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Plywood and OSB on exterior walls

Updated: Jan 8, 2019

I always get asked by my clients whether we should spec plywood or OSB (Oriented Strand Board) on the exterior wall of their new home. My answer to them is always the same. A resounding NO. (if you live in an area where the temperature drops below freezing)

Now don’t get me wrong, I’m not anti plywood or OSB. These are great products and can be used effectively in many other applications in the construction industry. They just don’t belong in exterior walls of homes where the temperature drops below freezing and here is why.

In the winter as water vapour (moisture) in the house migrates through the exterior walls because of temperature difference, most of it gets stopped by the sealed vapour barrier on the warm side of the wall. However, that sealed vapour barrier is never perfect and some of that moisture makes it through regardless.

As it migrates further into the wall it will reach a dew point. The location of that point is dependent on the temperature difference between the exterior and interior air temperatures. At that point it turns from water vapour to water. When and if the temperature difference increases that water will turn to ice under extreme conditions.

So how do you get rid of that water? You must allow it to escape the wall cavity. That means you can’t have any materials on the exterior of the wall that will restrict its flow to the exterior. Essentially you can’t have a material on the exterior of you wall that will act as a vapour barrier.

How do we know if a material acts as a vapour? Luckily the fine folks that write our building codes established a benchmark as to what constitutes a vapour barrier. Also called the permeance of a material which in scientific terms is the amount of water that material lets through and is measured as ng/(Pa x s x m2 ). Don’t worry about the science behind it. Explaining it would be beyond the scope of this article. The magic number to look for is 60 ng/(Pa x s x m2 ) also referred to as 1 PERM.

What that means is that if the number is more then 60 then the material does not act as a vapour barrier.

So here is where it gets tricky with plywood and OSB. Because of all the glue that is used when these products are manufactured their permeance is lover then the 1 PERM. Therefore, they act as vapour barriers thereby trapping moisture in the wall cavity and causing major problems which are self evident. Water in your wall, not a good thing. I have personally witness many walls completely saturated with water and ice during wither months.

So, what do I recommend to my clients? Rigid insulation that as a permeance of more then 1 PERM. (which incidentally is now required in some jurisdiction including Ontario).

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Mold in wall with OSB

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