Building Energy and Environmental Systems Laboratory (BEESL)
Hygrothermal (HT) Research Area
Manual of Test Methods and Standard Operating Procedures
Designation: HTTM-1
version 1 dated Jan 10, 2005
Standard Test Methods for Rating Water Vapor Permeance of Materials
1. Scope
1.1 These test methods cover the determination of water vapor permeance (WVP) as affected by interaction with liquid water when materials are exposed to air with high relative humidity or water. The information on WVP is of importance for materials such as water resistive barriers, plastic films or coatings, exterior gypsum, mineral or cellulose-based fiberboards, wood-based products, thermal insulating and finishing products
1.2 Two basic test methods, the Double Cup and Modified Inverted Cup are provided for the measurement of water vapor permeance. Test methods listed in this standard are limited to specimen thickness equal or less than 32 mm (11/4 in.), except as provided in Section 9. Results obtained with different WVP test methods[1] are not expected to agree; it is, therefore, important that selected test method corresponds to conditions of use.
1.3 The values stated in SI units are regarded as the standard. Inch-pound conversion factors for water vapor permeance and permeability are listed in Table 1. All conversions from Pa to mm Hg are performed with a reference temperature of 0°C.
1.4 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E 96 Standard Test Methods for Determination of Water Vapor Transmission of Materials[2]
C 168 Terminology Relating to Thermal Insulating Materials3
D 449 Specification for Asphalt Used in Dampproofing and Waterproofing[3]
D 2301 Specification for Vinyl Chloride Plastic Pressure-Sensitive Electrical Insulating Tape[4]
E 691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method[5]
3. Terminology
3.1 Definitions of terms used in this standard are included the standard C 168. This standard defines water vapor permeability as the time rate of water vapor transmission through unit area of flat material of unit thickness induced by a unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions.
3.2 Water vapor permeance is the time rate of water vapor transmission through unit area of flat material or construction induced by a unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions.
3.3 Discussion—Permeance is a performance characteristic and not a material property. Permeability is a material property, and it is the arithmetic product of permeance and material thickness.
3.4 Water vapor transmission rate is the steady water vapor flow in unit time through unit area of a body, normal to specific parallel surfaces, under specific conditions of temperature and humidity at each surface.
TABLE 1 SI system units and conversion factors[6]
Multiply by To obtain
Permeance
kg/Pa·s·m2 1.75x1010 1 Perm (inch-pound system)
1 Perm 5.72x10-11 kg/Pa·s·m2
Permeability
kg/Pa·s·m 6.88x1011 1 Perm inch
1 Perm inch 1.45x10-12 g/Pa·s·m
WVT
g/h·m2 1.43 grains/h·ft2
grains/h·ft2 0.697 g/h·m2
4. Summary of Test Methods
4.1 In the Double Cup Method the test specimen is placed between two vertically stacked test dishes each conforming to the requirements of ASTM E96 test method for Desiccant and Water methods respectively. This standard allows two methods of sealing specimens. For specimens with permeance of 57.2x10-12 kg/(m2sPa) i.e., 1 perm or lower, the specimen is sealed directly to the open mouth of a bottom test dish containing distilled water, and the assembly is sealed to the top test dish that contains desiccant. For specimens with permeance higher than of 57.2x10-12 kg/(m2sPa) i.e., 1 perm, the specimen is sealed between protective rings using wax and O-rings providing a seal between the dry and the wet dishes. For materials with permeance higher than 1 perm periodic weighing determines three quantities:
1. Mass decrease of the water container,
2. Mass change of the test specimen, and
3. Mass increase of the desiccant.
For materials with permeance lower than 57.2x10-12 kg/(m2sPa) i.e., 1 perm periodic weighing determines two quantities:
1. Mass decrease of the water container, and
2. Mass increase of the desiccant and specimen combined
These quantities permit determination of the rate of water vapor movement from near 100% RH (the water surface) through the specimen into near 0 %RH (the desiccant).
4.2 In the Modified Inverted Cup Method, 5 mm (3/16 in) thick layer of distilled water is placed directly on the top of the specimen in the top dish, while the bottom dish contains the desiccant. Periodic weighing are performed to determine the increase in the desiccant’s mass, which is used for calculating water vapor permeance between the water and the desiccant. The vapor pressure difference is nominally the same in both methods. In the MIC method, however, water is in contact with the material surface and may penetrate into the material.
