Vapor barrier commercial building solution or vapor permeable air barrier?

Are you sure you need a vapor barrier commercial building solution and not a vapor permeable air barrier? Read on to learn more about vapor permeability first before you make any decisions.

Depending on the design conditions of the building, a vapor barrier could contribute to trapping unwanted moisture inside a building or space, which could result in unhealthy indoor air quality conditions. Alternatively, a material with very low or no permeability may be desired for protecting moisture sensitive materials from an adjacent moisture environment. So, the answer to whether you specify and install a vapor permeable air barrier or a non-vapor permeable air barrier (aka vapor barrier), is really – it depends.

Moisture always flows from wet to dry, or from high vapor pressure to low vapor pressure. In everyday life, we observe this simply as “wet things drying out”. But for the building envelope the permeabilty of the wall components should be deliberately selected and aligned with design requirements of the building.

For water vapor to travel through solid material, there must be a difference in humidity on one side of the material versus the other and the material must have some degree of permeability to moisture vapor. Water vapor diffusion through solid material is typically a very slow process. The rate of water vapor diffusion through the material is governed primarily by two factors: 1) the vapor pressure difference and 2) the material’s inherent permeability to water vapor (permeance). The rate of water diffusion is proportional to the vapor pressure difference and the material’s inherent water vapor permeance.

Test Method for Measuring a Material’s Water Vapor Permeance

In the construction business, the universally accepted method of measuring water vapor diffusion through solid materials is ASTM E 96 “Standard Test Methods for Water Vapor Transmission of Materials”. The test method works by sealing a membrane or board sample to a metal cup. The cup is then placed in a controlled environment. Over time, the cup is weighed on a very sensitive scale to determine change in mass. The change in mass is attributed to water vapor diffusing either into or out of the cup. ASTM E96 contains two different methods for conducting the testing. These are the Desiccant Method and the Water Method.

As stated previously, water vapor transmission will not occur unless there is vapor pressure difference across the specimen. ASTM E 96 Method A or Method B maintains a steady vapor pressure difference by creating a stable environment within the metal cup and placing the metal cup in a controlled temperature/humidity chamber. The chamber is typically held at 23.0°C/50% RH.

With the desiccant method, the inside of the cup is dry and contains pellets of CaCl2 desiccant. Sealing the sample to the rim of the cup, the environment inside the cup is effectively 0% relative humidity. With the water method, the inside of the cup is wet, filled with enough water to cover the bottom of the cup. With the sample sealed to the rim of the cup, the environment inside the cup is saturated, effectively 100% relative humidity.

Either method A or method B maintains a steady 50% relative humidity difference. The duration of the test depends on a number of factors stated in the test method. Once the test is completed, water vapor transmission is calculated using the test data as follows:

Water vapor permeance is then calculated based on the vapor pressure difference created by the controlled test conditions. The following formula is used:

Although ASTM E 96 method A and method B impart the same vapor pressure difference across the specimen, test results derived from either method are not interchangeable. As method B exposes the material to a wetter environment, permeance results using method B are typically higher than those from method A. That is because many materials physically change and effectively become more permeable when wet. A material’s tendency to become more permeable when wet may be advantageous or undesirable depending on the assembly and the scenario.

So whether you’re looking for a vapor barrier commercial building solution or a vapor permeable air barrier solution, let your Henry rep help you select the right system for your next project.

Click here to read more about the diffences between vapor permeable and non vapor permeable air barriers.

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