WVTR: What Water Vapor Transmission Rate Is and How It Is Measured

The water vapor transmission rate (WVTR) is an important parameter for evaluating the moisture barrier properties of films and packaging materials. It describes how much water vapor passes through a material over a specified area and period under defined test conditions.

WVTR testing helps manufacturers, packaging developers and laboratories characterize materials, compare barrier performance and support material selection and quality control.

Understanding how WVTR is measured is essential because water vapor transmission can be influenced by temperature, relative humidity, material composition, film thickness and the test method. These factors must be considered when interpreting results or comparing different materials.

What Is Water Vapor Transmission Rate (WVTR)?

Water vapor transmission rate, commonly abbreviated as WVTR, measures the amount of water vapor that passes through a material per unit area over a defined period under specified conditions.

WVTR is used to characterize the moisture barrier performance of films and other materials. A lower WVTR generally indicates that less water vapor passes through the material under the stated test conditions.

However, a WVTR value should always be interpreted in context. Results depend on the material, the temperature and humidity conditions, the measurement method and the units used to report the value.

WVTR is particularly relevant when evaluating packaging materials intended to limit moisture transfer or when comparing alternative film structures.

Why Is WVTR Important?

Moisture transfer can influence product quality, material stability and packaging performance. Measuring WVTR helps determine how a film behaves as a barrier to water vapor under controlled conditions.

Moisture Protection

For products that are sensitive to moisture gain or loss, the moisture barrier properties of the packaging material can be an important part of packaging development.

WVTR testing provides data that can help compare materials and assess their suitability for a particular application.

Material Selection

Different polymers, coatings and multilayer structures can have different water vapor transmission characteristics. WVTR measurements support comparisons between candidate materials under defined conditions.

Packaging Development

When a film formulation, coating or layer structure changes, WVTR testing can help evaluate whether the change affects moisture barrier performance.

Quality Control

Consistent WVTR testing can help laboratories compare material samples, investigate differences between batches and monitor barrier properties over time.

WVTR is one input into packaging evaluation. A WVTR result alone does not establish product shelf life or guarantee the performance of a complete package under real storage conditions.

How Is WVTR Measured?

WVTR is determined by measuring the transmission of water vapor through a material under specified conditions. The exact procedure depends on the selected test method and measurement system.

In a typical film test, the sample is mounted between two chambers or environments with different water vapor conditions. This creates a driving force for water vapor to move through the material.

The instrument measures the water vapor passing through the sample or determines the transmission rate from the measured response, according to the method used.

The result is reported as a WVTR value using the appropriate units and calculation procedure.

Preparing the Sample

The film sample should be representative of the material being evaluated and prepared according to the selected test method.

Thickness, structure and sample condition should be documented where relevant. Folds, damage, contamination or poor mounting can affect the measurement, so consistent sample preparation is important when comparing materials.

For coated films and laminates, testing the complete structure can help characterize the moisture barrier performance of the finished material.

Establishing Temperature and Humidity Conditions

Temperature and relative humidity are key factors in water vapor transmission testing. The conditions on both sides of the sample influence the driving force for transmission and may affect the behavior of the material itself.

The selected conditions should reflect the purpose of the measurement and the requirements of the applicable test method.

When comparing WVTR values, confirm that temperature, relative humidity and other relevant parameters are comparable. Results obtained under different conditions should not automatically be treated as directly equivalent.

Measuring and Reporting the Result

The measurement system records the transmission behavior and determines the WVTR according to the selected method.

The reported value should be accompanied by the relevant units and test conditions. This information is essential for comparing samples and interpreting the material’s moisture barrier performance.

WVTR vs. Water Vapor Permeability

WVTR and water vapor permeability are related, but they describe different quantities.

Water Vapor Transmission Rate

WVTR measures the amount of water vapor passing through a specified area of material over a given period under defined conditions.

It is commonly used to characterize and compare the moisture barrier performance of films as tested.

Water Vapor Permeability

Water vapor permeability is a material property associated with the transport of water vapor through a material. Depending on the calculation method, a permeability coefficient accounts for factors such as the driving force for transport and material thickness.

Why the Difference Matters

WVTR and permeability values are not interchangeable. They can use different units and represent different physical quantities.

Before comparing results, check:

• Whether the reported parameter is WVTR or a permeability coefficient.
• The units and calculation method.
• The sample thickness and material structure.
• The temperature and relative humidity conditions.
• Whether the measurements were obtained using comparable test methods.

For a broader explanation of material barrier properties, see [Water Vapor Permeability and WVTR: Measurement and Analysis].

Which Factors Affect WVTR Results?

Several material and test-related factors can influence water vapor transmission measurements.

Temperature

Temperature can influence water vapor transport through a material. WVTR values obtained at different temperatures may therefore differ, even when the same film is tested.

Temperature should be controlled and documented according to the selected method.

