How It All Works
In traditional WPB fabrics, the path of water vapor from inside the jacket to outside the jacket is a very complicated process that involves phase changes, solid-state diffusion based on concentration gradients, and hydrophobic/hydrophilic interactions.
In an air perm garment, while these diffusion and hydrophobic/hydrophilic interactions still occur, they contribute little when it comes to water vapor transport. Instead, air perm garments function primarily through a different mass transfer mechanism—or, more specifically, convective mass transfer—that allows water vapor simply to pass through the fabric.
The nature of traditional WPB fabrics, on the other hand, prevents any such convective mass transfer. Why this is gets a little more complicated, but, generally speaking, it starts with polyurethane.
Polyurethane (& Perfume)
In the past, companies like Gore-Tex relied on technology like polyurethane (PU) coatings to protect their membranes. PU is a monolithic, non-porous layer, which means that water vapor transport can only occur by a diffusive mass transfer mechanism (again, as opposed to convective), which requires a strong driving force, in our case, the driving force is a concentration gradient.
Imagine spraying some perfume in a room. Initially there will be a high concentration of perfume in the air right where it was sprayed, but eventually the perfume will dissipate across the room. This is the work of diffusion driven by a concentration gradient. The perfume particles experience a driving force to travel from areas of high concentration to areas of low concentration, thus spreading throughout the room.
Similarly, in traditional WPBs, a rather large pressure, heat, and water vapor concentration gradient typically must form between the inside and the outside of the membrane in order for water vapor to make it through the non-porous PU later—that is, in order for the membrane to start “breathing.”
(And as you’ll know from having carefully studied my 101 article, when the concentration gradient is formed, each water vapor molecule adsorbs to the PU laminate surface, then is absorbed into the membrane and transported through the opposite side, driven by the concentration gradient between the water vapor on either side of the membrane.)
In other words, a traditional WPB only starts to breathe after you have worked up a bit of a sweat—which is often too late for you to stay dry.
The amount of water vapor transported out of the jacket is a function of water vapor pressure against the membrane. The more you sweat, the more the jacket will breathe.
And If U Ditch the PU?
Suppose, then, you could eliminate the need for the water vapor to pass through any sort of PU or non-porous layer? You would expect to get an improved flux of water vapor across the membrane—i.e., you’d increase breathability.
This is exactly what air perms do.
They eliminate any sort of non-porous layer, improving any diffusive mass transfer.
What’s more, breathability is further increased in air perms because the membrane’s pore sizes are large enough to allow a small amount of air to flow across the membrane. This makes convective mass transfer possible.
Convective mass transport works largely via advection, or the transport of mass through macroscopic fluid motion, typically in the form of currents. In our case, the fluid is air, and the mass being transported is water vapor (sweat). Water vapor simply passes through the fabric.
Put another way, convective mass transfer does not require a driving force or concentration gradient. The membrane is breathing as soon as you put it on and the amount of water vapor transported out of the jacket is not a function of water vapor pressure against the membrane, as it is in traditional WPBs. Air perm garments will breathe equally well no matter how much you are sweating (if we are only measuring the convective mass transfer, that is).
It is important here to reiterate the fact that no WPB membrane is “directional.” This means that there is no mechanism or force that keeps water vapor from transporting into your jacket from the outside. All WPB fabrics rely on the assumption that there is much less water vapor outside your jacket than inside. Let’s just say I don’t recommend standing in a hot tub with a pair of WPB waders on…
Air perms are obviously not completely windproof; there is a small amount of air that readily passes through the fabric. This small amount of air is practically undetectable to the wearer (I have never felt air move through an air perm membrane that I have worn), but air is moving all the same.
So, Why Now?
With the seemingly obvious advantages of air perms, one logical question is, “Why are air perms just now hitting the consumer market?”
First, as mentioned earlier, they are difficult to market. Windproof jackets that aren’t remotely waterproof have been out for years. But these new air perm fabrics are truly waterproof, though not absolutely windproof. And that can be confusing.
And that’s before we even bring up the inconsistency of how different brands define and rate breathability (see Outerwear 101).
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.