5. Significance and Use
5.1 The purpose of these tests is to measure, by means of simple apparatus, water vapor permeance through permeable and semi-permeable materials. These values are used in design, manufacture, and marketing. In contrast to test methods listed in ASTM standard E96, methods listed in this standard do not require the humidity control providing an advantage in quality assurance testing of moisture protective membranes and finishes.
5.2 As explained before, the permeance value calculated with one set of test conditions will not agree with the value calculated using different set of conditions. For this reason, the test conditions should be easy to correlate with those in service, while providing a reliable basis for comparison of moisture performance. Using the maximum driving force for water vapor transmission i.e., 100 % RH to 0 %RH at the reference temperature is definitely an easy way to compare performance of finishing and coating materials.
5.3 Two methods presented in this standard involve the same driving force for water vapor diffusion but differ with regard to contact between the material and the water. A thin layer of air separates water table from the material being tested in the Double Cup method, while water under 50 Pa of hydrostatic pressure is in contact with the material being tested in the Modified Inverted Cup (MIC) method. This makes the MIC test method particularly suitable for testing Water Resistive Barrier products.
6. Apparatus
6.1 Test Dish
The test dish shall be of any non-corroding material, impermeable to water vapor. A large, shallow dish is preferred, but its size and weight are limited when an analytical balance is chosen to detect small weight changes, therefore light-weight dish is desirable. The dish may be circular or rectangular. The mouth of the dish shall be as large as practical and at least 3000 mm2 (4.65 in2). The desiccant or water area shall be not less than the mouth area except if a grid is used, as provided in 12.1, its effective area shall not exceed 10 % of the mouth area. An external flange should be provided at the mouth of the dish as a mount for the pre-assembled specimen (specimen mounted between two nylon rings). This also provides some resistance against specimens shrinking and warping. When the specimen area is larger than the mouth area, the overlay upon the ledge is a source of error, particularly for thick specimens. This overlay material should be masked as described in 10.1 so that the mouth area defines the test area. Detailed description of the test cups and sealing methods for permeable and semi-permeable materials (WVP higher than 57.2x10-12 kg/(m2sPa) i.e., more than 1 perm are included in Appendix A and B.
The overlay material results in a positive error, indicating excessive water vapor transmission. The magnitude of the error is a complex function of the thickness, ledge width, mouth area, and the WVP. This type of error should be limited to less than 10%. For a 254 mm (10-in.) or larger mouth (square or circular), the ledge should not exceed 19 mm (3/4 in.), for a 76 mm (3-in.) mouth (square or circular) the ledge should not exceed 3 mm (1/8 in.). For further discussion of this error see paper[7].
Descriptions of dish design are provided in the ASTM standard E96. If a rim around the ledge is provided, it shall be not more than 6mm (1/4 in.) higher than the specimen. Cups with height other than indicated may be used. 19-mm (3/4-in) height, below the mouth, was found satisfactory for either method.
6.2 Temperature control
The room or cabinet where the assembled test dishes are to be placed shall have a controlled temperature. The temperature chosen shall be between 23 and 25°C (73 and 77°F), and shall be maintained constant within 0.1°C (0.2°F). The temperature stability inside the chamber shall be monitored recorded continuously (automatic cataloging capability) or measured as frequently as possible[8].
6.3 Balance and frequency of weighing
The balance shall be capable of measuring changes smaller than 1 % of the weight change during the test period. For example: 0.56 g/day passes though the specimen. In 18 days of steady state, the transfer is 10 g, thus the balance must have a sensitivity of 1 % of 10 g or 0.1 g and the weights must be accurate to 0.1 g. If, however, the balance has a sensitivity of 0.2 g or the weights are no better than 0.2 g, the requirement is met by extending the period of measurements to 36 days. This example highlights why for materials with WVP of 57.2x10-12 kg/(m2sPa) i.e., 1 perm or less the preferred method of weighing includes an analytical balance.
7. Materials
7.1 Desiccant and Water:
7.1.1 Anhydrous calcium chloride (desiccant) in the form of small granules that will pass through a No. 8 (2.36-mm) sieve. It must be free of fines that will pass a No. 30 (600-µm) sieve, shall be used as a desiccant[9]. It shall be dried at 200°C (392°F) before use.
