The standard "ASTM E96/E96M-22a Standard Test Methods for Gravimetric Determination of Water Vapor Transmission Rate of Materials" developed by the American Society for Testing and Materials (ASTM) describes test methods for determining the water vapor transmission rate of materials such as paper, plastic films, other sheet materials, coatings, foams, particle boards, plaster and stucco products, wood products, and plastics. The two basic methods for measuring the water vapor transmission rate are the desiccant method and the water method. These tests use desired temperature and adjacent humidity conditions with vapor passing through the sample. The test conditions used are at the discretion of the user, but are reported with the results in all cases.
For the purposes of this standard, in addition to the terms and definitions given in ASTM C168, the following definitions apply:
In the desiccant method, the test sample is placed in the open mouth of the last container containing the desiccant and the apparatus is placed in a controlled atmosphere. Periodic weighings are used to determine the rate of water vapor movement from the sample to the desiccant.
In the water method, the vessel contains distilled water and weights are used to determine the rate of vapor movement from the sample water into the controlled atmosphere.
Given a common controlled atmosphere, the vapor pressure difference is nominally the same in both methods, but the water method exposes the sample to higher humidity, potentially affecting water vapor transmission through the material. Controlled atmosphere temperature and humidity levels are selected by the user.
The purpose of these tests is to obtain, by means of a simple apparatus, reliable values for the rate of water vapour transfer through materials, expressed in suitable units. The value of the water vapour transmission rate obtained by one method under one set of test conditions does not necessarily indicate the value that would be obtained by another method under another set of conditions. A discussion of the determination of the dependence of the water vapour transmission rate on different relative humidity (at a given temperature) is included in the standard annex (Annex X3). Commonly used test conditions are listed in Annex X1, but selection of test conditions other than those listed and which approximate the exposure conditions of the material in actual use is permitted. When tests are performed for classification or compliance purposes, the environmental conditions are usually defined in the product standard specifications.
When performing tests, material shall be sampled in accordance with standard sampling methods applicable to the material or product being tested. For homogeneous materials, the sample shall be of uniform thickness. When opposite faces of a product are different, the two faces shall be identified by distinctive identification.
Test specimens shall be representative of the material being tested. When a product is intended for use in one position only, three specimens shall be tested by the same method with the steam flow in the specified direction. When the surfaces of a product are indistinguishable, three specimens shall be tested by the same method. When the surfaces of a product are different and the location of the steam source is independent of the installation, six specimens shall be tested by the same method, three with the steam flow in each direction, and reported as such. A blank specimen is required for both orientations.
Materials of homogeneous composition and physical structure should be tested at any thickness to determine water vapor permeability. When determining permeability, the sample should be at least 12,5 mm thick.
A sheet made of a single material whose physical structure varies from layer to layer, such as natural shell foam plastic or rubber, should be tested at its service thickness.
If a material has a pitted or textured surface, the overall tested thickness, if less than the service thickness, shall be at least five times the sum of the maximum pit depths on each face.
When testing joints sealed with pressure-sensitive adhesive used in insulation vapor retarder systems, specimens are prepared in accordance with ASTM C1809.
All tests require an additional blank sample to be tested, as do all others, except that no desiccant or water is added to the container. Failure to use this blank sample to determine the modified container weights may significantly increase the time required to complete the test. The time it takes for water vapor transmission to reach equilibrium increases with the square of the thickness. Some thick, particularly hygroscopic materials may take up to 60 days to reach equilibrium conditions.
The blank sample is used to eliminate the effects of barometric pressure changes and eliminates the need for buoyancy corrections, moisture changes of hygroscopic materials, and mass changes of uncured samples. This provides simpler, more accurate calculations and reduces the time to determine steady-state mass change.
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