What is WVTR?  

Water Vapour Transmission Rate (WVTR): is also known as moisture vapour transmission rate (MVTR), or vapour permeability.  It is the rate at which water as a vapour flows through a material.  It is always measured under specified temperature and pressure conditions. It uses a wide and confusing set of different units – see our Conversion Units page. Vapours can flow through (escape from) any material. 

WVTR  is a measure of the rate of vapour flow, rather than the resistance. When vapour permeability values increase, more of that vapour flows through the material. Flow resistance is the inverse of the vapour permeability.

Mathematically the process follows  Fick’s Law.

Why is WVTR Important?

Understanding and measuring Water vapour Transmission Rate (WVTR) is crucial for maintaining optimal moisture levels in a diverse range of products. Manufacturers spanning various industries rely on precise moisture control methods to safeguard the quality and longevity of their offerings, be it in the realms of food, pharmaceuticals, or construction materials.

In the construction sector, innovative coatings based on styrene-butadiene polymer emulsions, such as liquid applied membranes, have been developed to effectively manage moisture. Meanwhile, in sectors like food packaging, a combination of advanced barrier coatings and thoughtfully designed packaging materials are employed to achieve the desired moisture balance.

The techniques for measuring and controlling moisture levels vary significantly based on industry and application. Some industries utilize gravimetric methods, which assess moisture gain or loss by measuring mass, while others employ specialized instrumentation and adhere to industry-specific standards.

What are the applications?

WVTR and the permeability of other gases is important to almost every single non-service industry in the world.  Often it is critically important. A list of common applications is found here.  It affects everything from aerospace to zygology. 

It is frequently vital to safety, efficiency or processes.

It affects health, freshness and contamination.  It controls numerous industrial processes.  It can cause electronics to fail, pizza to go soggy or clothes to become uncomfortable to wear. 

How do you measure WVTR?

There are two main techniques for measuring any vapour’s permeability.  Instrumental and Gravimetric.

In the instrumental technique testing, is based on having a high vapour concentration on one side of a material and measuring how much vapour passes through to the other side. It is usually measured with electronic sensors.  These are fast and accurate. Many sensor types and techniques are available click here for details.  Results can be obtained in as little as 30 minutes, though they can take up to 24 hours or more.

What is the difference between WVTR, MVTR and Vapour Permeability?

Effectively, these are all the same thing. WVTR and MVTR refer to water vapour and Vapour Permeability is more general and covers the process for all gases and vapours, e.g. Oxygen, Hydrocarbons, CO₂ etc.  The words gases and vapours are synonymous.

What is Vapour Permeability

The Dangers and benefits of Vapour Permeability

 

The Maths and science behind Vapour Permeability