Plywood and OSB on exterior walls

I always get asked by my clients whether we ought to spec plywood or OSB (Oriented Strand Board) externally wall of their house floor plans Canada. My reply to them is definitely exactly the same. A convincing NO. (if you reside in a place where the temperature drops below freezing)

Now don’t misunderstand me, I’m not anti plywood or OSB. They're great products and could be used effectively in many other applications within the construction industry. They simply don’t belong in exterior walls of homes in which the temperature drops below freezing here is why.


During the cold months as water vapour (moisture) in the house migrates from the exterior walls as a result of temperature difference, the majority of it gets stopped from the sealed vapour barrier about the warm side from the wall. However, that sealed vapour barrier is rarely perfect and some of the moisture can make it through regardless.

Because it migrates further into the wall it's going to reach a dew point. The location of this point relies upon the temperature difference between the outside and interior air temperatures. At that time it turns from water vapour to water. When and when the temperature difference increases that water will consider ice under two extremes.

How do we eliminate that water? You need to give it time to escape the wall cavity. That means you can’t possess materials on the exterior of the wall that will restrict its flow towards the exterior. Essentially you can’t possess a material externally of you wall that will behave as a vapour barrier.

How can we determine if a material provides a vapour? Luckily the fine people that write our building codes established a benchmark as to what is really a vapour barrier. Also known as the permeance of a material which in scientific terms will be the level of water that material lets through and is also measured as ng/(Pa x s x m2 ). Don’t concern yourself with the science behind it. Explaining it could be at night scope want to know ,. This wonderful time number to look for is 60 ng/(Pa x s x m2 ) also called 1 PERM.

What which means is actually the quantity is a lot more then 60 then a material does not behave as a vapour barrier.

Here is where it gets tricky with plywood and OSB. As a result of every one of the glue that is used when the products are made their permeance is lover then the 1 PERM. Therefore, they act as vapour barriers thereby trapping moisture inside the wall cavity and causing major problems which can be self evident. Water within your wall, a bad thing. I have personally witness many walls completely saturated with water and ice during wither months.

So, exactly what do I suggest to my clients? Rigid insulation that being a permeance more then 1 PERM. (which incidentally is currently needed in some jurisdiction including Ontario).
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