Creating a fabric that blocks rain is easy. Creating a fabric that blocks rain yet still allows your sweat to escape is much more difficult. However all waterproof fabrics work on the same principle: diffusion. Molecules will move from areas of high concentration to areas of low concentration. Hence when you warm up and create moisture, it wants to pass through your waterproof jacket to reach the lower concentrations outside. Different waterproof materials achieve this in different ways
ePTFE Membranes with a PU coating (eg. GORE-TEX)
Traditional GORE-TEX (introduced in 1978) paved the way by using a stretched PTFE membrane containing 9 billion microscopic pores per square inch. These pores are 20,000 times smaller than a water droplet (so rain can’t penetrate) but also 700 times bigger than a molecule of moisture vapour (so vapour can escape). This ‘sieve’ of microscopic holes is backed by an ultra-thin oleophobic polyurethane coating which prevents body oils from contaminating the membrane. The PU coating also completely blocks the wind, which is what gives a GORE-TEX garment its windproof properties.
As you sweat, vapour condenses on the lining and passes through the polyurethane coating as a liquid, reaching the ePTFE membrane where it evaporates out through the network of microscopic holes. It is the differential between the warm, moist internal climate and the cooler, dryer external climate that drives vapour through the fabric’s pores away from your skin. So the harder you work, the warmer you get and the faster the whole process works.
ePTFE Membranes without a PU coating (eg. eVent / NeoShell)
These fabrics all utilise an ePTFE membrane without a protective PU film (or in some cases with a PU film that encircles the pores, leaving them uncovered). Since there’s no cover over the pores, the membrane is able to breathe more effectively. Fabrics like this can take air in as well as pushing moisture out. It’s a two-way air flow that disperses moisture by convection (air circulation) as well as diffusion (pressure gradients).
A downside of this system is that without a PU film the membrane may be more prone to contamination. It is possible to build an oleophobic treatment into the membrane but this may be less effective or durable than the traditional GORE-TEX method. Pores left open will breathe better but may become clogged with oils and dirt more easily, meaning you may need to wash the garment more regularly. Wearers may also experience more rapid cooling in high windchill situations though garments will still be roughly 95% windproof.