Plywood and OSB on exterior walls6831556

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I always get asked by my clients whether we have to spec plywood or OSB (Oriented Strand Board) on the outside of wall of these home building plans Canada. My reply to them is always exactly the same. A powerful NO. (if you reside in a place where the temperature drops below freezing)

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


In the winter months as water vapour (moisture) in the home migrates from the exterior walls as a result of temperature difference, nearly all of it gets stopped by the sealed vapour barrier about the warm side of the wall. However, that sealed vapour barrier isn't perfect and a few of this moisture causes it to be through regardless.

As it migrates further in to the wall it'll reach a dew point. The place of that point depends on the temperature distinction between the outside and interior air temperatures. At that point it turns from water vapour to water. When and if the temperature difference increases that water will consider ice under two opposites.

How do we eliminate that water? You have to let it escape the wall cavity. Meaning you can’t have materials on the exterior from the wall which will restrict its flow for the exterior. Essentially you can’t have a material on the exterior people wall that will behave as a vapour barrier.

How do we determine a material acts as a vapour? Luckily the fine people who write our building codes established a benchmark as to what produces a vapour barrier. Also called the permeance of your material which in scientific terms is the quantity of water that material lets through and is also measured as ng/(Pa x s x m2 ). Don’t worry about the science behind it. Explaining it might be beyond the scope want to know ,. This wonderful time number to find is 60 ng/(Pa x s x m2 ) also called 1 PERM.


What which means is when the amount is much more then 60 then the material does not act as a vapour barrier.

So now is where it gets tricky with plywood and OSB. Because of all the glue that is used when the products are manufactured their permeance is lover then the 1 PERM. Therefore, they behave as vapour barriers thereby trapping moisture inside the wall cavity and causing major problems which can be self evident. Water in 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 now needed in some jurisdiction including Ontario).