’s spread. The extended time period of safety afforded by the slowed fire spread can make all the difference to building occupants.

FRCC OSB may also be incorporated into structural insulated panels (SIPs), in which it may serve as one our both of the facing panels of an insulated ‘sandwich’.     

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