I remember a friend who came to see me one day showing me the overtrousers he had just bought for a trek in northern Laos. I had previously advised him to choose a waterproof and breathable garment. I think he had only remembered the first word, or hadn't understood the importance of the second. In the end, he had bought the cheapest product and ended up with overtrousers that had no breathability whatsoever!
Water from outside didn't get in, but perspiration on the inside wasn't expelled outward. It was the "pressure cooker" effect — but without the little steam vent! In the end, his hiking trousers underneath were permanently soaked. We've all made this kind of mistake when first buying gear, but don't fall into that trap — choose your waterproof hiking equipment (garments, single-wall tents, footwear, etc.) carefully to avoid spending money unnecessarily. The aim of this article is to help you do just that and give you the basic knowledge to find your way around.
What is breathability?
Breathability refers to a fabric's ability to expel water vapour. It can be measured in Thermal Evaporative Resistance (RET), though not all manufacturers necessarily use this standard — some instead display a figure in grams/m²/24h. In this case, the higher the figure, the better the moisture transfer. A key benchmark value to remember is 10,000 g/m²/24h.
- Below 10,000 g/m²/24h: average breathability
- Above 10,000 g/m²/24h: excellent breathability.
The higher a fabric's breathability, the lower its RET, and the more effectively it expels water vapour.
And in practice?
The more breathable a fabric is, the less you'll be dampened by your own perspiration when wearing it as a garment. If it's a tent wall, less condensation will form inside. Generally speaking, one might say that the more waterproof a fabric is, the less breathable it tends to be — and conversely, the more breathable it is, the less waterproof it is. In practice, however, modern technologies now make it possible to have products and garments that are both waterproof and highly breathable. But you need to make the right choice, resist going for the cheapest option, and be prepared to spend a little more for greater comfort. When choosing a garment, always take its waterproofness into account and select the product based on a balanced waterproofness/breathability trade-off suited to your activity.
But how does a fabric become breathable?
The breathability of a waterproof fabric generally comes, in simplified terms, from two processes:
The first process can be described as "mechanical": the pores in the membrane or coating of the fabric are large enough to allow water vapour molecules to pass through, but too small to let liquid water in. It is therefore the portion of your perspiration that is in a gaseous state that can be expelled. This is the principle behind Gore-Tex, for example. A second process is more "chemical": hydrophilic molecules are incorporated into the membrane or coating of the fabric and draw water vapour from the inner side, pushing it outward thanks to the differential in temperature and relative humidity on each side of the fabric.
A few tips and common mistakes to avoid
It's worth knowing that with extremely breathable garments, you can still end up damp from your own perspiration. For example, on a rainy day, you keep your fleece jacket on under your waterproof and breathable jacket during a climb where you're working hard (you're very hot), and you can't be bothered to take it off: you'll regret it when you end up wet and cold on the descent. I'm sure it's happened to you before… it's certainly happened to me!
In fact, buying breathable and waterproof garments doesn't mean you can stop thinking about and consistently applying (and adjusting according to context) the three-layer rule. To avoid sweating heavily under waterproof garments, you need to regulate your body temperature by managing your layers correctly and making use of ventilation options, for example.
All ventilation systems (zips, mesh, etc.) are, in fact, important elements to consider when choosing waterproof gear.
It's worth knowing that when a waterproof and breathable fabric becomes saturated with water (the outer surface is completely wet), the fabric can no longer breathe and moisture vapour will no longer be expelled. You must therefore prevent your perspiration from condensing inside the fabric. If this happens, wipe or blot the inside of the fabric. DWR treatments now help, in part, to address these issues, by helping the outer surface of the fabric to avoid becoming saturated with water. Overall, you can see that the breathability of a fabric or garment is essential when it comes to sport or travel. It's a matter of your comfort and wellbeing, so don't overlook this point when making purchases — and pay attention to the tips and advice we've shared here.
