Two numbers decide whether a flexible package protects your product: OTR and WVTR. They appear on every technical data sheet, they drive a large part of the material cost, and they are routinely misused in sourcing conversations — usually by comparing figures that were never measured under the same conditions.

This guide explains what each number measures, how to reason from shelf life back to a structure, and what to insist on when a supplier quotes you a barrier value.

What the two numbers measure

OTR — Oxygen Transmission Rate. How much oxygen passes through a given area of film in a given time. Reported in cubic centimetres per square metre per day (cc/m²·day). Lower is better. This is the number that matters for oxidation: rancidity in fats and oils, colour loss, flavour degradation, vitamin loss.

WVTR — Water Vapour Transmission Rate. How much moisture passes through, reported in grams per square metre per day (g/m²·day). Lower is better. This is the number for texture: crisp products going soft, powders caking, hygroscopic ingredients clumping.

They are independent. A structure can be excellent at one and mediocre at the other, which is why a single “high barrier” claim tells you almost nothing on its own.

Which one matters for your product

Product type	Primary concern	Drives
Roasted coffee, nuts, fried snacks	Oxidation and aroma loss	OTR
Biscuits, crisps, crackers	Loss of crispness	WVTR
Milk and protein powders	Caking and clumping	WVTR
Cured meats, cheese	Colour and rancidity	OTR
Effervescent tablets, some supplements	Moisture reaction	WVTR, often severely
Pet food with high fat content	Rancidity plus grease resistance	OTR, plus sealant choice

Work out which of the two governs your product before you compare quotes. Buyers frequently pay for oxygen barrier on a product whose real enemy is moisture.

Reading barrier levels without being misled

Published screening bands for flat films, in cc/m²·day under comparable conditions, are commonly grouped roughly as follows: ultra-high barrier at or below 1; high barrier above 1 up to 10; moderate above 10 up to 100; and basic above 100.

Treat those bands as a first screen and nothing more. The industry sources that publish them attach the same warning we would: they are not shelf-life guarantees and not a substitute for an application-specific specification, and isolated numbers copied from different technical data sheets should never be compared directly.

The reason is that a barrier figure is only meaningful with its test conditions attached. Temperature and relative humidity both change the result, sometimes dramatically — several common barrier materials lose a great deal of their oxygen performance as humidity rises. A number quoted without conditions is marketing, not data.

The material families, in order

Roughly from most to least protective, with the trade-off each one carries:

Aluminium foil laminates. The reference point for barrier, and effectively opaque to both oxygen and moisture. Costs more, cannot be seen through, and creases can compromise it. Covered in our guide to foil stand-up pouches.
Metallised films (VMPET and similar). A vacuum-deposited metal layer gives a large barrier improvement over the base film at lower cost and weight than foil, with a metallic appearance.
Coated barrier films — EVOH co-extrusions, PVDC coatings, oxide-coated PET. High oxygen barrier while remaining transparent, which is why they dominate where the customer must see the product. EVOH in particular is humidity-sensitive and is normally buried between moisture-resistant layers.
Nylon (PA). Moderate oxygen barrier, excellent puncture resistance. Often chosen for the mechanical property with the barrier as a bonus.
PET and BOPP. Structural and printable outer layers with modest barrier.
PE. Good moisture barrier, poor oxygen barrier, and the standard sealant layer.

If transparency matters to your brand, the decision usually comes down to a coated transparent structure versus metallised or foil. Our comparison of frosted, clear and foil finishes covers the visual side of that choice.

Working backwards from shelf life

The correct sequence is not “pick a barrier and see what shelf life you get”. It is:

State the shelf life you need, with the storage conditions it must hold under — ambient in a hot humid market is a different problem from air-conditioned retail.
Identify the failure mode. What actually goes wrong first: rancidity, softening, colour, aroma?
Establish the tolerance. How much oxygen or moisture pickup can the product absorb before it fails sensory or safety criteria? This usually requires your own product data.
Match a structure to that tolerance, including headspace, pack size and the surface-area-to-content ratio — a small sachet has proportionally far more film per gram of product than a large pouch.
Validate with real storage trials on the finished pack.

Steps 3 and 5 are the ones buyers skip, and they are the ones that determine whether the pack actually works.

Why lab numbers and shelf performance diverge

A film’s measured barrier is a ceiling, not a promise. In a finished package the weak points are usually elsewhere:

Seals. A channel leak in a seal makes the film’s rating irrelevant.
Fitments. Zippers, spouts and valves are separate paths for gas transfer.
Creases and flex cracking. Foil and some coated films lose performance where they are repeatedly folded — relevant for anything that travels far or is handled a lot.
Headspace. Oxygen already inside the pack at sealing does its damage regardless of the barrier.
Storage reality. A container crossing the equator sees temperatures and humidity no data sheet assumed.

What to ask a supplier

What are the OTR and WVTR of this exact structure, and at what temperature and relative humidity were they measured?
Which test standard was used?
Is the figure for the film alone or for the finished pack including seals and fitments?
How does the value change at the humidity of my destination market?
Can you supply the test report rather than a summary line in a quotation?

A supplier who can answer all five is quoting from data. One who answers with an adjective is quoting from hope.

Getting a structure specified

If you can tell us the product, the shelf life you need, the destination market and the pack format, we can propose a structure and say plainly which of the two barrier numbers is doing the work. Message us on WhatsApp or send the details through the enquiry form.

For the format decision that sits alongside this one, see our guides to stand-up pouches and choosing a packaging bag.