What Happens When OSB Gets Wet?

We occasionally hear the suggestion that OSB is especially prone to decay and is associated with the moisture failures that increasingly plague homeowners across North America. This is an unfortunate association arising from OSB's success at gaining market share at a time when the proportion of leaky buildings has been increasing as a result of other changes in construction practice. In other words, as the percentage of buildings with OSB rises, and the proportion of leaky buildings rises, so does the chance of finding OSB when a house leaks. There is no data to indicate that OSB bears any particular responsibility for moisture failures.

This is not to say that OSB has no problem whatsoever when exposed to excessive moisture. Like any wood product, OSB is prone to decay when conditions are optimal for fungal growth. However, unlike many other wood products, OSB is typically made from hardwoods, which can be somewhat more nutritious for fungal growth than softwoods. This means OSB may degrade marginally more rapidly than other wood products, however the difference is probably not noticeable in practice. It's the leak that's the problem, not the choice of product - a leaky building of any construction will quickly experience damage. Careful decisions in design, construction and maintenance will prevent building leaks and other errors from causing water to go where it's not intended.

OSB's other moisture challenge has to do with dimensional stability - in other words, OSB can irreversibly swell, especially at the edges, when it gets too wet. This is not a decay issue, but swelling can cause other difficulties in construction. Please see "Proper Handling" below for tips on preventing swelling. If handled properly, OSB can accommodate exposure to some water without swelling. It has some inherent moisture resistance, due to the resins which hold together the strands, along with some wax which is added to the product specifically for this purpose. Most panels are also treated with a sealant on the panel edges to retard moisture penetration. In addition, OSB is bonded with waterproof adhesives. It is important to note that OSB is not extremely sensitive to small amounts of water, and it absorbs water slowly. However, it is also slow to dry out.

Preservative Treatment for OSB

When marginal moisture conditions are anticipated, the addition of low levels of low-toxicity fungicides can be used to upgrade the durability of OSB. For equivalent fungal resistance to Douglas-fir-faced plywood, Forintek recommends a combination of 0.2% zinc borate added during manufacture and 10 mg/cm2 Oxine copper sprayed on the surface.

For applications where the moisture content remains above 28% for extended periods, such as siding, higher levels of preservative (0.75% zinc borate) are added to OSB during the manufacturing process. Zinc borate additionally provides termite resistance. A higher level of dimensional stability can also be designed into the product if these conditions are anticipated. Preservative-treated OSB and wood composite siding are now available in the marketplace.

Proper Handling of OSB on the Job Site

Panels should be protected from excessive wetting during storage and construction. The panel edges are especially vulnerable and therefore manufacturers protect the edges with paint or a special sealer. This sealer is usually coloured. If a field cut of a panel slices away a sealed edge, take extra care to protect this now vulnerable side during construction. Manufacturers also recommend a 2mm (1/8") gap be left between panels during installation to accommodate expansion. Do not allow panels to sit in pools of water. Store panels indoors or under cover, with enough support to keep panels flat. Schedule delivery as close as possible to time of use and close in the structure from the weather as quickly as practical.

Appropriate OSB Applications

Conventional OSB is suitable for all dry sheathing applications and treated, stabilised OSB should be used where moisture tolerance may be required. There are some applications where OSB is not advised by manufacturers. OSB is not recommended for use where it would be in sustained contact with a moisture source, such as soil or concrete within 150mm of soil level.

Like most other wood products, OSB is designed for use under dry service conditions. OSB is manufactured dry and should be transported, stored and, ideally, installed dry. Installing dry is virtually impossible in rainy climates, however, brief periods of wetting should not cause decay problems, provided OSB dries below 16% before the air barrier and cladding are both added. Interestingly, in above-ground field tests of small boards, OSB fully exposed to drying on both sides remains free from decay for many years. Once the building is closed in, the ability of the wall to dry is dramatically reduced. This means the OSB - and all other components of the framing - should be kept dry through the life of the wall. This is true no matter what the choice of construction materials.