Plywood and OSB on exterior walls9945700

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I get asked by my clients whether we ought to spec plywood or OSB (Oriented Strand Board) on the exterior wall of the new house plans Canada. My reply to them is always the same. A powerful NO. (if you reside in a place in which the temperature drops below freezing)

Now don’t get me wrong, I’m not anti plywood or OSB. These are great products and could be used effectively in many other applications in 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 house migrates with the exterior walls as a result of temperature difference, most of it gets stopped from the sealed vapour barrier around the warm side from the wall. However, that sealed vapour barrier isn't perfect plus some of the moisture can make it through regardless.

Because it migrates further to the wall it'll reach a dew point. The positioning of that point depends on the temperature distinction between the surface and interior air temperatures. When this occurs it turns from water vapour to water. When and if the temperature difference increases that water will consider ice under two opposites.

Exactly how do you eliminate that water? You have to let it escape the wall cavity. That means you can’t have any materials on the exterior from the wall that may restrict its flow to the exterior. Essentially you can’t possess a material on the exterior people wall that may act as a vapour barrier.

How can we know if a fabric behaves as a vapour? Luckily the fine folks that write our building codes established a benchmark about what produces a vapour barrier. Also referred to as the permeance of a material which in scientific terms may be the level 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 might be beyond the scope want to know ,. The magic number to consider is 60 ng/(Pa x s x m2 ) also known as 1 PERM.


What meaning is when the amount is more then 60 then your material does not behave as a vapour barrier.

Here is when it gets tricky with plywood and OSB. Due to all of the glue which is used when the products are produced their permeance is lover then a 1 PERM. Therefore, they work as vapour barriers thereby trapping moisture within the wall cavity and causing major problems which are self evident. Water in your wall, a bad thing. We have personally witness many walls completely saturated with water and ice during wither months.

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