Air, Vapor or Water Resistive Barriers?

When it comes to the building envelope, air, vapor or water-resistive barriers can all play an important role. Unfortunately, the specific role that each barrier plays can be confusing. Even the experts sometimes use these terms interchangeably, which is incorrect. Also, the fact that a single material can simultaneously function as an air, vapor and water-resistive barrier only adds to the confusion.

Understanding the differences (and similarities) between these barrier types is essential to designing and installing an effective building envelope. With that in mind, here’s a look at how the different types of barriers function within the building envelope. Let’s start with some basic definitions:

Air barrier: A system that forms a continuous plane around a building to prevent uncontrolled air leakage into and out of the building envelope, or between conditioned and unconditioned spaces.

Vapor barrier: A material installed in a wall assembly that limits the migration of water vapor through walls caused by different vapor pressures. As with an air barrier, vapor barriers must form a continuous plane.

Water-resistive barrier (WRB): A material installed on a substrate to keep bulk, or liquid water from entering the building enclosure. When used with flashing and sealants, a WRB provides a fully sealed assembly that directs water to the building’s exterior.

Air and Vapor Barrier Confusion

A vapor barrier is intended to help stop the migration of water vapor through a wall assembly. It is not typically intended to stop the migration of air – this is the function of air barriers. A vapor barrier is designed to restrict the flow of water vapor through a material, just the same as an air barrier material restricts the flow of air through a material. Air leakage – not vapor diffusion – is the real concern because air leakage accounts for over 200 times the amount of moisture transmitted by diffusion.

A single material can function as all three, an air, vapor and water resistive barrier. Some materials function only as a water resistive barrier that are not air barriers. It is important to understand the function of the materials used, to ensure proper material selection relative to project requirements and building performance.

Have more building envelope questions?

Understanding the differences between air, vapor or water resistive barriers is crucial for effective building envelope performance, but it’s just the first step. If you need more information about air barriers or any component in the building envelope, Henry Company is always here help – just schedule a meeting with one of our expert advisers today.