What Does Waterproof Breathable Mean?

“Waterproof Breathable” is a term that raises questions. Is it waterproof AND breathable or merely waterproof OR breathable?

Membranes like Gore-Tex, Neo Shell, and eVent serve as a vital layer for many when the weather is less than ideal. However, despite their name, these waterproof breathable membranes often fail to perform well in wet conditions, and physics plays a significant role in this limitation.

So, what exactly are these membranes? There are two primary types of waterproof jackets: membranes and coatings. Membranes bond directly to an outer material (often layered with others), while coatings are simply a substance applied to the outer material, sometimes featuring a protective inner layer. Membranes such as Gore-Tex and eVent typically offer better breathability. However, they tend to be heavier, less packable, and more costly than coatings. On the other hand, coatings like Marmot's NanoPro or Pertex Shield+ are lighter and more affordable but often lack breathability and durability compared to membranes.

The Waterproof Breathable Problem

Waterproof breathable membranes and coatings function based on the principle of diffusion. Diffusion requires a humidity difference between the two sides of the membrane to facilitate moisture transfer. Greater differences yield higher diffusion rates. When exercising, the interior of your jacket becomes humid, making it most effective at transferring sweat to the outside when conditions are dry.

However, in heavy rain, the external humidity is typically high, which reduces the diffusion driving pressure to low or zero. Thus, while your jacket may be waterproof, its breathability is negligible during these conditions. If you sweat while it pours, chances are, you'll end up getting wet. When humidity levels outside match those inside the jacket, sweat has nowhere to go. Unless there is some fantastic solution, it seems impossible for a breathable jacket to keep you dry while working hard in a humid environment. My experience in the exceedingly wet coastal mountains of British Columbia supports this observation.

If you're looking for more information regarding how waterproof breathable membranes and coatings work, I recommend checking out REI's informative article.

Ultimately, when it rains, you have three realistic options:

1. Forget about staying dry and get wet in the rain, which is viable in warm conditions.
2. Wear a waterproof shell, exert yourself, and risk sweating more than you’d like, resulting in a colder experience when you stop moving.
3. Don a waterproof shell, realize that even the fanciest models struggle with breathability due to similar humidity levels inside and outside, and slow your pace to manage sweat production. If you start sweating too much, slow down.

Notably, there's no solution for staying dry while working hard in the rain. Unfortunately, nothing currently on the market can accomplish this effectively.