Water vapor transmission rate (WVTR) testing quantifies how much moisture permeates through packaging films, laminates, and finished containers per unit area per unit time, and the reliability of every result depends on strict control of temperature, relative humidity, and surrounding laboratory conditions. This article explains the environmental requirements defined in international standards such as ASTM E96, ASTM F1249, ISO 15106, and GB/T 21529, covering chamber parameters, sample conditioning, carrier gas quality, and best practices for reproducible measurement in accredited third-party laboratories.
Technical Overview
Water vapor transmission rate (WVTR), also known as moisture vapor transmission rate (MVTR), expresses the steady-state mass of water vapor that permeates through a unit area of a material per unit time under a specified humidity gradient, typically reported in g/m²·24h. Because moisture diffusion through polymers is a thermally activated process, WVTR is not an intrinsic constant of a material; it is a conditional property that is only meaningful when the test temperature, the relative humidity on each side of the specimen, and the conditioning history are fully specified.
Temperature and humidity control are therefore the foundation of every WVTR determination. The permeation rate follows an approximately exponential relationship with temperature, so a deviation of only 1 °C can shift a reported WVTR value by several percent. Likewise, the partial pressure difference of water vapor across the specimen, which is the true driving force of permeation, is a direct function of the relative humidity maintained on the wet side and the dry side. For this reason, major standards including ASTM E96/E96M, ASTM F1249, ISO 15106 (Parts 1–3), ISO 2528, JIS Z 0208, GB/T 1037, GB/T 21529, and GB/T 26253 all prescribe precise environmental conditions and tolerances that the test chamber and the laboratory must maintain throughout the measurement.
Testing Samples
Flexible Plastic Films and Laminates
Monolayer and multilayer films such as PE, PP, PET, PA, PVDC-coated films, EVOH-containing structures, metallized films, and aluminum foil laminates are the most frequently tested sample category. Specimens must be flat, free of wrinkles and pinholes, and representative of production material.
Paper-Based and Composite Materials
Coated papers, paper-plastic laminates, and foil composites used in food, tobacco, and pharmaceutical packaging are tested to verify moisture barrier performance after coating or lamination.
Sheets, Membranes, and Building Products
Polymer sheets, waterproofing membranes, roofing underlayments, and breathable construction membranes are evaluated at condition sets relevant to their service environment.
Finished Packages and Containers
Bottles, blister packs, pouches, closures, and complete medical device trays can be tested in whole-package mode to assess total moisture ingress rather than material permeability alone.
Testing Items
Typical test items provided by accredited third-party laboratories include the following:
- WVTR at standard conditions, commonly 23 °C / 85 % RH or 23 °C / 90 % RH
- WVTR at accelerated or tropical conditions, commonly 38 °C / 90 % RH or 40 °C / 90 % RH
- WVTR at high-temperature or saturated conditions for demanding applications
- Water vapor permeance for thin films where thickness effects are excluded
- Water vapor permeability coefficient for homogeneous materials
- Whole-package moisture transmission for sealed containers
Environmental Requirements for the Test Chamber and Laboratory
Temperature Control and Stability
The test chamber must hold the specimen at the specified temperature with a tolerance typically no wider than ±0.5 °C, and tighter control is required for high-barrier or high-precision measurements. The saturation vapor pressure of water rises steeply with temperature, and the diffusion coefficient of the polymer also increases exponentially, so even small thermal drift produces significant measurement error. Temperature must be verified with calibrated sensors and recorded for the full duration of the test.
Relative Humidity Control on Both Sides of the Specimen
The humidity gradient is the driving force of the test. Commonly specified wet-side conditions include 85 % RH, 90 % RH, and saturated conditions (100 % RH, achieved by direct water contact or a saturated salt solution), while the dry side is maintained near 0 % RH using a desiccant or a stream of dry carrier gas. Relative humidity tolerances of ±2 % to ±5 % RH apply depending on the method. Both the set values and the achieved stability must be reported with the results.
Ambient Laboratory Conditions and Sample Conditioning
The laboratory itself should be maintained at a standard atmosphere of 23 °C ± 2 °C and 50 % ± 10 % RH in accordance with ISO 291, GB/T 2918, or ASTM D618 practice. Prior to testing, specimens are normally conditioned in this atmosphere for at least 24 hours, extended to 48 hours or more for hygroscopic or thick materials, so that the sample reaches moisture equilibrium and its initial state is reproducible. Conditioning time and atmosphere must be documented.