Relative Humidity

Relative humidity affects the water vapor conditions surrounding the sample and can influence the driving force for transmission. Some materials may also respond to moisture in ways that affect their barrier properties.

The relative humidity conditions used during testing are therefore essential when interpreting and comparing WVTR results.

Material Composition

Different polymers and barrier materials exhibit different water vapor transmission characteristics. The chemical and physical properties of the material influence how water vapor interacts with and moves through it.

Film Thickness

Film thickness can affect water vapor transmission. However, the relationship between thickness and the measured result depends on the material, structure and test conditions.

Comparisons should account for thickness and should distinguish WVTR from permeability coefficients.

Coatings and Multilayer Structures

Coatings, laminates and multilayer films combine materials with different properties. Their overall moisture barrier performance depends on the complete structure and the integrity of its layers.

Testing the finished film helps characterize the structure as a whole.

Sample Preparation and Test Method

Sample damage, inadequate mounting or inconsistent preparation can influence measurement reliability. Differences in test methods or reporting conventions can also affect whether results are comparable.

Following a consistent procedure helps support reliable material evaluation.

Which Materials Can Be Tested?

WVTR testing can be relevant to a variety of films and barrier materials, depending on the instrument configuration and selected method.

Examples include:

• Single-layer plastic films.
• Coated films.
• Metallized films.
• Multilayer films and laminates.
• Other moisture barrier materials suitable for the test configuration.

The suitability of a material depends on its characteristics, the expected transmission values and the capabilities of the measurement system.

For related applications, explore Permeability Testing of Plastic Films and Permeability Testing of Multilayer Films and Laminates.

WVTR Testing in Packaging Applications

Water vapor transmission testing can support packaging development in applications where moisture control is an important consideration.

Food Packaging

Moisture barrier properties may be relevant when packaging products that can be affected by moisture gain or loss. WVTR measurements help characterize film performance under defined conditions.

Explore Permeability Testing for Food Packaging.

Flexible Packaging

Flexible packaging may use coatings, laminates and multilayer films to achieve specific barrier properties. WVTR testing helps compare these materials and evaluate the moisture barrier performance of a complete film structure.

Explore Permeability Testing of Flexible Packaging.

Pharmaceutical Packaging

Depending on the product and packaging design, moisture barrier performance may be an important characteristic to evaluate.

WVTR measurements can contribute to material characterization, but they do not by themselves establish product safety or regulatory compliance.

Explore Barrier Testing for Pharmaceutical Packaging.

How to Choose a WVTR Testing System

Choosing a suitable measurement system begins with identifying the required parameter, sample type and test conditions.

Important considerations include:

• Measurement objective: determine whether WVTR or a water vapor permeability coefficient is required.
• Material characteristics: consider composition, thickness, coatings and layer structure.
• Expected transmission values: confirm that the instrument and method are suitable for the expected range.
• Temperature and humidity: identify the conditions required for the application and selected method.
• Sample configuration: confirm that the material is compatible with the instrument’s test configuration.
• Reporting requirements: establish the units and information needed for reliable comparisons.
• Laboratory workflow: consider the number of samples and the required testing process.

For laboratories that need to evaluate moisture barrier properties alongside oxygen or carbon dioxide transmission, a system capable of measuring multiple parameters may be worth considering.

The final selection should be based on the intended application, the test method and confirmed instrument capabilities.

ExtraSolution Instruments for WVTR Testing

ExtraSolution offers solutions for measuring water vapor transmission through films and barrier materials. The appropriate instrument depends on the required measurement parameters, the material being tested and the intended test conditions.

PermeH2O

PermeH2O is designed for water vapor transmission rate (WVTR) measurement.

Explore PermeH2O.

MultiPerm O2-H2O

MultiPerm O2-H2O is designed for oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) measurements.

Explore MultiPerm O2-H2O.

MultiPerm H2O-CO2

MultiPerm H2O-CO2 is designed for water vapor transmission rate (WVTR) and carbon dioxide transmission rate (CO₂TR) measurements.

Explore MultiPerm H2O-CO2.

TotalPerm

TotalPerm combines OTR, CO₂TR and WVTR measurement capabilities in one integrated system.

Explore TotalPerm.

The most suitable solution depends on the sample, the required parameters and the measurement conditions. Contact ExtraSolution to discuss your application and confirm instrument compatibility.

Conclusion

The water vapor transmission rate (WVTR) is a key parameter for characterizing the moisture barrier performance of films and packaging materials. It measures water vapor transmission under defined conditions and helps laboratories compare materials for packaging development and quality control.

Reliable interpretation requires attention to temperature, relative humidity, sample characteristics, units and the selected test method. WVTR should also be distinguished from water vapor permeability, as the two parameters represent different quantities.

ExtraSolution provides solutions for measuring water vapor transmission through films and barrier materials.

Contact ExtraSolution to discuss your WVTR testing requirements and identify a suitable measurement system.