Quick answer: Water vapour transmission rate is not one measurement. ASTM F1249 uses a modulated infrared sensor on flat film and reports a rate per unit area. ASTM E96 is a gravimetric cup method, also per unit area, and the cup variant used changes the answer. ASTM D7709 is gravimetric with desiccant inside the container and reports a rate per container — but it is written for pharmaceutical bottles and blisters, not flexible pouches or sachets. A barrier figure quoted without temperature, relative humidity and method cannot be compared against another supplier's figure. And for desiccant dosage you need grams of water per package per day, which no per-area film number gives you directly, because seals and closures leak more than film does.
Ask a packaging engineer what they most want from a supplier and the answer is usually barrier data. Ask what most often disappoints them and it is the same thing: a single number on a datasheet with no conditions attached.
Three methods, three questions
ASTM F1249 — instrumented, flat film
A specimen separates a dry chamber from a humid one, and a modulated infrared sensor measures the vapour arriving on the dry side. Fast, sensitive, and the default for comparing film structures. It answers: how good is this material?
ASTM E96 / E96M — the cup method
A dish containing either desiccant or water is sealed with the specimen and weighed over time. Simple, widely available, and it works on materials an instrument struggles with. The catch is that E96 covers more than one procedure — the desiccant method and the water method do not produce the same figure, and a report that does not say which was used is incomplete.
ASTM D7709 — pharmaceutical bottles and blisters
Desiccant is placed inside the container, the container is conditioned, and it is weighed over time. The result is a rate per container rather than per square metre, which is exactly the shape of number a dosage calculation wants. The scope is the thing to check: D7709 is written for pharmaceutical bottles and formed blisters, and its conditions sit in the accelerated-stability range. It is the right standard to cite for a bottle or a blister. It is the wrong standard to cite for a flexible pouch, and a laboratory will say so.
Why the per-package number is the one that matters
Desiccant dosage is calculated against the total moisture that has to be handled over the service life: what was sealed in, what permeates in, and what enters at each opening. The permeation term needs grams of water per package per day. A per-area figure has to be converted using surface area, and that conversion silently assumes the seals and the closure behave like the film.
They usually do not. A zipper, a spout, an induction seal or a badly-set jaw temperature can dominate a package's real transmission. This is the most common reason a dosage calculated from a datasheet turns out to be undersized in the field — the arithmetic is covered in desiccant dosage by MVTR.
The conditions that must travel with the number
- Temperature and relative humidity. The classic 38 °C / 90% RH and 23 °C / 85% RH conditions produce very different figures for the same film. A number without conditions is not a number.
- Specimen. Bare film, laminate, or the converted pack with seals.
- Method and variant. Standard, revision, and for E96 the procedure.
- Units. g/m²/day and g/pack/day differ by orders of magnitude and are frequently mixed up in the same email thread.
- Laboratory and date. Because the film you buy in 2026 is not necessarily the film that was tested in 2021.
How to ask for it
For a rigid pharmaceutical container, name the standard: "Please provide WVTR for the finished bottle to ASTM D7709, with method, conditions and the laboratory report attached."
For a flexible pouch, do not cite D7709. Ask for what you actually need: "Please provide a gravimetric water-gain measurement on the converted pouch, including seals and closure, at 38 °C / 90% RH, reported in g/pack/day, with the protocol and laboratory report attached." Most contract laboratories will run this as a desiccant weight-gain study; the point is that the specimen is the finished pack, not the film.
If only material-level data exists, ask for F1249 or E96 with conditions and specimen stated, and then treat the derived per-package figure as an optimistic estimate rather than a design value.
The recyclability squeeze
There is a reason this question has become more urgent in 2026. Moving from a multilayer laminate to a mono-material recycle-ready structure generally costs barrier performance. More vapour gets in over the same shelf life, and the desiccant dose has to rise to compensate. Teams making that switch on the basis of a per-area film figure, without re-testing the converted pack, are the ones who see stability surprises — the trade-off set out in why weaker barriers mean more desiccant, not less.
Frequently asked questions
Is WVTR the same as MVTR?
The terms are used interchangeably in packaging. What differs between quotes is the method and conditions, not the acronym.
Which method should we specify for a stability submission?
For pharmaceutical containers, the relevant compendial chapters govern — our note on USP <670> and <671> covers what those chapters actually test, and ASTM D7709 sits alongside them for bottles and blisters. For a flexible pack, specify a gravimetric measurement on the converted pouch and state the conditions yourself.
Can you convert between per-area and per-package figures?
Arithmetically yes, using surface area. Practically it is an estimate, because the conversion ignores seal and closure contributions.
Our supplier only has a number with no conditions. Now what?
Treat it as marketing, not data, and either commission a test on the converted pack or apply a safety factor and validate through stability. A supplier who cannot produce the report behind a figure is telling you something useful about the rest of their documentation, as discussed in eight supplier due-diligence questions.
Does the desiccant type change the barrier requirement?
No — barrier is a property of the pack. But a two-way desiccant that holds a band rather than driving towards zero changes what "enough" means, which is worth settling before the dosage calculation starts.
Send us your barrier data and we will size the dose
Whatever method your WVTR figure came from, ATMOSIScience will tell you what it implies for desiccant dosage — and flag where the number is likely to be optimistic.
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