Waterproof breathable hard shells are one of the most commonly discussed features in a clothing system. While they can be a very useful piece of equipment, they are far less necessary than manufacturers would have us believe. Due to questionable if not false advertising, there are a great number of myths that circulate regarding hard shells. The following will describe the design and purpose behind the leading waterproof breathable membranes and suggest how they can be used to your benefit as part of a clothing system.
Defining “Waterproof” and “Breathable”
While you’d think that a “waterproof” jacket would, by definition, allow no water to pass through it, that isn’t necessarily the case. A truly waterproof material like polyurethane won’t allow water to pass in through it, but won’t allow any to pass out either. The result is saturation from our own moisture.
The most common method for determining water resistance is measured in terms of hydraulic pressure applied to a given area, and is recorded in millimeters (aka the “hydrostatic head”). The higher the value, the greater hydraulic pressure is required to overcome the water resistance of a membrane.
Under lab conditions, the breathability of a fabric is generally measured in terms of a water vapor transmission rate. Simply put, it is the rate at which a fabric transmits water vapor from one side to the other. There are a number of ways to measure breathability that favor one membrane type over another, and the value most favorable for a given brand will often be quoted by the manufacturer. Therefore, it is important to consider field reports and the basic design of the membranes to determine what products will breathe the best in real world scenarios.
Gore and PTFE’s
PTFE, literally Teflon, is the most common and well known waterproof breathable membrane (Gore-tex is a PTFE). It consists of a Teflon layer full of micropores that are designed to allow moisture to be repelled from the outside, but transported from the inside. The outer layer of the membrane is very water repellent on a molecular level, while the inner face is not. Liquid water is resistant to this hydrophobic surface environment, and thus will not pass through the membrane until a certain level of pressure is reached, or the surface layer loses its water resistance. Water vapor can be passed to the outside of the membrane via capillary action or by simply escaping through the microscopic pores therein. As time goes on, a bare PTFE membrane will become contaminated by dirt, oil from skin, etc. This reduces the hydrophobicity of the outer layer of the membrane to the point of leakage.
To remedy the problem Gore added a polyurethane coating to the PTFE to protect it from contaminants. The coating negates the PTFE’s ability to allow water vapor to escape directly through the membrane. This is what we know today as a standard Gore-tex membrane. Since polyurethane is not water permeable, it was altered to have hydrophilic (water absorbing properties). Therefore, it absorbs each water molecule that it contacts on the inner surface of the outerwear and diffuses it to the outer layer of the fabric. For this diffusion to occur, the outer layer of the fabric must be dry to create a moisture gradient.
By reducing the thickness of the polyurethane layer on the inside of the membrane, Gore increased breathability by an advertised 25% over the previous generation. This conformation was called Gore-tex XCR. It still employs same basic design, but with minor improvements.
Gore-tex Pro Shell is Gore’s latest entrance into the market of waterproof breathable mountain wear. It uses the same polyurethane coated PTFE, but includes some fabric alterations. The inner fabric is lighter and more tightly woven and the seam taping is slightly narrower. Gore claims better breathability, but does not offer how, as there is still a PU layer impeding moisture transfer. Most likely, they have simply made that layer thinner than before and perhaps used a better wicking outer material.
The traditional Gore-tex that we are all familiar with is now called Performance Shell, but is unchanged from the basic design. Gore-tex Paclite offers a lighter interior coating, but is basically the same membrane and PU combination used in Performance Shell.
Most of the common proprietary waterproof breathable membranes from major outdoor apparel manufacturers are some variation of the PTFE bonded to PU. While there are a few other types of waterproof breathable products on the market, PTFE and the EPTFE (eVent) are most common and most available in high end designs and outerwear construction.
eVent, an EPTFE
eVent is an expanded PTFE. It has all of the benefits of a true PTFE without a polyurethane coating, and shows no significant drawbacks. This is accomplished by creating an oil and dirt repellent interior membrane. Since there is no PU layer to limit function, breathability is much higher than any other waterproof breathable, and exponentially higher at low humidity levels. Since it moves moisture equally well when the inside of the fabric is relatively dry as when it is damp (Gore-tex products work best when you are already wet), you stay drier in a wider range of uses and conditions.
While Gore is still the most common name in the waterproof breathable product market, eVent has become immensely popular with virtually everyone who has used it. It is truly a cut above any other product of its kind. As serious outdoor companies like Rab and Integral Designs have begun to use eVent, a number of excellent designs have been made available.
Hard Shell Jacket and Pant Construction
A hard shell jacket for mountain hunting must be light and durable. The “one to one” woven fabric designs of Rab and Westcomb’s high end jackets exemplify this trait, coming in at approximately 1 pound. If you hunt in a wet environment and plant on wearing a shell for a large portion of your trip, durability is very important. For many hunters, a shell may only be used for a few days in a season. An ultralight eVent shell weighing between 10-13 ounces may be a better choice for those users. These lightweight jackets are still durable enough for hard use and will take up less space and weight in your pack.
A hard shell should be sized to fit over your active layers, but not over your insulating belay jacket. It should have a hood with draw cord, and a draw cord at the hem. The number and style of pockets is personal preference. It should be cut to move with you and not feel restrictive.
Recently, some manufacturers have started to produce “hybrid shells” jackets that strategically mate a hard shell fabric and soft shell fabric for optimum comfort, breathability, and weather protection. More on this can be found in the “Hybrid Shells” article.
A hard shell pant should also be very durable, as a pant seems to take the bulk of outerwear abuse. At least ¾ if not full length side zips are very helpful and allow you to put on and remove layers without taking off you boots. The convenience is easily worth the added weight of the zipper. They should be sized to fit over your active pant and under a belay pant.
A Durable Water Repellent (DWR) treatment on the outside of a shell fabric is very important to its performance. A DWR causes water to bead up and roll off of the shell, keeping it from becoming saturated. A saturated shell will not breathe well, as the moisture gradient vital to transport will no longer exist. DWR can be re-applied through washing, using specifically designed products from McNett and Nikwax. It is also important to only wash technical outerwear in detergent designed for that purpose. This will minimize loss of the DWR and will keep the membrane from becoming contaminated.
Hard shells, while a valuable piece of gear, should be used judiciously in drier climates. When the situation calls for a waterproof breathable layer, eVent is by far the best choice, and can be found in a durable lightweight package. By using your hard shell effectively, you can maximize your ability to stay dry and comfortable in difficult weather conditions.