Plywood and OSB on exterior walls4384756

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I usually get asked by my clients whether we ought to spec plywood or OSB (Oriented Strand Board) externally wall of these dream home design Canada. My reply to them is definitely exactly the same. A powerful NO. (if you live in a location the location 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 several other applications inside the construction industry. They only don’t belong in exterior walls of homes the location where the temperature drops below freezing here is why.


In the winter as water vapour (moisture) in the home 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 with the wall. However, that sealed vapour barrier isn't perfect and some of the moisture can make it through regardless.

Since it migrates further into the wall it's going to reach a dew point. The place of that point depends on the temperature among the surface and interior air temperatures. At that point it turns from water vapour to water. When and when the temperature difference increases that water will consider ice under extreme conditions.

How do we get rid of that water? You have to let it escape the wall cavity. That means you can’t possess materials on the exterior from the wall which will restrict its flow towards the exterior. Essentially you can’t have a material externally of you wall that may act as a vapour barrier.

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


What which means is when the number is more then 60 then a material will not work as a vapour barrier.

Here is how it gets tricky with plywood and OSB. Due to every one of the glue that is utilized when these items are manufactured their permeance is lover then your 1 PERM. Therefore, they work as vapour barriers thereby trapping moisture inside the wall cavity and causing major problems that are self evident. Water inside your wall, wii thing. We have personally witness many walls completely saturated with water and ice during wither months.

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