Water Vapor Transmission Testing by the Cup Method: Principles, Standards, and Applications

Water vapor transmission testing by the cup method, also known as the gravimetric or dish method, is a fundamental laboratory technique for evaluating the moisture-barrier performance of plastic films, packaging laminates, and sheet materials. This technical article explains the test principle, the governing standards such as ASTM E96/E96M, ISO 2528, and GB/T 1037, the calculation of WVTR, permeance, and the water vapor permeability coefficient, and the practical applications of the method across the packaging, pharmaceutical, and construction industries.

Technical Overview

The cup method is the oldest and most widely recognized gravimetric technique for measuring how quickly water vapor passes through a barrier material. A circular specimen is sealed over the mouth of a shallow test cup (permeation dish) that has been charged with either a desiccant or distilled water. The sealed assembly is placed in a climate-controlled chamber maintained at a specified temperature and relative humidity, which establishes a stable water vapor pressure differential across the specimen.

Water vapor migrates through the specimen from the high-humidity side to the low-humidity side. The cup is removed and weighed at regular intervals, and the change in mass—gain in the desiccant method or loss in the water method—is plotted against time. Once the system reaches steady state, the slope of the linear portion of the curve, divided by the exposed test area, gives the water vapor transmission rate (WVTR).

From the WVTR, two additional quantities can be derived: water vapor permeance, which normalizes the result to the vapor pressure differential, and the water vapor permeability coefficient, which further normalizes to material thickness and represents an intrinsic property of homogeneous materials. Cup method testing is standardized internationally in ASTM E96/E96M, ISO 2528, DIN 53122-1, JIS Z 0208, and GB/T 1037, and is routinely performed by manufacturers, research institutions, and accredited third-party laboratories.

Testing Samples

Plastic Films and Sheets

Single-layer and multilayer films produced from polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), ethylene-vinyl alcohol copolymer (EVOH), and their metallized variants. Rigid sheets up to a few millimeters in thickness can also be evaluated, provided an effective seal can be achieved at the cup rim.

Flexible Packaging Laminates

Composite structures such as PET/AL/PE, BOPP/VMPET/CPP, PA/PE, and paper/AL/PE laminates used for food, pharmaceutical, and medical device packaging. The cup method is particularly valuable for finished laminate structures because it measures the performance of the complete construction, including adhesives, coatings, and metallized layers.

Paper, Paperboard, and Coated Substrates

Moisture-resistant papers, wax-coated or polymer-coated boards, and release liners. ISO 2528 was developed specifically for sheet materials of this type and remains the reference method in many packaging specifications.

Textiles, Nonwovens, and Functional Membranes

Waterproof-breathable membranes, hydrophilic films used in protective clothing, and technical nonwovens, where moderate to high water vapor transmission is a desired functional property rather than a defect.

Building and Industrial Barrier Materials

Vapor barrier films and membranes for roofing and wall assemblies, insulation facing materials, and other construction products whose moisture-control performance is specified in building codes and product standards.

Testing Items

Depending on the product specification and the applicable standard, cup method testing typically reports one or more of the following quantities:

• Water vapor transmission rate (WVTR), expressed in g/m²·24h, measured under defined conditions such as 38°C/90% RH or 23°C/50% RH.
• Water vapor permeance, expressed in g/(m²·24h·kPa) or g/(m²·s·Pa), obtained by dividing the WVTR by the water vapor pressure differential across the specimen.
• Water vapor permeability coefficient, expressed in g·cm/(cm²·s·Pa), obtained by multiplying the permeance by the specimen thickness; applicable to homogeneous monolayer materials.
• Comparative WVTR testing before and after treatment, for example sterilization, thermal aging, flexing, or accelerated weathering, to assess the durability of barrier performance.

Testing Methods

Two fundamental variants of the cup method are defined by the standards, distinguished by what is placed inside the cup and, consequently, the direction of the humidity gradient.

Desiccant Method

The cup is filled to within a few millimeters of the rim with a conditioned desiccant—typically anhydrous calcium chloride or a molecular sieve—and the specimen is sealed over the mouth. The assembly is placed in a chamber at the specified temperature and relative humidity, for example 38°C/90% RH. The desiccant absorbs permeating vapor and maintains a near-zero humidity condition on the inner side of the specimen, and the cup gains mass over time. The desiccant method provides a consistent dry sink and is favored for comparing materials under standardized conditions.

Water Method

The cup contains distilled water, creating a saturated, near-100% RH condition on the inner surface of the specimen, and is placed in a chamber at a lower relative humidity, for example 23°C/50% RH. Water vapor diffuses outward and the cup loses mass. A widely used variant, the inverted water method, places the water in direct contact with the specimen, representing the fully saturated conditions found in liquid-filled packages.

Specimen Preparation and Sealing

Specimens are cut as discs larger than the cup mouth—commonly 70 to 100 mm in diameter depending on the cup design—conditioned to equilibrium (commonly 23°C/50% RH in accordance with ISO 291 or ASTM D618), and inspected for pinholes, wrinkles, and other defects. Sealing quality is the single most critical experimental factor: any leak at the rim invalidates the result. Specimens are secured with molten wax blends such as microcrystalline wax and paraffin, gasketed retaining rings, or clamping flanges as prescribed by the standard in use. A minimum of three replicate specimens is customary, and additional replicates are recommended for materials with high transmission variability.

