The Test Methods
D1653 vs. E96
There are two primary ASTM test methods used to calculate the passage of water vapor through materials: ASTM D1653, Water Vapor Transmission of Organic Coating Films and ASTM E96, Water Vapor Transmission of Materials. Often times, you’ll see them referred as D1653-08 or E96-05. In ASTM convention, the number following the test method number is the revision year.
So what’s the difference between the two test methods? In all honesty, there are too many to list here, but the main difference lies in what materials are typically “covered” by each method. D1653 states that “these test methods cover the determination of the rate at which water vapor passes through films of paint, varnish, lacquer, and other organic coatings. The films may be free films or they may be applied to porous substrates”. E96 states “these test methods cover the determination of water vapor transmission (WVT) of materials through which the passage of water vapor may be of importance, such as paper, plastic films, other sheet materials, fiberboards, gypsum and plaster products, wood products, and plastics.” In our lab, D1653 is used for free paint films and E96 is used for thicker materials or coated materials, eg: paint on drywall. However, there is nothing stated directly in E96 that it can’t also be used for free films. That’s the beauty of ASTM methods: specific in some regards, vague in others.
Dry Perms vs Wet Perms
It gets even better – both D1653 and E96 cover both the “Dry Cup” and the “Wet Cup” method. We tend to refer to these methods as “dry perms” and “wet perms”, respectively. The primary difference between dry cup and wet cup methods is in direction of water vapor “flow”. In the dry cup method, the sample is adhered to a test dish containing a suitable desiccant and the whole apparatus is placed in a controlled temperature and humidity chamber. Water vapor therefore “flows” from the chamber side of the test specimen (high humidity) and into the desiccant (low humidity). In the wet cup method, the sample is adhered to a test dish containing water, so that water vapor “flows” from the water side (high humidity) through the test specimen and out into the chamber (low humidity).
ASTM states that the dry cup method should be used when the test material in question is not expected to experience any high relative humidities. Conversely, the wet cup method is used when high relative humidities in the vicinity of the test specimen are expected. In general, a coating will have a higher water vapor transmission rate by the wet cup method than by the dry cup method. This is caused by the fact that most coatings that allow water to permeate will absorb more water at high humidities and therefore allow for a easier passage of water vapor through the coating. Therefore, it cannot be expected to directly compare results obtained by the dry cup method to those obtained by the wet cup method.