How to identify and when to select a commercial vapor barrier

Commercial vapor barriers limit the amount of water vapor diffusing through the wall as a result of different vapor pressures. With the advent of modern building science, it has been found that air leakage – and not vapor diffusion – is the real problem.

Permeability – or perms – is the measurement used to determine the effectiveness of a material to control diffusion. A perm is defined as the ability to pass one grain of water vapor per hour through one square foot of flat material at one inch of mercury (gr/h*ft²*in.Hg). One grain of water is 1/7000 of a pound or 0.0022 ounces of water. Many building materials are tested to measure permeability, the result of this test is perm rating.

A perm rating is a standard measure of the water vapor permeability of a material. The higher the number, the more readily water vapor (in the gaseous state) can diffuse through the material. A perm rating of less than 0.1 is considered a commercial vapor barrier; perm between 0.1 and 1 is considered a vapor retarder; a perm between 1 and 10 is semi-permeable; and a perm rating greater than 10 is considered permeable. The four general classes of materials based on permeance are as follows:

Vapor impermeable: 0.1 perm or less
Vapor semi-impermeable: 1.0 perm or less and greater than 0.1 perm
Vapor semi-permeable: 10 perms or less and greater than 1.0 perm
Vapor permeable: greater than 10 perms

Air/vapor barrier material manufacturers all report the permeability of their products. Design professionals use that information to specify materials.

Understanding the difference between vapor barrier and air barrier
A vapor barrier is meant to hold back the migration of water vapor and is designed to restrict the flow of water vapor through a materials. An air barrier is meant to hold back the migration of air and is designed restrict the flow of air through a material. Remember that air leakage – and not vapor diffusion – is the real concern because air leakage accounts for over 200 times the amount of moisture transmitted by diffusion.

Understanding vapor barrier in wall design
When you are designing a wall assembly, it is important to use products that can allow vapor to drain out of the building if it gets in. This is not simple, as sometimes the best strategies around keeping water vapor out of a building lead to that water vapor being trapped inside the building.

Climate can play a critical role in how a building manages water vapor. We all know that when water vapor moves from the warm side of building to the cold side, it creates water vapor. But what is challenging is deploying the right solution for the particular climate in which a building is being built.

If you need additional support around whether to select or specify a commercial vapor barrier or impermeable air barrier, Henry Company can help. Schedule a meeting with one of our expert advisers today!

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