The Waterproof Breathable Conundrum

Almost every motorcycle rider has been caught out in the rain — it's inevitable. Manufacturers make a variety of waterproof gear to keep you dry in almost any weather condition. This gear is often described as waterproof and breathable - or as waterproof, windproof, and breathable. But what does that really mean?

From Merriam-Webster:
Definition of breathable
2: allowing air to pass through : porous
We'll get back to that definition in a moment.

When you think of waterproof gear, you may think of the premiere brand name GORE-TEX®. About a year ago I took a day-long tour of the GORE facility in Newark, DE and learned the history of the company, the products, and saw first hand some of the manufacturing and quality assurance processes. During a Q&A session with their textile engineers, I was also able to clear up some of my own confusion about the product's advertised attributes compared to my real-world experience with it, having owned a number of products that utilize GORE-TEX® myself. GORE-TEX® is a membrane that is used by a number of different manufacturers in their gear. Other manufacturers may have similar membranes of their own, like Hydratex®. We will focus our example on the GORE-TEX® membrane, however this will apply to most any material that works in the same manner.

First off, the GORE-TEX® membrane IS completely waterproof. Liquid water cannot pass through the membrane unless there is damage or a defect. You can fill a GORE-TEX® boot with water and none will leak out. It is also completely windproof, meaning that it cannot pass air. In fact, if you take some of the membrane material and fill it like a balloon, you can actually pressurize it to a low PSI — air will not leak out. In the GORE factory this is one of the ways they test and quality control the product, such as glove linings.

So that brings us to the breathable part. How can a fabric be windproof — passing no air at all — and still be breathable according to our definition above? Short answer — it can't. OK, maybe Merriam-Webster's definition of breathable is incorrect. Let's double-check with the Cambridge Dictionary definition:
breathable adjective
Breathable fabrics for clothes allow air to pass through them.

Looks like we still have a conundrum — you simply cannot have a fabric that both passes air and does not pass air. What is going on? "Breathable" is a misused term which causes confusion among consumers.

What is really meant by "breathable" in the context of waterproof gear (for any active wear, not just motorcycling) is that water vapor is able to pass through. Not liquid water, and NOT AIR as we breath it and which can cool us, but just water vapor — the gaseous phase of water. These water molecules (H2O) are not bound together like they are in water's liquid or solid (ice) phases, and according to the Ideal Gas Law, water vapor is lighter and less dense than air (think of clouds in the sky). This is what allows these molecules to permeate the membrane.

In the lingo of waterproof fabric manufacturers, and only there, the definition of "breathable" has morphed away from what we are used to — the actual definition, but is referring only to water vapor. This sets unrealistic consumer expectations. If you want to have some gear which is truly breathable in the traditional sense, don't be lured in by "breathable". What you need to look for in that case is "venting". In waterproof motorcycle gear, venting is what will allow actual air to get through a garment and reach your skin to help cool you. But understand, of course, that if you open a vent you have now defeated any waterproof capability of the gear in question. The vents themselves are potential weak points in the waterproofness of a garment, and so are generally kept to a minimum.

Humans perspire, and perspiration is meant to cool your body through its evaporation to the air (enthalpy of vaporization). If you don't have any air movement, very little perspiration will evaporate. If it's dry out, you can simply open the vents in your gear to create air movement which will help keep you cool and evaporate your sweat. But when you're riding in the rain, it's important to manage your temperature so as to not create excess perspiration, as it will be difficult to expel through your waterproof gear.

Non-waterproof textiles, on the other hand, can be truly breathable in the traditional sense — they are air-permeable. Mesh gear is the most obvious form of non-waterproof, fully-vented or air-permeable gear, and many other textiles that are not specifically labeled as waterproof or windproof will have some air-permeability — they will be properly breathable in the expected way. This gear may help keep you cooler and evaporate your perspiration on hot days better than any waterproof gear. These are among the tradeoffs that you must consider when purchasing — there is no magic bullet, and non-waterproof gear is not necessarily inferior for your intended use.

Back to the waterproof, windproof, "breathable" membranes. One property not yet discussed is that of directionality. The membrane works the same in both directions, it is not a one-way valve for water vapor. The water vapor will penetrate the membrane according to a difference in "vapor pressure" — think of potential difference, like electrical voltage or hot to cold. The side with the higher vapor pressure will allow water vapor to pass to the side with lower vapor pressure.

The extreme version of this example is the classic demonstration of putting a wet hand into a GORE-TEX® glove liner, and then putting the hand in ice water (as per this video). The heat generated from the hand heats up the small amount of water inside the liner, and the contrasting cold ice water pulls the vapor right out of the liner, drying the hand within seconds. If dipped in warm water this would not work the same way, it is demonstrating the most ideal circumstance to show the properties of the membrane. However, these are not normal circumstances when riding in rain. Water vapor will go equally well through the membrane in the opposite direction if there is a greater "vapor pressure" on the outside of the garments. Think heated grips in the rain, or riding through fog while you're not perspiring. There are scenarios where the properties of the membrane will actually work against you, or won't work well enough to keep up with your rate of perspiration.

These unique circumstances aren't a failure of the gear or a defect, just another property that riders need to be aware of — you may still become wet in waterproof gear. If you've ever worn full rubber rain gear, you will know how clammy it feels being sealed in a fully waterproof suit with no way to get the water vapor out.

— Greg Hassler