Meaning and Purpose
5.1 One of the factors affecting the performance provided by organic coatings is its ability to resist or help water vapor pass through. In some services, such as exterior wood and masonry, the coating needs to allow the right amount of water vapor to pass through the film without damaging it. Therefore, the water vapor transmission characteristics of coatings are very important to evaluate their practical performance.

5.2 The purpose of these test methods is to obtain the value of water vapor transfer through the coating, which ranges from high to low permeability. These values are used for design, manufacturing, and marketing.

5.3 Water vapor transmittance is not a linear function of film thickness, temperature, or relative humidity.

5.4 The values of water vapor transmittance (WVT) and water vapor permeability (WVP) can be used for the relative rating of the coating only if the coating is tested under the same tightly controlled temperature and relative humidity conditions and the thickness is equal.

5.5 Test Method A – The Cheers method is a selective test method to obtain values associated with conventional dwellings that are not expected to have high relative humidity.

5.6 Test Method B – Wet cup method is the test method of choice to obtain values associated with applications with high expected relative humidity near the barrier material. In general, the more permeable the coating is to water passage, which is typical of many water-reduced coatings, the greater the affinity for water and the greater the transmittance when tested and exposed to high humidity conditions. Water absorption may reduce the density of the coating compared to a dry environment, thus allowing moisture to easily diffuse and leading to a higher water vapor transmittance (WVTR).

ASTM D1653-2021: Test Method for Water Vapor Permeability of Organic Coatings

Scope of
1.1 These test methods cover the determination of the rate at which water vapor passes through paint, varnish, varnish and other organic-coated films. Films can be free films or applied to porous substrates.

1.2 A similar but more generally applicable test method is test method E96, which should be considered when other materials are involved.

1.3 Test Methods Two test methods are introduced in D1653, as follows:

1.3.1 Test Method A – Cheers method, and

1.3.2 Test Method B – Wet (Penn) cup method.

1.3.3 Consistency should not be expected between the results obtained by different methods or test conditions. A method closer to the conditions of use should be chosen.

1.4 Values expressed in inch-pounds should be considered standard values. The values given in parentheses are mathematical transformations of SI units and are for reference only and are not considered standard. The conversion coefficients are shown in 13.2.1.2 and 13.2.2.2.

1.5 There are instruments on the market that claim to measure the water vapor transport of thin films more easily and rapidly than those described in test methods D1653 and E96. They run essentially the same type of test as the ASTM method, but in an instrumental manner. However, there does not appear to have been a side-by-side test to compare the measurements of such instruments with the precision and accuracy of these ASTM methods.

ASTM D1653-2021: Test Method for Water Vapor Permeability of Organic Coatings

1.6 This standard is not intended to address the safety issues, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health and environmental practices and to determine the applicability of regulatory restrictions prior to use.

1.7 This international Standard has been developed in accordance with the internationally recognized standardization principles established in the Decision on Principles for the Development of International Standards, Guidelines and Recommendations issued by the Committee on Technical Barriers to Trade of the World Trade Organization.