Cold rating of the Everest boot

What modern testing techniques indicate about the successful design and production of the 1953 Everest boot.

by Maria Agueda Sanchez

Mankind’s history is all about facing up to a challenge – about pushing the boundaries to go a step further and explore the unknown. Some 60 years ago, two men stood on top of the world for the very first time, stretching the limits of human capabilities. It was 11:30am on Friday 29th May 1953 when New Zealander Edmund Hillary and Sherpa Tenzing Norgay made their way to the summit of Mount Everest – the highest mountain on Earth, which stands 29,029 feet (8,848m) above sea level.

This great feat was only possible with the correct equipment. According to members of the expedition, footwear was second only to oxygen equipment in importance. Lack of oxygen and frostbite damage to feet were the obstacles that prevented previous expeditions from succeeding and needed to be tackled. To address the latter problem, scientists from the British Everest Expedition team worked with SATRA to design, construct and deliver the high altitude boots that would later be worn at the top of the world.

Ensuring footwear is fit for purpose

It is obviously important that the design of any type of footwear is in context with its intended use. Different applications call for different materials and designs and, as such, a shoemaker needs to know the conditions to which the product will be exposed. For example, materials used in a running shoe intended to be used on a sports track will differ from those selected for footwear conceived for trail running in the mountains. The trail running shoe will impact on a harder and much rougher surface on every strike, so it will need a soling material that is able to provide a higher degree of support and ground insulation.

Making sure that footwear is ‘fit for purpose’ becomes even more critical when the wearer is going to be exposed to conditions that pose a real risk to human wellbeing – such as the environment to be encountered in the Himalayas. In May 1953, temperatures in the region of -20°C to -30°C and 50mph winds were expected during the ascent. The combination of these two factors gave a resultant thermal sensation, (often referred to as the ‘wind chill index’) as low as -50°C. Very high thermal insulation and lightweight footwear was essential. The team would have to carry the boots with the rest of the supplies many miles to their base camp. Essentially, the boots were disposable items designed for only at most a few weeks’ wear for the final assault to the summit, and only at an altitude above 23,000 feet.

The boots, weighing just 1.9kg per pair, were far lighter than any other mountaineering footwear available at the time. Although liquid water was not likely to be present at such high altitudes, some degree of water resistance was also important. The glace kid upper leather was treated with a silicone compound to increase its water resistance. As additional protection, an outer cover made from a polyurethane-coated fabric was also incorporated.

Protection against cold or thermal insulation in footwear is primarily achieved by trapping and keeping dry air, which is known to be a good insulator. Low density materials that contain space for air (such as nonwoven textiles) are most suitable for this purpose. A 25mm thick nonwoven web of kapok fibres (cotton-like fluff obtained from the seed pods of the tree Ceiba Pentandra) was chosen to procure the thermal insulating properties to the Everest boot. However, selecting the correct materials does not guarantee that the desired overall thermal performance will be achieved. This is because such performance is also influenced by other contributing factors which need to be carefully considered. If a material becomes compressed, it will reduce its volume and thus less of it will be available to trap air. This could potentially jeopardise the insulating properties of the material, hence the importance of a good fitting and lasting process in the shoemaking.

Likewise, the ability to oppose heat loss will be affected if the insulating material gets damp, either from water ingress or perspiration. The reason for this is that water is a good heat conductor, and so is very efficient at transferring away heat. This means that damp materials will feel cold and, more importantly, they will make wearers vulnerable to frostbite in cold conditions. It is clear from SATRA’s archives and the remaining boots in our possession that the designers (and shoemakers) took great care to ensure the insulation potential of the kapok layer was not unduly compromised, either in shoemaking or while the boots were being worn. Compression of the layer of kapok in shoemaking had to be minimised. This was achieved by ensuring that the patterns provided sufficient allowance for thickness of the kapok and by careful control in the hand-lasting process.

Providing a means of keeping the layer dry in wear was critical. The designers realised that the key threat would come from within the footwear rather than from outside. This is because at the temperatures at which the footwear was to be worn, the climbers’ strenuous activity would inevitably lead to the generation of foot perspiration. A barrier in the upper between the foot and the layer of kapok was needed, and this was provided by a rubberised fabric, referred to as a ‘vapour barrier’. It was recognised that the wearer’s hose may have become wet as a result but, in this very particular set of circumstances, this was not considered important provided the wearer’s feet were warm.

Assessing the whole shoe

As previously mentioned, there are many factors that come into play when it comes to delivering an expected performance level. Thus, it is important to test not only the thermal insulation properties of the materials, but also of the whole shoe. Testing the insulating properties of whole footwear can be done by subjecting humans to a variety of activities in cold temperatures. This is carried out in a climatic chamber, which replicates the predicted conditions that will be faced in wear. The Everest boots were successfully tested in this way in the 1950s as a means to assess their suitability for cold weather. However, this method poses some disadvantages. The main drawbacks are the high cost associated with a test of this nature, and its reliability on human subjects.

