Vapour permeability units and conversions

Water Permeability units and conversion between units

The very wide set of different units that are used for vapour permeability often make it incredibly difficult to compare samples, gazes and results. Not only are different sets of units used in different geographical regions, but also in different applications!

A material does not have a single vapour permeability for all gazes / vapours the transmission rate depends critically on the specific vapour, and sometimes on a vapour's specific isotopes. For example, water vapour may flow easily through one material, but that same material may put up much greater resistance to another vapour - such as oxygen, carbon dioxide or a hydrocarbon. The pressure difference of the vapour across the material can also make a big difference, as can temperature. Thus, the vapour permeability of a material is given for specific materials at specific temperatures, specific material thicknesses and specific vapour partial pressures.

The permeability coefficient measures relative permeation behaviour and is used to compare the permeability of different polymers.

In an effort to clarify the differences between the units we hope the following information is useful though, in a note from the lawyers, although we believe all of the information is accurate, you use these conversions entirely at your own risk

The normalized units for vapour permeability are called the vapour transmission rate, and are expressed as g mm/m2 day in standard SI (System International) nomenclature. However, in practice, for largely historic reasons, a wide and confusing variety of different units are commonly used.  This page gives the conversion factors between these vapour permeability units Please not that temperature is a major factor in the permeation rate and it should always be quoted.

According to the definition given in EN 31092:1993, water-vapour permeability is "a characteristic of a textile material or composite depending on water vapour resistance and temperature
Technically the vapour permeability of a material is given by the equation

dMgas/ dt = P x A dp /l

where:-

P is the vapour permeability of the barrier
dMgas is the amount of gas that flows through the barrier
dt is the time it takes
A is the area of the barrier
l is the thickness of the barrier
dp is the partial pressure difference across the barrier

Effectively a moisture vapour transmission rate (MVTR) is a measure of the passage of vapour through a material (such as a vapour barrier, film or fabric) of a given unit area per unit time under pre-specified temperature and humidity conditions.

WVTR is the same as MVTR when the vapour it water vapour

The lower the MVTR the smaller the amount of moisture passes through in a given time.

In metric units, MVTR and WVTR are expressed in kilograms, grams, milligrams or nanograms per square meter per 24 hours (g/m2 /day). In the US, however, it is often expressed in grams per 100 square inches per 24 hours (g/100 in2 /day).

The SI unit for permanence is the kilogram (or gram or nanogram) per second per square meter per pascal.  For example, 1 kg s−1 m−2 Pa−1 ≈ 1.74784x1010 US perms≈ 1.15191x1010 metric perms.  However, when it is also expressed as nanograms per second per square meter per pascal.  In this case 1 ng·s-1 m−2·Pa−1 ≈ 0.0174784 US perms≈ 0.0115191 metric perms

The US perm is defined as 1 grain of water vapour per hour, per square foot, per inch of mercury.  1 US perm = 0.659045 metric perms≈ 57.2135 ng·s-1 m−2·Pa−1

The metric perm (which is not a SI unit) is defined as 1 gram of water vapour per day, per square meter, per millimeter of mercury.  1 metric perm = 1.51735 US perms ≈ 86.8127 ng·s-1 m−2·Pa−1

The Units conversion tables below are displayed as a graphic so that it works on all devices. A PDF of it, thogewther with the information above can be downloaded below

Vapour permeability conversion factors

Note 1. To convert units in the first column to units in the heading multiply by the factor given at the intersection of the row and column
Note 2. This was the term used by the building industry
Note 3. No conversion from temperate to tropical is shown for reasons given in clause 46 of BS 2972 : 1975
Note 4. The symbol gr refers to grams

Disclaimer: Although every effort has been made to ensure the figures are accurate, you use it at your own risk

 

Download PDF of Vapour Permeability Units and conversion tables.