Classified to BS EN 300, boards graded as OSB2 (sometimes written as OSB/2) are designed to be used only in dry, internal conditions.
OSB3, or OSB/3, products are similarly load-bearing but can be used in humid conditions, where there is elevated moisture content in the air. However, it is still expected that the boards are protected from external exposure. This can be done by application of waterproof paints, installation of traditional waterproofing membranes or specification of Arctek performance overlay technology.
The first two traditional weatherproofing methods for OSB can be time consuming and if the installation takes place on site, they become weather dependant. In contrast, Arctek overlay solutions are applied to the OSBs during manufacture and the boards are dispatched to the site already protected. The thermally fused overlay has a strong bond with the substrate which results in high performance that keeps the moisture from penetrating the boards even during extreme weather conditions.
Why might waterproof OSB be required?
The range of applications in which OSB, and OSB3 specifically, can be used results in significant variation of exposure to external conditions and sources of moisture. Although it’s an engineered product, OSB is still subject to the effects of moisture given the right combination of circumstances.
The timber strands on the surface, and particularly on the edges of the boards can swell and shrink in response to both liquid water and moisture vapour in the air. With sufficient exposure, water can eventually penetrate through the layered strands and cause the board to fail.
Some applications for OSB intentionally place the boards in external conditions, with site hoarding and fencing being the best examples.
In other applications – such as wall sheathing, roof sheathing and SIPs – delays following the OSB’s installation might unintentionally expose the boards to prolonged or heavy rain that they were not designed for. OSB3 might be capable of standing up to a light shower, though it cannot be guaranteed, but if there is any risk of extended rainfall exposure then a weatherproofing treatment should be considered.
Ultimately, it is impossible to say definitively what an acceptable level of exposure is, because the “humid conditions” that OSB3 is designed for relates to moisture vapour in the air and not liquid water.
Can painted OSB be exposed to rain and used outside?
Painting OSB can help it to achieve some additional weatherproofing but should not be relied upon to act as a waterproof finish. It’s helpful to first prime the boards in order to avoid the timber absorbing large quantities of paint during application.
Of course, painting is as much about any required decoration as it is about waterproofing. Paint should therefore be finished with a protective varnish or coating to achieve the desired level of waterproofing.
Applying paint, coatings or membranes on site is one way to achieve a weatherproof OSB product, but it is time consuming and subject to the same risks discussed above – namely that the ideal conditions for working cannot be guaranteed. As a result, it is hard to achieve a consistent quality of application.
About Arclin’s Arctek overlay technology
One way to overcome this is to use boards that have been treated with a factory-applied overlay technology such as Arclin’s Arctek performance overlay technology. Manufacturers of OSB3 products offer their boards with a proven and reliable weatherproof overlay.
The overlay is applied to manufactured OSB products in controlled factory conditions, ensuring consistency of application and performance across all treated boards. As a contractor, you can therefore be confident that any treated board you use will deliver reliable and high-quality weatherproofing across a range of applications.
Use our directory of OSB manufacturers and local suppliers to find stockists of boards that have been treated with Arctek performance overlay technology. You can also learn more about the complete range of Arctek overlay technologies.
