Water vapor transmission (WVT) is the rate of water transmission through a material of a constant thickness at a given time. Water Vapor Transmission is calculated using the formula: WVT = G / t A= (G / t) / A (G=weight gain; t=time tested; A=area of the test area or dish opening)
Two of the methods to test the water vapor transmission of a material are desiccant (dry cup) and water (wet cup). These two tests are similar in setup but the service conditions are different and the results are not comparable in any way. The dry cup is designed to simulate a heated dry building during a pouring rain, measuring the drive into the building. The water, or wet, cup measures the vapor drive moving in the opposite direction.
For assembling these tests (shown in diagram 1 below), the desiccant or water is placed in a dish leaving a small gap (0.25” to 0.75”) of air space between it and the material. The test chamber is maintained at a constant temperature of 23°C (73.4°F) and a relative humidity of 50 ± 2%. An initial weight is taken of the apparatus and during the course of the test the weight change of the complete test assembly is measured until the results become linear.
The water method assembly measures weight loss due to water vapor from the cup transmitting through the material to the test atmosphere as well as the humidity of the test chamber.
The desiccant method assembly measures weight gain due to water vapor from the chamber transmitting through the material due to the desiccant absorbing any moisture from the material sample and the humidity from the test chamber that is being absorbed by the material.
Based on the equilibrium change of weight, a water vapor transmission value is calculated, which provides an average permeance for the tested material. The product’s permeability may be derived from this calculation* as well as if a material is permeable or non –permeable.
Permeance = WVT/ Δp= WVT/S (R1 –R2)
Δp = vapor pressure difference, in. Hg,
S = saturation vapor pressure at test temperature, in. Hg,
R1 = relative humidity at the source expressed as a fraction (the test chamber for desiccant method; in the dish for water method), and
R2= relative humidity at the vapor sink expressed as a fraction.
Here are the various permeability ratings:
Vapor impermeable: 0.1 perm or less
Vapor semi-impermeable: 1.0 perm or less and greater than 0.1 perm
Vapor semi-permeable: 10 perms or less and greater than 1.0 perm
Vapor permeable: greater than 10 perms