Water vapor transmission rate

Moisture vapor transmission rate (MVTR), also water vapor transmission rate (WVTR), is a measure of the passage of water vapor through a substance.

There are many industries where moisture control is critical. Moisture sensitive foods and pharmaceuticals are put in packaging with controlled MVTR to achieve the required quality, safety, and shelf life. In clothing, MVTR as a measure of breathability has contributed to greater comfort for wearers of clothing for outdoor activity. The building materials industry also manages the moisture barrier properties in architectural components to ensure the correct moisture levels in the internal spaces of buildings. Optoelectronic devices based on organic material, generally named OLEDs, need an encapsulation with low values of WVTR to guarantee same performances over the device lifetime.

There are various techniques to measure MVTR, ranging from gravimetric techniques that measure the gain or loss of moisture by mass, to highly sophisticated instrumental techniques that in some designs can measure extremely low transmission rates. Special care has to be taken in measuring porous substances such as fabrics, as some techniques are not appropriate. For very low levels, many techniques do not have adequate resolution. Numerous standard methods are described in ISO, ASTM, BS, DIN etc.—these are quite often industry-specific. Instrument manufacturers are often able to provide test methods developed to fully exploit the specific design which they are selling. The search for the most appropriate instrument is a zealous task which is in itself part of the measurement.

The conditions under which the measurement is made has a considerable influence on the result. Both the temperature of and humidity gradient across the sample need to be measured, controlled and recorded with the result. An MVTR result without specifying these conditions is almost meaningless. Certainly no two results should be compared unless the conditions are known. The most common international unit for the MVTR is g/m²/day. In the USA, g/100in²/day is also in use, which is 0.064516 (approximately 1/15) of the value of g/m²/day units. Typical rates in aluminium foil laminates may be as low as 0.001 g/m²/day, whereas the rate in fabrics can measure up to several thousand g/m²/day.

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