OTR and WVTR Explained: Understanding Oxygen and Moisture Barrier
Why does coffee go stale, a cracker go soft, or a chip lose its crunch? In most cases the culprit is the slow movement of oxygen and water vapor through the packaging. Two numbers describe how well a package resists this movement: OTR and WVTR. Understanding them is the key to choosing the right barrier for your product.
What Is OTR?
OTR (Oxygen Transmission Rate) measures how much oxygen passes through a given area of film in a given time. It is typically reported in cc/m²/day (cubic centimeters of oxygen per square meter per day) at standard test conditions, often 23 °C and 0% relative humidity. A lower OTR means better oxygen barrier.
Oxygen drives oxidation — rancidity in oils, fading of colors, loss of vitamins, and microbial growth. Products sensitive to oxygen, such as nuts, meat and coffee, need low-OTR structures.
What Is WVTR?
WVTR (Water Vapor Transmission Rate) measures how much water vapor passes through the film, usually in g/m²/day (grams per square meter per day). A lower WVTR means better moisture barrier.
Moisture causes crisp products to soften, powders to cake, and dried goods to spoil. Products that must stay dry — crackers, snacks, powders — need low-WVTR structures. Note that test conditions matter: WVTR depends heavily on relative humidity, so always compare numbers measured under the same conditions.
How to Read the Units
| Property | Common Unit | Imperial Unit |
|---|---|---|
| OTR | cc/m²/day | cc/100in²/day |
| WVTR | g/m²/day | g/100in²/day |
A "high barrier" film might have an OTR below 1 cc/m²/day and a WVTR below 1 g/m²/day. A plain PE film might be hundreds of times more permeable.
What Affects Barrier Performance?
- Material: aluminum foil and EVOH block oxygen far better than PE or PET.
- Thickness: for a given material, a thicker film transmits less — the relationship is roughly inversely proportional to thickness.
- Temperature and humidity: permeability rises with temperature, and some materials (like EVOH and nylon) lose oxygen barrier when wet.
- Defects and seals: pinholes, poor seals and wrinkles can leak far more than the material's rated transmission.
Barrier in a Laminate: Layers Add Up
When several films are laminated together, the layers act like resistors in series. The total barrier of the laminate is always better than any single layer, and you can estimate it from the transmission rate and thickness of each layer. Rather than doing the arithmetic by hand, use the ElagentPak Barrier Calculator to compute the total OTR and WVTR of a multi-layer structure in seconds.
Practical Takeaway
Match the barrier to the shelf-life you need, not to a marketing claim. Over-engineering a barrier wastes money; under-engineering it wastes product. Start with your required shelf life and sensitivity, then choose the lightest, cheapest structure that meets the OTR and WVTR targets.