Carrier Gas Quality for Instrumental Methods
Sensor-based methods (infrared, electrolytic, and humidity sensor) require a supply of dry nitrogen as the carrier and purge gas. Purity of 99.9 % or higher is typical, with a dew point low enough to guarantee an effectively 0 % RH dry side. Carrier gas flow rate must be regulated within the range stated by the method, and the gas path must be leak-tight so that ambient moisture cannot contaminate the measurement.
Airflow, Vibration, and External Interference
For gravimetric cup methods, controlled and uniform air velocity across the cups is required so that the boundary layer does not limit the transmission rate. For all methods, the instrument should be shielded from direct sunlight, drafts, nearby heat sources, and mechanical vibration, and supplied with stable electrical power. Regular calibration using certified reference films traceable to national metrology institutes, performed at the same temperature and humidity conditions as the test, is essential for long-term accuracy.
Testing Methods
Gravimetric Cup Method
Described in ASTM E96/E96M, ISO 2528, GB/T 1037, and JIS Z 0208, the cup method seals the specimen over a cup containing either desiccant (dry cup) or water (wet cup). The cup is placed in a controlled temperature and humidity chamber, and the weight gain or loss is tracked gravimetrically until steady state is reached. This method depends heavily on chamber stability because every weighing interval exposes the result to environmental drift.
Infrared Sensor Method
Specified in ASTM F1249, ISO 15106-2, and GB/T 26253, this technique carries permeated water vapor with dry nitrogen to a modulated infrared detector. It offers high sensitivity, short test cycles, and excellent suitability for medium- and high-barrier films, provided the chamber temperature and carrier gas dryness are rigorously controlled.
Electrolytic (Coulometric) Sensor Method
Covered by ISO 15106-3 and GB/T 21529, the electrolytic method absorbs permeated water vapor in a phosphorus pentoxide cell and quantifies it by electrolytic current. It provides very high sensitivity and is widely used for high-barrier materials.
Dynamic Relative Humidity Method
Defined in ISO 15106-1, this approach monitors the humidity change on the dry side of the specimen with calibrated humidity sensors, and is particularly useful for measuring moderate transmission rates under rapidly stabilized conditions.
Applications
Reliable WVTR data obtained under controlled environmental conditions support critical decisions across many industries:
- Food and beverage packaging: shelf-life modeling, moisture-sensitive products such as snacks, powdered drinks, and dried foods
- Pharmaceutical packaging: blister films, sachets, and bottle systems for effervescent tablets and hygroscopic drug products
- Electronics and displays: ultra-low WVTR verification for OLED and photovoltaic encapsulation barriers
- Medical devices: sterile barrier systems and moisture-protective tray laminates
- Construction: breather membranes, vapor barriers, and roofing underlayments evaluated at service-relevant conditions
- Hygiene and consumer products: backsheet films and absorbent article components
FAQ
Why is temperature control so critical in WVTR testing?
Moisture permeation is a thermally activated diffusion process. Both the diffusion coefficient of the material and the saturation vapor pressure of water increase steeply with temperature, so a small thermal deviation can change the measured WVTR by several percent. Tolerances of ±0.5 °C or tighter are therefore standard practice.
What are the most commonly specified test conditions?
Frequently used condition sets include 23 °C / 85 % RH, 23 °C / 90 % RH, 38 °C / 90 % RH, and 40 °C / 90 % RH. The selection depends on the product's intended storage environment and the applicable regulatory or industry standard.
How long should samples be conditioned before testing?
Most standards require conditioning at 23 °C / 50 % RH for at least 24 hours, with longer periods (48 hours or more) for hygroscopic, coated, or thick specimens. The conditioning atmosphere and duration must be reported together with the test results.
Can WVTR results measured under different conditions be directly compared?
No. WVTR values are only comparable when the temperature, humidity gradient, and method are identical. Because permeation behavior varies nonlinearly with temperature and humidity, converting results between condition sets without dedicated testing at each condition is not scientifically reliable.
Does the ambient laboratory environment affect instrumental test results?
Yes. Although the specimen sits inside a controlled chamber, ambient temperature swings, humidity excursions, vibration, and carrier gas contamination all influence instrument stability and baseline drift. A controlled laboratory atmosphere, leak-tight gas paths, and scheduled calibration with certified reference films are required for accurate measurement.
