ASTM F1249 Water Vapor Transmission Rate
ASTM F1249: Water Vapor Transmission Rate of Flexible Materials
Gravimetric techniques involve weighing a sample and waiting. For a metallic film or coated barrier film, the weight change after a week may fall within the balance’s noise level, so you never get a reliable answer. ASTM F1249 uses infrared detection to measure water vapor in a carrier gas, achieving transmission rates many times lower than those measurable by weighing, and in a much shorter time frame.
MatX Lab performs WVTR testing on packaging films, laminates, coated substrates, and barrier structures.
How the Measurement Works
The specimen is clamped between two halves of a diffusion cell, sealing them together.
One side holds a controlled humidity, generated by a wetted sponge or a humidified gas stream. The other side is swept with dry nitrogen. Water vapor permeating through the film joins the nitrogen and is carried to a modulated infrared sensor, which responds to the water vapor concentration in the stream.
There is continuous product output. This means there is no need to measure the mass difference after two weighings; an increasing signal marks the process due to permeation until it reaches a stable value. Thus, the equilibrium is reached visibly rather than hypothetically.
Standard Conditions
| Parameter | Typical value |
| Test temperature | 37.8 °C (100 °F), other temperatures on request |
| Driving humidity | 90% RH or 100% RH depending on the configuration |
| Carrier gas | Dry nitrogen at controlled flow |
| Reported unit | g/(m²·day) at the stated conditions |
| Test area | Commonly 50 cm², masked down for high barrier films |
| Detector | Modulated infrared sensor |
The 37.8 °C and high humidity combination is the packaging industry convention, chosen to accelerate permeation while staying below temperatures that would alter the film. Results at these conditions do not transfer directly to ambient or chilled service, since permeation is strongly temperature-dependent.
Sensitivity and Its Limits
This test has broad range coverage. Ordinary polyolefin films pass a few grams per square meter per day, and the tests can be completed in no time. Barrier films and metalized films pass very low amounts, but high-barrier films require many days to test.
Two factors influence the lowest level measurable in practice. The first is system background—the amount of vapor reaching the sensor through paths other than the sample film. The second is test duration, since very slow permeation requires more time to distinguish a true plateau from instrument drift.
A calibration film with a known transmission rate verifies instrument response, and running one alongside a barrier program separates instrument issues from material results.
What Goes Wrong
- Seal leakage. There is only one reason the result will be unsatisfactory: the obstruction value is always higher than what actually happens. Grease lubrication, clamp force, and sample flatness are some of those factors.
- Pinholes and flex cracks. A defect anywhere in the test area dominates the measurement. That is useful information when you are screening for defects and misleading when you believe you are measuring a material.
- Calling steady state early. The film used for the high barrier will ascend gradually over several hours. If you report before the plateau, you will report a rising number and underestimate the true value.
- Conditioning. Polyamide and EVOH barrier layers are humidity-sensitive, and a specimen that has not equilibrated at the test condition gives a transient rather than a property.
- Thickness normalization. Division by the total thickness is justifiable for a monolith film but not for a multilayer film where only one layer acts as the barrier.
Choosing Between F1249 and Gravimetric Methods
| ASTM F1249 | Gravimetric, ASTM E96 | |
| Detection | Infrared sensor on carrier gas | Mass change of a sealed dish |
| Best suited to | Flexible films, laminates, barrier structures | Building materials and moderate barriers |
| Sensitivity | Reaches low transmission rates | Limited by balance resolution and internal air gap |
| Duration | Hours for ordinary films | Days to weeks |
| Sample form | Flat flexible specimen clamped in a cell | Sheet or board sealed to a dish |
Oxygen transmission on the same films is measured separately under ASTM D3985, and most barrier programs need both, since a structure can perform well against one gas and poorly against the other.
Specimen Requirements
Flatten film or laminate that is oversized for the cell and interleaved for shipping. A crease intersecting the test area is a defect.
Determine the product contact surface. Coated and laminated films are non-symmetric and may show varying transmission depending on the measurement direction.
Send enough for replicates plus spares. On high barrier material, one anomalous specimen is usually a defect rather than the material, and distinguishing the two needs more than one result.
ASTM F1249 FAQs
Why is weighing a dish not enough?
The first reason is that mass changes are very minute in barrier film transmission. Second, air in the dish adds resistance in series with the sample.
How long does a barrier film take?
Ordinary films reach steady state within hours. High barrier structures can take a day or more, and reporting before the signal plateaus produces a value that understates transmission.