How Waterproof/Breathable Fabrics Work

Until the latter stages of the 20^th century, the term “waterproof/breathable” didn’t get a great deal of circulation in hiking circles. Upon its introduction, it was viewed with suspicion — how, after all, could a fabric let anything out without simultaneously letting other stuff (i.e. rain) in?

Fast forward to the enlightened days of the early 21^st Century and you can’t escape it appearing everywhere in the clothing aisle of your local outdoor store, so much so that a few of us are inclined to wonder how in the heck the magic is done.

For most of us, taking the wonders of our waterproof/breathable backpacking rain pants at face value is good enough, but learning a little about their workings and mysterious ways can go a long way towards helping us choose the best products out there for our activity type and MO in the outdoors.

Below, we’ll break down the secrets behind the functioning of the most breathable fabrics that are also waterproof into bite-sized, easily digestible chunks, starting with a brief overview of what these products are and do before delving into a dash of science.

What are These Fabrics?

To offer a broad definition, a waterproof/breathable fabric (abbreviated to “WP/BR”) is one that, to varying degrees, combines both the ability to prevent external moisture (i.e. rain and snow) from entering while permitting or actively encouraging internal moisture (i.e. sweat) to seep outward and evaporate on the fabric’s surface. This is similar but subtly different to water-resistant clothing which usually only uses a DWR coating (What’s the difference between water-resistant and waterproof you ask?).

The term itself is, in fact, something of a misnomer, with the “fabric” in question most often composed of a duo or trio of very thin layers that would make the plural — fabrics — more accurate.

Breathable waterproof fabrics first hit the shelves in the late 1970s with the introduction of Gore-Tex’s (then) groundbreaking laminate membranes. These days, Gore-Tex no longer rules the roost of waterproof breathable fabrics as comprehensively as it once did and many other forms of WP/BR products — eVent, Sympatex, MemBrain Strata, HyVent — are now making huge inroads into Gore’s one-time monopoly

Waterproof/Breathable Fabrics – The Science

The magic of waterproof/breathable technology is achieved by using either a laminate membrane or a liquid coating on the interior of the garment:

Laminates  

The most common forms of laminate membranes are made with either expanded polytetrafluoroethylene (or “ePTFE”, a.k.a. Teflon), polyurethane (PU) films, or polyester films.

This membrane usually measures somewhere between 7 and 30 microns thick and is bonded to the interior of a garment’s outer like a second skin. To give you some idea of scale, one micron is one-millionth of a meter and a human hair measures around 100 microns.

ePTFE membranes contain a multitude of microscopic pores (W. L. Gore, the maker of Gore-Tex, estimates about 9 billion per square inch) which are responsible for the fabric’s ability to resist penetration by rainwater while simultaneously allowing sweat vapor molecules to escape outward to the fabric’s surface.

It may seem counter-intuitive to make a waterproof garment porous, and the instinctive line of thinking is that this abundance of microscopic holes/pores should make an ePTFE sure to leak, but these membranes work because said pores/holes are far smaller than even the tiniest raindrop but big enough to let water vapor molecules (which are much smaller) seep through.

Additionally, ePTFE membranes possess what in scientific lingo is known as “low surface tension,” meaning the membrane can only be penetrated by other fluids with equally low surface tension. Liquids with a “high surface tension,” such as rainwater, pool together into beads or globules on the membrane’s surface and slide off instead of penetrating or saturating the membrane.

In the case of Gore-Tex WP/BR fabrics, the ePTFE membrane is attached to an incredibly thin protective polyurethane (PU) film to create what is known as a bicomponent laminate. This secondary layer protects the ePTFE from contaminants such as sunscreen, body oils, or insect repellent, which can cause a membrane’s efficiency to deteriorate with time.

Water vapor transfer is permitted by making the polyurethane film hydrophilic (meaning it attracts water) with water-attracting chemicals or by using other hydrophilic materials such as polyethylene oxide.

Sweat molecules are drawn to the hydrophilic film in a process known as adsorption and eventually seep through as a result of the differential pressure on either side of the film. In a nutshell, the hot air and vapor on the inside of the jacket move towards the cooler and drier surface of the jacket, jumping from one hydrophilic polyurethane molecule to the next in a microscopic but very intricate and longwinded game of hopscotch. The game reaches its final stage when the molecules reach the outside of the PU film, where they then evaporate and seep through the ePTFE membrane as a gas, leaving the inside of the garment dry.

Liquid Coatings

Liquid coatings are solutions applied to the interior of a garment to provide WP/BR laminate-like properties. Generally speaking, liquid-coated WP/BR fabrics aren’t as dynamic and don’t perform quite as well as laminate membranes, but usually come in at a much lower price. As such, they are most commonly found in entry or mid-level rain shells or those intended for less extreme activities.

Some examples of brands that use coatings instead of (or as well as) laminates are Marmot, Rab, and Mountain Equipment.

Liquid polyurethane coatings can take one of two forms: microporous coatings and monolithic coatings.

Microporous Coatings

These work much like the laminate membranes mentioned above — by using a microscopic network of channels that are too small for exterior water to penetrate, but large enough to allow vapor from sweat to escape.

The porous quality of these coatings is made in one of two ways: either with a foaming agent that forms gas bubbles that expand inside the coating or with microscopic particles that are mixed into the coating solution in order to allow the formation of minuscule cracks and openings. During the process of drying and solidification, both methods create a network of tiny conduits in the coating through which water vapor molecules can escape.

Monolithic Coating

This form of coating works by creating a solid, hydrophilic (water-attracting) layer that conveys moisture by a trio of processes known as adsorption, diffusion, and desorption.

In brief, these processes work as follows:

Adsorption: The monolithic coating draws water molecules to itself owing to its hydrophilic properties

Diffusion: The liquid seeps through the coating owing to differential pressure. High pressure seeks low pressure and vice-versa, so the high pressure inside the jacket naturally gravitates outward to meet the lower pressure on the jacket’s surface.

Desorption: The vapor molecules evaporate and escape through the outer layer as a gas, completing the process of “water vapor transfer” that is measured in the WVTR, or “Water Vapor Transfer Rate,” now found in the product descriptions of some WP/BR products.