But the main reason we haven’t seen air perms on the market, until now, is because the technology simply wasn’t there. In the past, as I mentioned, companies relied on technology like PU coatings to protect their WPB membranes—always tweaking, but never innovating.
In the past couple of years, however, technological advances have allowed companies to create membranes with very little in common with their PU-coated ancestors. What these new membranes look like and how they are protected varies between manufacturers, but none of them rely on monolithic-PU protection schemes


Traditional WPBs are breathable in that they transmit water vapor through the membrane via a single mechanism: solid-state diffusion. (More details on this below, but for a more thorough explanation, I’d recommend hopping over to the Outerwear 101 article.)
Air perms, on the other hand, breathe via two mechanisms: solid-state diffusion and, to a much larger extent (and as the name implies) by moving air directly through the membrane. This is a convective mechanism.
And that is the primary reason for the “enhanced” breathability of air perms: in addition to the diffusive mechanism, air perms also have a convective mechanism that contributes to their breathability.