Perm converter
Convert vapor permeance between US perms and SI units, and see which vapor control class a material falls into.
That's 1.25× as open as vapor retarder paint, while plasterboard is 30× higher. That is semi-permeable: a wall built with it can still dry.
What it means
How much vapor gets through?
Permeance belongs to a material of a stated thickness. Permeability — perm-inches — belongs to the material itself, and must be divided by the thickness.
Every unit
Your permeance in every unit
Learn
Three things worth knowing
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1 US perm = 57.45 ng/(Pa·s·m²)
And a metric perm is about 1.51 US perms, since the two were defined from different units of pressure and time.
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Grains through a square foot
The US perm was defined as one grain of water vapor per hour, per square foot, per inch of mercury — three imperial units stacked together, which is why the SI conversion is so untidy.
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A wall must dry somehow
Putting a barrier on both sides traps whatever moisture gets in. Modern practice is one barrier and one breathable side, so the wall can dry in at least one direction.
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Background
About perms
Permeance says how readily water vapor passes through a building material. Under the US classes, below 0.1 perm is a vapor barrier, 0.1 to 1 a retarder, and above 10 the material is considered vapor open.
Getting it wrong traps moisture inside a wall, which is how walls rot from the inside.