7.1.2 Distilled water shall be used in the water test dish.
7.2 Sealant — The sealant used for attaching the specimen to the dish, must be highly resistant to the passage of water vapor (and water). It must not lose weight to, or gain weight from, the atmosphere in an amount, over the required period of time, that would affect the test result by more than 2 %. It must not affect the vapor pressure in a water-filled dish. Molten asphalt or wax is required for permeance tests below 2.3x10-10 kg·/( m2 s Pa) or 4 perms. Sealing methods are discussed in ASTM standard E96.
8. Sampling
8.1 The material shall be sampled in accordance with standard methods of sampling applicable to the material under test. The sample shall be of uniform thickness. If the material is of nonsymmetrical construction, the two faces shall be designated by distinguishing marks (for example, on a one side-
coated sample, “A” for the coated side and “B” for the uncoated side).
9. Test procedure
9.1. Preparation of test specimens
9.1.1 Test specimens shall be representative of the material tested. When a product is designed for use in only one orientation towards moisture flow, three specimens shall be tested with the vapor flowing in the designated direction. When the sides of a product are different and either side may face the vapor source, four specimens shall be tested by the same method, two being tested with the vapor flow in each direction and so reported.
9.1.2 A slab with characteristics of a laminate (such as a foamed plastic with natural “skins”) may be tested in the actual thickness of use. Alternatively, it may be sliced into two or more sheets, each being separately tested and so reported as provided in 9.4, provided also, that the “overlay upon the cup ledge” (6.1) of any laminate shall not exceed 1/8 in. (3 mm).
9.1.3 When the material as used has a pitted or textured surface, the tested thickness shall be that of use. When it is homogeneous, however, a thinner slice of the slab may be tested as provided in 9.4.
9.1.4 In either case (9.2 or 9.3), the tested overall thickness, if less than that of use, shall be at least five times the sum of the maximum pit depths in both its faces, and its tested permeance shall not be greater than of 2.9x10-11 kg/(m2sPa) i.e., 5 perms.
9.1.5 The overall thickness of each specimen shall be measured at the center of each quadrant and the results averaged. Specimens 3 mm (0.125 in) or less in thickness shall be measured to the nearest 0.03 mm (0.0012 in). Thicker specimens shall be measured to the nearest 0.05 mm (0.002 in).
9.1.6 When testing any material with a permeance less than 0.1 perm or when testing a low permeance material that may be expected to lose or gain weight throughout the test (as a result of evaporation or oxidation), it is required that an additional specimen, or “dummy,” be tested exactly like the
others, except that no desiccant or water is put in the dish. It is strongly recommended that a dummy cup be included in every test of materials with permeance less than 1 perm.
9.2 Attachment of specimen to the test dish
9.2.1 Mask the specimen surfaces so that its exposure duplicates the mouth shape and size and is facing it directly. Use a template to prepare the mask. Thoroughly seal the edges of the specimen to prevent the passage of vapor into, or out of, or around the specimen edges or any portion thereof. The same assurance must apply to any part of the specimen faces outside their defined areas. Attach the specimen to the dish by sealing (or clamping) in such a manner that the mouth of the dish coincides with the area of the specimen exposed to the vapor pressure difference. Suggested methods of attachment are described in ASTM standard E96 and in the Appendix A.
9.3. Preparing dish with a desiccant
9.3.1 Fill the test dish with a desiccant within 6 mm (1/4 in) of the specimen. Leave enough space to allow the dish to be shaken to mix the desiccant (after weighing).
9.3.2 Attach the specimen to the dish (see 9.2.1) and weigh the dish assembly periodically, often enough to provide at least seven data points during the test. A data point is the mass at a particular time. The period between two consecutive measurements should be stated with a precision of approximately 0.1 %. Thus, if weighing is made every hour, record the time to the nearest 3 s; if recordings are made every day, a time to the nearest 1 min suffices. Initially, a rapid weight change may occur. However, when a steady state is reached, the rate of change should stabilized and become constant. Weighing should be accomplished without removal of the test dishes from the controlled atmosphere, but if the removal is necessary, the time the specimens are kept at different conditions should be kept to a minimum.
When WVP is less than 5.7x10-12 kg/(m2sPa) i.e., 0.1 perm, the use of a dummy specimen and continuous recording of the barometric pressure are strongly recommended to increase the reliability of measurements. The effects of changes in temperature or in buoyancy caused by barometric pressure fluctuation can be determined on the dummy specimens and tarred out of the measured mass changes. Analyze the results as prescribed in 13.1.
9.3.3 Change the desiccant before the mass increase exceeds 10 % of its starting weight. This limit cannot be exactly determined and the experimenter’s judgment is required.
