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Mountain shoes: differences between waterproof membranes

The waterproofing of mountaineering shoes is one of the most important elements for those who practice winter trekking, mountaineering or trail running.

The technology that ensures waterproofing is often provided by “ingredient” brands (Gore-Tex is the best known, but also Sympatex, OutDry, HDry and others), manufacturers of membranes or waterproofing systems.

But what do we expect from waterproof mountaineering footwear? The most obvious answer is that they should keep our feet dry. But that is not enough.

One requirement that is certainly fundamental is breathability. Today, all branded membranes have a good level of breathability. It can be slightly higher or lower, depending on the purpose of the shoe.

When it comes to waterproofing, however, it is important to note that the technology has evolved a lot in recent years and there are now different waterproofing “philosophies”, each with its pros and cons.

In addition, the effectiveness of the waterproofing is not only dependent on a single component (the membrane), but also on a number of other factors. The main ones are: the outer materials of the upper, the number and position of the seams on the upper, and the design of the shoe. The position of the membrane in the shoe is also important.

Upper: materials and construction

Leather and upper materials are normally not resistant to water penetration, and therefore water-repellent treatments are usually applied to their surface. These treatments make the footwear resistant to light, short wetting, but cannot prevent water penetration over time.

Recently, these treatments have become “special observed” from a sustainability perspective because they often contain environmentally hazardous and persistent PFCs (perfluorocarbons), while more environmentally friendly alternatives are less effective.

In addition, the water-repellent effect of these treatments tends to fade over time and must be restored by the user with special sprays that must be purchased separately.

Another weakness is the holes in the seams that join the upper materials together. These holes provide a direct route for water to enter the footwear. To solve this problem, waxed threads are available to reduce moisture transport through the seam, or liquid sealants or thermo-adhesive tapes are applied directly into the seams. However, this is generally not enough to prevent water from passing through the seam holes, especially in areas of the shoe that are subject to bending, such as the area immediately behind the toe.

Paradoxically, it is the finest rain or dew that has the greatest wetting effect. In fact, the very small size of these raindrops increases their wetting capacity and renders water-repellent treatments ineffective.

The absorption and subsequent penetration of water through the upper is a problem for the user, even if, as we shall see, the presence of a membrane prevents the water from reaching the foot.

In fact, water trapped between the upper and the inner membrane increases the weight of the footwear, reducing thermal insulation and overall comfort.

Design of waterproof mountaineering footwear

In addition to aesthetics, the design of a shoe has a direct impact on its functionality, including waterproofing.

Certain design tricks can improve performance in this respect. For example, as we have seen, seams are an easy way for water to get in, so the designer should try to keep them to a minimum. Or to place them outside the flex lines of the footwear.

There are other important design factors, such as using a gusseted tongue under the lacing to increase the height at which the footwear remains waterproof. This is important when fording streams and rivers.

Membranes and application methods

Waterproof and breathable membranes – that are capable of blocking water but allowing perspiration vapor to pass through – are a very effective system for making waterproof mountaineering footwear.

These membranes are inherently thin and fragile and almost always need to be supported by another layer of material, usually textile. They are therefore commonly available as ‘sandwich’ laminates, where the membrane is protected on both sides. Most membranes now provide an excellent level of waterproofing.

However, there are several methods of applying membranes to footwear.

The “bootie” system

The most common and widespread method of applying waterproof and breathable membranes (used by Gore-Tex and Sympatex, for example) is to combine them with a lining material, which then becomes the side in contact with the foot. In this way, the membrane and lining take the form of a “sock”, cut and assembled with seams that are then sealed with thermo-adhesive tapes. The technical term for this process is “bootie”.

The “bootie”, which acts as a waterproof lining, is then inserted into the upper (while remaining separate from it) and the shoe is completed by fitting the last and applying the sole.

The ‘bootie’ system is popular and used by footwear manufacturers because it is easy to make and offers consistent quality, but it has one drawback. Although the foot remains dry (because it is inside the waterproof lining), water can still penetrate the footwear through the material and seams of the upper, stopping only when it meets the membrane.

site source Gore-Tex

It is therefore essential to minimize water penetration and stagnation between the upper and the waterproof lining, as this makes the boot heavy and the foot cold. If it manages to get through the upper, water can even run down and collect under the foot in the hollows of the sole.

Another disadvantage is that if water gets under the upper and stays there, it takes longer and harder for the footwear to dry. Before the upper can dry, the water must evaporate completely and this can take several days.

The lamination system

A newer alternative to the “bootie” system is a technology where the waterproof, breathable membrane is laminated directly into the upper material. Basically, the membrane is bonded to the inner surface of the upper with microdots when the upper is assembled and sewn. This provides a 3D conformation of the membrane with respect to the shoe.

Site source HDry

This technology was introduced a few years ago by OutDry and then further developed by HDry. Gore-Tex has also recently gone down this route with its “Invisible Fit“, but this is limited to 2D applications, i.e. uppers that can be laid flat, and basically to low, lightweight footwear. In fact, this technology has not yet been widely used because it requires rather complex and expensive machinery, which is why it is mainly used on high-end technical footwear.

The main advantage of this method of membrane application is that it effectively seals the upper and all the seams in a single operation. This eliminates the possibility of water penetration and accumulation between the outer material and the membrane. It also eliminates all the associated disadvantages such as heaviness, loss of insulation and long drying times.

As a result, there is less need to seal seams and treat the outer fabric with water-repellent products. The latter is now an important advantage from a sustainability point of view because, as mentioned above, these products are often based on environmentally harmful fluorinated compounds (PFCs).

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