Photo: Ron Lach

Does OSB warp with water?

Boards may be briefly leaned against a structure at a 70° angle, but to prevent buckling and warping they must be stacked flat on a pallet or wooden spacers. Contact with the ground must be avoided, as rising moisture can also damage them. Boards should ideally be stored in dry conditions beneath a roof.

swisskrono.com - OSB and Moisture – How Much Moisture Do OSB Boards Tolerate?

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SWISS KRONO OSB excels across a diverse spectrum of applications: as a loadbearing timber construction material, as reinforcing planks, for making concrete formwork, as roof boards, for dry screed, for making shipping crates and furniture, and even for decorative purposes. The excellent technical properties and sealed surfaces of SWISS KRONO OSB make it superbly suited for modernisation and renovation projects as well as for erecting new, eco-friendly buildings. Yet it consists to 97% of wood, a natural raw material, so it seems logical that it ought to be impervious to moisture. As there is considerable disagreement about where and how OSB may be used, this article seeks to answer the questions “how much moisture does OSB tolerate?”, “where should which kinds of panel be used?” and “how can exposure to moisture be prevented?” OSB – which stands for “oriented strand board” – was invented in the mid-1960s by technology pioneer Armin Elmendorf, who originally had interior finishing applications in mind. Today this popular engineered wood product has a vast range of uses. OSB’s high strength and ability to reinforce other structures, its relatively low mass and the fact that it halts penetration of water vapour all combine to make it a highly versatile, universal building material.

OSB Grades – Which Panel Is Suitable for What?

First of all, not every kind of OSB is suited for every application! To help us tell them apart, the European EN 300 standard defines various grades and use classes. Based on their mechanical performance and relative resistance to moisture, OSB panels are assigned to four different grades:

OSB/1

This covers general-purpose, non-loadbearing panels for interior fitments and furniture in dry conditions. This grade is not sold in Germany.

OSB/2

These panels are very strong and may therefore be used for loadbearing purposes where they will remain dry (utilisation class 1). It is rare for them to be used in construction, however.

OSB/3

Also for loadbearing purposes, but as these panels are made with water-resistant binders they may also be used in humid conditions (utilisation class 2).

OSB/4

This classification designates heavy-duty boards for use in both dry and humid conditions (utilisation class 2). The rule of thumb is that panels of higher grades are stronger and more resistant to moisture. In addition, the standard defines various use classes based on dimensional changes, mechanical properties and performance:

1: Dry conditions

This class covers completed enclosed, heatable spaces. The average equilibrium moisture content is about 12% at an ambient temperature of 20°C, with relative humidity of 65% that is only exceeded for a few weeks each year.

2: Humid conditions

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This class refers to roofed-over but open constructions which are not directly exposed to the elements. Solid wood components reach an average equilibrium moisture content of about 20% at an ambient temperature of 20°C and relative humidity of predominantly 85%. These values may only be exceeded during a few weeks each year, however. For engineered wood products, the value may not be greater than 18%, which is not due to stricter requirements but rather to the fact that engineered wood have a lower equilibrium moisture content under conditions that are otherwise the same.

3: Outdoor conditions

This class covers components that are directly exposed to the weather, resulting in an equilibrium moisture content greater than 20%. Examples are bridges, towers and similar structures. It is important to take the climatic conditions at the installation site into account. For example, loadbearing boards used in the enclosures of indoor swimming pools can develop an equilibrium moisture content greater than 12% owing to the high relative humidity of the air, despite the fact that an enclosed and heated space is involved. This example therefore doesn’t match class 1. It is essential to make sure that conditions at the installation site will remain constant throughout the useful life of the loadbearing structure. If this is not the case, a higher use class should be assumed. Engineered wood panels are described and classified by DIN EN 13986. They must belong to use class 3 for exterior use if they will be exposed to the weather. OSB/3 and OSB/4 boards must always be shielded from the weather. This means that, while they may definitely be used as structural building elements, they must be additionally protected by some kind of covering, such as a curtain wall. DIN 68800-2:2012-02 defines “protected from the weather” as meaning permanently and effectively covered or located beneath roofing or an overhang inside a line drawn inward and down at a 60% angle from the outer edge. To prevent damage, it’s vital to make sure that at the time of installation, wood has a moisture content which is already very close to the equilibrium moisture content that will develop over time. Especially components intended for later use in class 1 or 2 applications must be protected during transport and installation to prevent them from being directly exposed to the elements and absorbing large amounts of moisture.

swisskrono.com - OSB and Moisture – How Much Moisture Do OSB Boards Tolerate?

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