This will be the first of a two-part article talking about how materials are tested for water vapor transmission and how to design a building envelope with the known values.
There are three primary terms used when talking about measuring water vapor through building materials that is important to our industry. These terms are defined.
Water vapor permeability is time rate of water vapor transmission through a material at a specific thickness. This would be most applicable to materials that can vary in thickness such as fluid-applied air barriers and spray polyurethane foam (SPF)
Water vapor permeance is the time rate of water vapor transmission through a material of a unit thickness. Permeance is a performance evaluation and not a property of a material. This would be most applicable to materials such as self-adhered sheet materials, insulating board stock and mechanically fastened building wraps.
Water vapor transmission is the rate of water transmission through a material of a constant thickness at a given time.
ASTM E96 - Test Method of Water Vapor Transmission
ASTM E96 desiccant (dry cup or method A) test
Picture Credit: PRI Construction Materials Technologies
The method for testing water vapor permeance of a building material is ASTM E96 - Standard Test Methods for Water Vapor Transmission of Materials. There are two methods to test the water vapor transmission (WVT) of a material, desiccant (dry cup or method A) or water (wet cup or method B) methods. Both of these tests are similar in setup though the service conditions are different and the values obtained from both methods cannot be compared in any way.
The water or desiccant is placed on the bottom of the dish within 1/4" of the material being tested which is attached to the mouth of the dish. The test chamber is maintained at a constant temperature of 73.4°F and a relative humidity of 50 ± 2%. During the course of the test, the weight change of the test assembly (material, cup and contents) is measured. The water method assembly will lose weight due to water vapor from the cup transmitting through the material to the material absorbing moisture from the material sample as well as the humidity of the test chamber. The desiccant method assembly will gain weight due to water vapor from the chamber transmitting through the material due to the desiccant absorbing any moisture from the material sample and the humidity from the test chamber that is being absorbed by the material. Based on the change of weight, a water vapor transmission value is calculated for materials using both methods.
The designer or specifier shall ensure that when comparing WVT values of multiple materials, they are always comparing the identical methods of this test (water or desiccant). The best practice for the industry is for manufacturers of a fluid-applied material to list the WVT thickness at their specified installation thickness as the thickness of the material will determine the WVT value obtained.
When reviewing a test table of WVT of thickness and values of a material, there may be a correlation between the two but the results are not linear. Therefore, if a material was tested at 'X' thickness and the WVT is 'Y', a designer cannot calculate and/or assume that if a material was specified and applied at half the thickness the WVT will be double, or if the thickness the WVT is double the WVP will be halved.
See the ABAA website for a listing of ASTM E96 water and desiccant method values at given tested thickness for all ABAA Evaluated Materials.
Colin Szewaga, C.E.T.
David Beauchamp, B.Sc.
Stay tuned for the second article in an upcoming ABAA Newsletter on how to use these ASTM E96 test values in your building envelope design analysis using a psychrometric chart or WUFI.