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 only remembered the first word, or hadn't grasped the importance of the second. In the end, he had bought the cheapest option and ended up with overtrousers that were not breathable at all!
Water from outside couldn't get in, but the sweat on the inside wasn't being pushed out. It was the "pressure cooker" effect — but without even a small steam vent! In the end, he had a hiking trouser underneath that was permanently soaked. That said, we've all made this kind of mistake on our first purchases — but don't fall into this trap. Choose your waterproof hiking gear carefully (clothing, single-wall tents, footwear, etc.) to avoid wasting money. The aim of this article is to help you do exactly 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 Evaporative Thermal Resistance (RET), though not all manufacturers use this and some instead display a grams/m²/24h figure. In this case, the higher the number, the better the moisture transfer. A useful benchmark value is 10,000 g/m²/24h.
- Below 10,000 g/m²/24h: average breathability
- Above 10,000 g/m²/24h: excellent breathability.
The higher the breathability of a fabric, the lower its RET value and the more effectively the fabric expels water vapour.
And in practice?
The more breathable a fabric is, the less you'll be soaked by your own sweat if it's a garment. If it's a tent wall, there will be less condensation forming on the inside. In general, one could 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. In reality, however, modern technologies now make it possible to have waterproof and highly breathable garments and products. You just need to make the right choice, resist going for the cheapest option, and be prepared to spend a little more for extra 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, broadly speaking, from two processes:
The first process can be described as "mechanical": the holes in the membrane or fabric coating are large enough to let water vapour molecules pass through, but too small to allow liquid water to penetrate. It is therefore the gaseous portion of your perspiration that can be expelled. This is the principle behind Gore-Tex, for example. A second process is more "chemical": hydrophilic molecules are embedded in the membrane or fabric coating and attract water vapour from the inner side, pushing it outward thanks to the temperature and relative humidity differential on each side of the fabric.
A few tips and mistakes to avoid
Bear in mind that with extremely breathable garments, you can still end up wet from your own sweat. For example, on a rainy day, if you keep your fleece jacket under your waterproof and breathable jacket during an uphill section where you're exerting yourself (you're very warm) and you "can't be bothered" to take it off — you'll regret it when you're soaked and get cold on the way back down. I'm sure that's happened to you before… As it has to me!
In fact, buying breathable and waterproof garments doesn't mean you can stop thinking about — and consistently applying (while adjusting to the context) — the 3-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 openings, for example.
All ventilation systems (zips, mesh, etc.) are, in fact, important elements to consider when choosing waterproof gear.
One final point to remember: when a waterproof and breathable fabric becomes saturated with water (the outer surface is completely wet), the fabric can no longer breathe and water vapour will no longer be expelled. You should therefore avoid letting your sweat condense on the inside of the fabric. If this happens, wipe or blot the inside of the fabric. Water-repellent treatments now help, in part, to address these issues, as they help the outer surface of the fabric resist 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 when making purchases, and pay attention to the tips and advice we've shared here.
