Selecting the right air barrier
Understanding how much water vapor can pass through various materials used in wall assemblies is an important factor in selecting an air barrier to meet a specific wall assembly design and climatic considerations. Once again, there’s a standardized method for measuring this: ASTM E 96. “Standard Test Methods for Water Vapor Transmission of Materials.”
ASTM E 96-16 actually defines two different ways to perform this measurement: the desiccant method and the water method (also referred to as dry cup and wet cup methods).
In the desiccant method, a specimen of the material to be tested is sealed to a dish containing a desiccant and the entire assembly is weighed and then placed in a test chamber with a controlled atmosphere (73˚F and 50% RH) and circulating air. The desiccant creates a low vapor pressure environment within the dish. The pressure differential between the atmosphere inside the dish and outside the test assembly causes water vapor molecules to pass through the material, adding mass to the inside space. This change in mass is measured periodically during the test. At the conclusion of the test, the mass over time is plotted on a graph. With this method, we would expect to see mass increase in the dish as water vapor is transmitted from the atmosphere to the dish.
The water method effectively turns this around, creating a high vapor pressure environment within the dish, by filling the dish with distilled water and placing it in a controlled atmosphere of 73˚F and 50% RH. Again, weight measurements are made periodically and charted on a graph. With this method, we would expect to see mass decrease as water vapor escapes to the atmosphere outside the dish.
Either method will provide a measurement of the tested material’s vapor permeance, or the rate of water vapor transmission through a material over time. This measurement is expressed as “Perms” with a Perm equal to 5.72X10-9 grams of water vapor per pascal (Pa) x second x meters squared. It is calculated by dividing the slope of the graphed results (change in mass over time) by the exposed area of the material tested divided by the pressure difference.
This measurement enables calculation of the material’s vapor permeability, which is its permeance as a function of the material’s thickness (Perm x thickness) and may be used to assess a specific product’s suitability for a particular application.
ASTM E 96 can be used to measure a wide range of vapor permeable and impermeable building envelope materials, from plywood and gypsum to a variety of air barrier membranes. While it provides valuable information for understanding the relative vapor permeance of different materials, it is important to remember that test results are obtained under laboratory conditions; actual vapor permeance of a material may vary in the field due to differing temperature and humidity.
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