Diffusion of a substance through a layer is described by Fick’s 1st and 2nd Laws, which relate to the steady state and nonsteadystate condition, respectively. Fick’s main hypothesis states that the rate of transfer through unit area of a section, F(x), is proportional to the concentration gradient measured normal to the section, i.e.

F(x) = 2D(hc/hx)        (1)

where c is the concentration of diffusing substance, x is the space co-ordinate normal to the section and D is the diffusion coefficient. The water vapour transmission (WVT) properties of WBF membranes can be derived from eqn (1) using a different proportionality constant (k) to relate WVT with the pressure difference across the membrane surfaces (p1/ p2) and its thickness d.

m/At = k(p1- p2)/d (2)

Many recognized breathability tests simply measure the amount of water vapour (m) passing through a fixed area of material (A) over time period (t) by gravimetry. The WVT rate (m/At) obtained under steady state conditions has an SI quantity of kg mˉ2 sˉ1, although the trivial unit g mˉ2 dayˉ1 is most widely recognised by this industry.