9.4 Preparing dish with water for the double cup method
9.4.1 Fill the test dish with distilled water to a level ranging between 6 and 12 mm (1/4 to 1/2 in) below the specimen. The air space thus allowed has a small vapor resistance, but it is necessary to reduce the risk of water splashing on the surface of the specimen when the dish is removed for weighing. Such a splash would invalidate the test on some hygroscopic materials.
The water height shall not be less than 3 mm (1/8 in) to ensure coverage of the dish bottom throughout the test. Water surges may be reduced in placing a grid of light non-corroding material in the dish to break the water surface. This grid shall be at least 6 mm (1/4 in.) below the specimen, and it shall not reduce the water surface by more than 10 %.
9.4.2 Attach the specimen to the dish (see 9.2.1). Coating an empty dish with sealant before assembly is another option. Water may be added after the specimen is attached, through a small sealable hole in the dish above the water line.
9.4.3 Weigh the dish and continue to assembly the set-up. When WVP results of water vapor transmission are expected to be less than 5.7x10-12 kg/(m2sPa) i.e., 0.1 perm, a dummy specimen is strongly recommended. Such a dummy specimen should be attached to an empty cup in the normal manner. The effects of changes in temperature or in buoyancy caused by barometric pressure fluctuation can be determined on the dummy specimens and tarred out of the measured mass changes. Analyze the results as prescribed in 13.1.
9.4.4 Where water is expected to be in contact with the barrier in service, use MIC method. Assembly the lower part of the set-up and place 5 mm (3/16 in.) thick layer on the top of the test material.
10. Calculation and Analysis of Results
Because time to reach equilibrium of water permeance increases as the square of thickness, thick, hygroscopic, materials may take as long as 60 days to reach equilibrium conditions.
10.1 The results of the rate of water vapor permeance shall be determined numerically.
10.1.1 Dummy Specimen—If a dummy specimen has been used to compensate for variability in test conditions, due to temperature or barometric pressure, or both, the daily recorded weights can be adjusted by calculating the weight change from initial to time of weighing. This adjustment is made by
reversing the direction of the dummy’s weight change, relative to its initial weight, and modifying all the appropriate specimen weight(s) recorded at this time. An alternate procedure, particularly for tests of long duration and more than seven weighings, is to subtract the arithmetic mean slope of the rate of weight change of the dummy specimen from the arithmetic mean slope of each similar specimen to get an effective rate of weight change. These procedures are also desirable if the specimen is changing weight due to a curing process while under test.
10.1.2 Numerical Analysis—A least squares regression analysis of the mass increase divided by the time interval between measurements. The regression should be modified by the dummy specimen when used, and this gives the rate of water vapor transmission. An uncertainty, or standard deviation of this rate, can also be calculated to define the confidence band. For very low permeability materials, this method can be used to determine the results after 30 to 60 days when using an analytical balance, even if the weight change does not meet the 100 times the sensitivity requirement of 6.3. Specimens analyzed in this manner must be clearly identified in the report.
10.2 Calculate the water vapor transmission, WVT, and permeance as follows:
10.2.1 Water Vapor Transmission:
WVT = G/DtA (1)
where:
G = weight change, in kilograms,
Dt = time during which G occurred, in seconds,
A = test area (cup mouth area), m2, and
WVT = rate of water vapor transmission, kg/m2s
10.2.2 Permeance:
WVP= WVT/Dp = WVT/S (2)
where:
Dp = vapor pressure difference, in Pa,
S = saturation vapor pressure at test temperature, in Pa,
Note: In the dish the relative humidity is nominally 0 % for the desiccant and 100 % for the water. These values are usually within 3 % relative humidity of the actual relative humidity for specimens
below 4 perms.
11 Report
11.1 The report shall include the following:
11.1.1 Identification of the material tested, including its thickness.
11.1.2 Test method used
11.1.3 Test temperature.
11.1.5 Permeance of each specimen in perms (to two significant figures).
11.1.6 The side of each specimen on which the higher vapor pressure was applied. (The sides shall be distinguished as “side A” and “side B” when there is no obvious difference between them. When there is an obvious difference, this difference shall also be stated, such as “side A waxed” and “side B unwaxed.”).
