Deconstructing the Membrane
Before technical fabrics became commonplace, most of us had a plastic-like waterproof jacket. It might have done a great job of keeping out the rain, but it also trapped body moisture inside. As a result, we’d get cold and shiver anyway.
Modern outdoor clothing is designed to work in moisture wicking layers to prevent that uncomfortable, clammy feeling. In wet conditions, the outermost layer has to be waterproof and that’s where a breathable membrane becomes essential. In basic terms, it stops water from getting in, while at the same time allowing it to get out. As a result, your body stays dry and warm.
How it’s made
Membrane fabrics are usually made of two or three layers. The inner layer protects the membrane from damage while soaking up the moisture from your body.
The middle layer is the membrane proper and is made of a microporous material such as polytetrafluoroethylene. It sounds a little terrifying but it’s nothing unusual. In fact, it’s the same as the Teflon on your frying pans. It allows sweat (in the form of steam) to pass through the tiny holes in the fabric, but prevents rainwater (in the form of liquid) from getting in.
The two layers are then laminated to the outermost shell of nylon or polyester, which provides the fabric with strength and durability.
How it works
Baselayers and midlayers absorb sweat and simply wick it away from your body. In a scenario when you’re not wearing a waterproof shell, it evaporates freely. Met with a breathable waterproof membrane, it depends on osmotic pressure to get out.
Osmotic pressure occurs when there’s a difference in steam concentration on either side of the membrane: high on the inside, low on the outside. To even it out, steam trapped in your clothing wants to get out, so it squeezes through the tiny pores in the material. You’re left dry, happy and in awe at the wonderful laws of physics. Thank you, Newton!
What you need to know
Waterproof outdoor clothing is labeled with two ratings: for example, 20K mm, 20 g/m2. The first part is expressed in thousands of millimeters of water and refers to how much rain a fabric can withstand. The second measurement is given in thousands of grams of vapor per square meter and is an indication of breathability.
A 5-10K mm jacket will protect you against rain showers or dry snow. If you’re searching for something that can withstand long periods of heavy rain, look for a garment rated 15-20K mm, or even >20K mm for expedition level protection. Breathability wise, 10-15 g/m2 is usually enough for medium intensity exercise such as easy hiking or trail skiing. For more intense activities, like mountaineering or ski touring, choose a higher rating of 15-20 g/m2 or even more.
A great example of a high-end, breathable waterproof garment is the Arc’teryx Stikine Jacket. Complete with a GORE-TEX® membrane and Climashield insulation, it’s ready for any backcountry adventure.