OSB3 - The Weetabix Theory - True or False?

I read an article in an online architectural magazine recently. The false claims made in the article inspired me to write this blog to set the record straight.

It was alleged that OSB was like Weetabix, and when it got wet from just a few days exposure to rain it would swell and turn to mush.

Looking at the two side by side there is no doubt they do look a little similar, however that doesn't mean the physical properties are the same! Please read on to find out the facts.

The OSB used in the manufacture of our SIPs, Structural Insulated Panels, and in most SIPs and timber frame construction in the UK is BBA certified OSB3.

OSB3 is certified for use in load-bearing walls, in dry and humid conditions.

During manufacture the individual strands of wood are coated with a Formaldehyde Free, Water resistant synthetic resin. I have had the fortunate experience of seeing the production process first hand during a tour of the mill where the OSB is manufactured.

When Egger were asked “How long could OSB boards stay uncovered (exposed), without the risk of altering their stiffness and bending potential? Egger said “It depends on the climatic conditions that are specific to each country, on the type of boards used, and of course, on the duration and intensity of any unexpected rain, that can range from episodic shower to unstopped raining days. From our experience gained by weathering tests (2 months), max. 8 weeks open construction (exposure to weathering)"

A typical SIPs house kit will be erected in under a week. The walls and roof are covered with a breathable membrane protecting the outer OSB sheathing from the elements until the final roof covering and wall cladding is installed.

It is therefore highly unlikely that the structural integrity of the OSB facings will be compromised during construction.

Thanks For Reading.

Martin