11.1.7 The average permeance of all specimens tested in each position.
11.1.8 The permeability of each specimen, and the average permeability of all specimens tested.
11.1.9 Include a portion of the plot indicating the section of the curve used to calculate permeability.
11.1.10 State design of cup and type or composition of sealant.
12. Precision and Bias
12.1 Precision—Table 2 is based on an interlaboratory tests conducted in 1988 and 1991[10]. In 1988 four materials (A, B, C, D) were tested using the desiccant method and the water method in triplicate. Fifteen laboratories contributed data, with full results secured from four laboratories. In 1991 ten laboratories
contributed data for material E, using triplicate specimens, again using both the dessicant method and the water method.
12.1.1 Test results were analyzed using Practice E 691.
12.2 Bias—This test method has no bias because water vapor transmission of materials is defined in terms of this test method.
13. Keywords
16.1 Water vapor permeability; water resistive barriers, plastic sheet and film; sheet material; thermal-insulating materials; thermal insulation permeability films; water vapor transmission (WVT)
TABLE 2 Precision Results from Interlaboratory Testing For Desiccant Method at 23.7 oC (73.4°F)
|
|
||||||||
|
TABLE 2 Precision Results from Interlaboratory Testing |
||||||||
|
For Desiccant Method at 73.4°F: Material Thickness, in. |
WVP (mean), perm |
|
RepeatabilityA |
|
|
ReproducibilityA |
|
|
|
S |
CV % |
LSD |
S |
CV % |
LSD |
|||
|
A |
0.001 |
0.606 |
0.0166 |
2.70 |
0.047 |
0.098 |
15.0 |
0.278 |
|
B |
0.0055 |
0.0129 |
0.0028 |
22.1 |
0.008 |
0.0055 |
42.6 |
0.016 |
|
C |
0.5 |
0.0613 |
0.0044 |
7.22 |
0.012 |
0.0185 |
30.6 |
0.052 |
|
D |
1.0 |
0.783 |
0.0259 |
3.30 |
0.073 |
0.0613 |
7.8 |
0.174 |
|
E |
0.014 |
0.0461 |
0.0023 |
4.99 |
0.007 |
0.0054 |
11.7 |
0.015 |
|
For Water Method at 73.4°F: Material Thickness, in. |
WVP (mean), perm |
|
RepeatabilityA |
|
|
ReproducibilityA |
|
|
|
S |
CV % |
LSD |
S |
CV % |
LSD |
|||
|
A |
0.001 |
0.715 |
0.0134 |
1.95 |
0.039 |
0.156 |
21.9 |
0.44 |
|
B |
0.0055 |
0.0157 |
0.0022 |
13.8 |
0.0062 |
0.0021 |
19.4 |
0.006 |
|
C |
0.5 |
0.097 |
0.0055 |
5.7 |
0.016 |
0.0195 |
20.9 |
0.055 |
|
D |
1.0 |
1.04 |
0.0192 |
1.8 |
0.054 |
0.217 |
20.9 |
0.62 |
|
E |
0.014 |
0.0594 |
0.0034 |
5.7 |
0.010 |
0.0082 |
13.8 |
0.023 |
S = standard deviation,
CV = percent coefficient of variation (S 3 100/mean), and
LSD = least significant difference between two individual test results based on a 95 % confidence level = 2 =2S .
BMaterial B was Teflon9 PTFE fluorocarbon resin brand of tetrafluoroethylene. It was extremely difficult to provide a seal to this sample, which accounts for the poor repeatability.
APPENDIX A: Design of dishes for Double Cup (DC) for permeable and semi-permeable materials
(Non-mandatory Information)
Figures 1 through 3 show schematic design, components and construction drawings of test set-up used for double cup.
Figure 1. Schematic set-up for Double Cup method; a container with water will be placed in the bottom cup on the base plate so that water level is between 6 and 12 mm (1/4 and ˝ in.) below the test specimen and container with a desiccant is placed in the top cup.
Figure 2. Components in the Double Cup set-up
Figure 3. Construction drawings of double cup test set-up
APPENDIX B: Design of dishes for Modified Inverted Cup (MIC) for permeable and semi-permeable materials
Figures 4 through 6 show schematic design, components, and construction drawings of test set-up used for modified inverted cup.
Figure 4. Schematic set-up for Modified Inverted Cup method; a container with desiccant water will be placed in the bottom cup on the base plate so that desiccant level is between 6 and 12 mm (1/4 and ˝ in.) below the test specimen and 5 mm thick water layer is placed in the top of the specimen.
Figure 5. Components in the Modified Inverted Cup set-up
Figure 6. Construction drawings of Modified Inverted Cup test set-up