Test Conditions

Standard temperature and humidity combinations include the temperate condition of 23°C/50% RH and the tropical condition of 38°C/90% RH; other combinations such as 40°C/90% RH per JIS Z 0208 may apply when required by product specifications. Chamber temperature is typically controlled within ±1°C or tighter, and relative humidity within approximately ±2 to 3% RH, depending on the standard. The saturated water vapor pressures at 23°C and 38°C are approximately 2.81 kPa and 6.63 kPa respectively, and these values enter the calculation of permeance.

Measurement Procedure

The sealed cups are placed in the conditioned chamber and weighed at defined intervals, usually every 24 hours, on an analytical balance with a readability of 0.1 mg to 1 mg depending on the expected transmission level. Each weighing must be rapid and consistent to minimize disturbance of the thermal equilibrium. Testing continues until steady state is reached—that is, until a sufficient number of consecutive data points define a straight-line relationship between mass change and elapsed time—typically requiring 3 to 14 days for most materials and longer for high-barrier structures. The WVTR is then calculated from the slope of the fitted straight line.

Calculation of Results

The water vapor transmission rate is calculated as WVTR = (G/t) × 24 / A, where G/t is the slope of the mass-change-versus-time curve in grams per hour and A is the exposed test area in square meters; the result is reported in g/m²·24h.

Permeance is calculated by dividing the WVTR by the water vapor pressure differential Δp across the specimen: permeance = WVTR / Δp, expressed in g/(m²·24h·kPa). For the 38°C/90% RH desiccant method, for example, Δp = 0.90 × 6.63 kPa ≈ 5.97 kPa. The water vapor permeability coefficient is obtained by multiplying permeance by the average specimen thickness, yielding g·cm/(cm²·s·Pa). This coefficient is meaningful only for homogeneous, non-perforated materials, since laminates and coated structures act as composite barriers.

Factors Affecting Accuracy

Principal error sources include leaks or moisture ingress at the seal edge, drift or instability of chamber temperature and humidity, condensation on the cup during handling, electrostatic effects on the balance, desiccant saturation during long runs, and terminating the test before steady state is established. Strict adherence to the conditioning, sealing, and weighing requirements of the governing standard, together with replicate testing, keeps reproducibility within the between-laboratory tolerances published in ASTM E96/E96M and ISO 2528.

Applications

Cup method WVTR data underpin material selection, quality control, and compliance verification across many industries:

• Food packaging: shelf-life modeling for dry foods, snacks, powdered beverages, and moisture-sensitive products; verification of high-barrier laminates containing aluminum foil, metallized films, EVOH, or PVDC layers.
• Pharmaceutical and medical packaging: qualification of blister films, pouches, and sterile barrier systems against moisture ingress limits defined in pharmacopeial and regulatory guidance.
• Electronics packaging: evaluation of barrier bags and moisture-barrier packaging for moisture-sensitive devices and components.
• Construction: vapor barriers, breather membranes, and insulation facings rated for specific vapor permeance classes within building envelope assemblies.
• Textiles and personal protection: waterproof-breathable membranes characterized for wearing comfort in outdoor apparel and protective clothing.
• Research and development: comparison of resin grades, coating formulations, and converting parameters during new product development.

FAQ

What is the difference between the desiccant method and the water method?

In the desiccant method, the desiccant inside the cup creates a dry interior and the cup gains weight; in the water method, the water inside the cup creates a saturated interior and the cup loses weight. The two variants subject the specimen to different humidity gradients and can yield different values for the same material, so the variant and test conditions must always accompany the reported result.

Which standards govern cup method testing?

The principal standards are ASTM E96/E96M, ISO 2528, DIN 53122-1, JIS Z 0208, and GB/T 1037. All define the same gravimetric principle with minor differences in cup design, conditioning, sealing technique, and the statistical treatment of the mass-change data; the standard applied must be clearly stated in the test report.

How do WVTR, permeance, and the permeability coefficient differ?

WVTR describes the mass of vapor passing through unit area per unit time under specific conditions and therefore depends on the test climate. Permeance normalizes WVTR to the vapor pressure differential, making results obtained under different climates comparable. The permeability coefficient additionally removes thickness dependence and characterizes the intrinsic moisture permeation behavior of a homogeneous material.

How long does a cup method test take?

Typical runs last from three days to two weeks. High-transmission materials such as breathable films reach steady state quickly, while high-barrier laminates may require extended runs to accumulate a measurable, statistically valid mass change. The test must continue until the mass-change-versus-time plot is demonstrably linear.

Can the cup method measure high-barrier materials?

The practical lower detection limit of the cup method is on the order of 0.1 to 1 g/m²·24h, constrained by balance resolution and the practical run length. For ultra-high-barrier materials such as foil laminates or transparent oxide-coated films, instrumental sensor methods—for example ASTM F1249 modulated infrared sensor testing or electrolytic detector methods under the ISO 15106 series—are generally preferred.

What specimen size and quantity are required?

Circular specimens somewhat larger than the cup opening—commonly 70 to 100 mm in diameter for cups with exposed test areas of roughly 25 to 100 cm²—are required, with a minimum of three specimens per test condition. Additional material should be supplied to allow for conditioning and remounting in case of sealing defects.