Ensuring breathability of waterproof motorcycle boots

Bruno Valeri

Gore Tex is designed to allow moisture vapor to flow while preventing access to water droplets. The ability to transfer moisture vapor and to provide waterproofing are often opposing requirements. On the one hand, moisture vapor needs to be transfered outward. On the other hand, water must be prevented from migrating inwards. One requires flow-through, the other requires a barrier.

Though high quality materials can allow this within certain parameters, understanding how to keep this breathable and waterproof system working will go a long ways to achieving the desired result and to maintain this performance over time.

note: Manufacturing a waterproof motorcycle boot is not difficult. For example, rubber boots are very waterproof. Unfortunately, they are low on the comfort scale. They offer no permeability to vent moisture, no breathing. The challenge becomes to manufacture a waterproof motorcycle boot that is also comfortable to wear.

So what is Gore Tex?

Gore Tex is a waterproof inner membrane that is composed of holes, much like perfed leather. In a Gore Tex membrane, the holes are big enough to allow vapor molecules through but are too small to allow water molecules to penetrate. So it's really like a filter or screening mechanism. If you're a vapor molecule, you get a green light. If you're water molecule you get a red light.

What's in a name?

First a note on breathable membranes. Not all breathable membranes are created equal. Gore Tex is an established brand that has performed over time. There are other quality breathable membranes that also perform well. But some just carry the a designation stating breathable and waterproof, while performing quite poorly when judged on their ability to transfer water vapour. So buyer beware.

General principle

For a Gore Tex membrane to perform optimally and as intended, it must work as part of a system. Internal moisture must have an open path that vents to atmosphere and there must be positive pressure on the inside to push the moisture outwards.

So the process is:

1) Body emits moisture: this heated moisture increases the pressure on inner side of the Gore Tex membrane. A Gore Tex membrane relies on this positive pressure to drive the moisture through the membrane and on to the next stage.
2) Moisture is pushed through the Gore Tex waterproof membrane. The principle is that the hotter you are, the more heat and moisture that you generate. This increases the pressure available to pump the moisture through the Gore Tex membrane. No positive pressure equals no push.
3) Moisture is then is absorbed by leather and wicked to the outer surface where it is evaporated to atmosphere.

If there is an obstruction impeding the flow in this process, performance will decrease.

Considerations for Gore Tex boots:

Typically, waterproof leather will be used in Gore Tex boot applications. This leather limits the amount of water that it absorbs and, more importantly, allows it to flow moisture vapor (what we refer to as being breathable).

A Gore Tex membrane will only work within a system where all the components are permeable to moisture vapor ie the Gore Tex membrane, the inner liner, and the leather. Clogging one of these components, will significantly reduce the effectiveness of the Gore Tex membrane.

Think of this like the exhaust generated by an engine. The harder an engine works, the more exhaust it produces and the more pressure this exhaust gas will generate. You can have a tuned and high performance header system and pipes, but if you obstruct the exhaust cannister, you get an increase in back pressure. The more the exhaust cannister is obstructed, the higher the back pressure. An effective exhaust is all about flow. In the same way, effective transfer of moisture vapor is also about flow.

If we apply traditional waterproofing agents on waterproof leather ie oils, creams, or greases, we obstruct this breathability by clogging the pores of the leather. This is similar to obstructing the exhaust cannister.

With vapor having nowhere to go when it hits the now clogged leather surface, pressure will build between in the inside and outside of the Gore Tex membrane, impeding directional flow. The inside moisture ie next to your socks, will condense, leaving you with clammy-feeling wet socks. This is similar to what happens when you wear a pair of rubber boots. So, in effect, you lose the comfort advantage of breathable waterproof leather boots. The leather may then become somewhat more waterproof but at the detriment of having wet feet resulting from inner vapor condensation.

Making your leather waterproof by applying a product that clogs the leather pores (ie provides a physical barrier to water) tends to nullify much of the benefits offered by the Gore Tex membrane. So, what to do?