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 pharmeceuticals 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.
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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. Note that special care has to be taken in measuring porous substances such as fabrics as some techniques are not appropriate. Likewise for very low levels, many techniques would not have the resolution to provide a reliable result. Numerous standard methods are described in ISO, ASTM, BS, DIN etc., -- these are quite often industry-specific. Instrument manufacturers will often be able to provide test methods developed to fully exploit the specific design which they are selling.
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 approximately 1/15 of the value of g/m²/day units. (More precisely, the ratio is 1/15.500031, or very close to 2/31.) 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.
Often, testing is conducted on a sheet of material. Calculations based on that can be useful when designing completed structures (packages, clothing, etc.). Seams and seals are also very important to end-use performance; performance verification and validation of complete containers or irregular objects is often recommended.