Familiarity with the type of product under test is important in order to give a well-founded opinion on the performance of the shoe in subject trials. For instance, when testing a basketball shoe, ideally only those acquainted with this type of footwear would provide feedback, as their comments are based on previous experience and they know what to expect from the product. This represents an ideal scenario, yet it is difficult to achieve in practice. In order to eliminate such subjectivity from the test, SATRA later developed an alternative test method to assess the insulating properties of whole footwear.

Testing with SATRA TM436

SATRA TM436:2010 – ‘Determination of whole shoe thermal insulation value and cold rating’ is a controlled laboratory test based on the principle of heat transfer. A foot form is heated and maintained at 38°C throughout the test, which is carried out at a 23°C ambient temperature. Therefore, there is a difference in temperature between the foot and the surrounding environment – the latter being colder – just like when are wearer is exposed to cold environments. Heat behaves in such a way that it flows from high temperature regions to low temperatures regions. The rate at which this occurs depends, among other factors, on the temperature difference between them. The more marked this difference in temperature, the higher the rate at which heat will be transferred.

The shoe being tested is fitted onto the foot form and the energy required to maintain the temperature of the artificial foot is measured. This input of energy relates to the thermal insulating properties of the shoe, defined by its ‘R value’. The R value is a measure of resistance to heat flow through a given thickness of material. So the higher the R value, the more thermal resistance the material has and, therefore, the better its insulating properties. The higher the thermal insulation value is, the less energy the system needs and the more efficient the shoe is at offering resistance to heat flow. In contrast, a low R value indicates a higher input of energy into the system.

Does a high R value mean a better performance of a product? Not necessarily, as it will depend on the uses for which the footwear was conceived. A product with a low thermal insulation value might be more suitable for a certain application than a product with a high R value. The outside temperature and the level of activity will define the amount of thermal insulation to incorporate to the product. A visual example is depicted in a football pitch in wintertime, where two different activity level scenarios are found – very low for the standing crowd and high for the players. Consequently, the amount of thermal insulation required to feel comfortable will differ in each case and, as such, the crowd will wear clothes that provide higher thermal insulation than the ones worn by the players.

The concept of different activity levels gives SATRA TM436 a much broader scope. The test not only provides the thermal insulation value of the whole shoe, but also puts it into context by predicting the ambient temperature at which one should feel comfortable – called the ‘cold rating’. Three levels of activity are defined – ‘very low’, ‘low/medium’ and ‘high’ – each of these giving the outside temperature that would make the wearer feel comfortable. The way ‘comfort’ is perceived will vary from person to person. This ‘cold rating’ is thus intended to provide a comfort temperature guide and not an absolute minimum temperature to which the wearer can be exposed. Defining the ‘correct’ level of insulation in accordance with the intended use becomes vital in order to offer and maintain a thermal comfort that avoids stress to the body. As pointed out above, better insulation does not necessarily translate to better performance, and a very well insulating shoe might cause as much discomfort as one that is poor at insulating.

A shoe with very good insulating properties might feel warm on the wearer’s feet if it is not being used in an application for which it was designed. In this case, the feet will perspire in an attempt to lose heat, causing an excess of moisture in the shoe. As previously explained, dampness feels cold and might lead to a sense of discomfort in the wearer. As a result, the product would be having an opposite effect and behave contrary to that for which it was intended.

SATRA TM436 is a modern test and was not available at the time that the Everest boot was developed in the 1950s. The Everest boot needed to withstand temperatures down to -50°C and overcome the frostbite problem common to all previous expeditions.

Modern evaluation of a 1953 Everest boot

It is not always possible to subject such an historic artefact to modern tests, to see how today’s results compare to the figures obtained in the 1950s. Thankfully, among all the sketches, photographs and other documents related to the design of the Everest boot, there was an unused example in a reasonably good condition that had been brought all the way back to England. The opportunity to test it under modern 21st century non-destructive techniques was not to be missed. The boot was assessed in line with the SATRA TM436 test method and was cold rated. The results confirmed that at the highly demanding activity levels faced by the team that climbed Mount Everest, the ambient temperature at which they would feel thermally comfortable was around -50°C – just as expected (figure 1). However, an even more remarkable fact corroborated the idea behind cold rating, as stated in a letter from one of the members of the expedition, who had to wear the SATRA boots for much longer than expected due to the early failure of his low-altitude boots. He remarked that, because SATRA’s Everest boots were designed only to be worn in the freezing temperatures in altitudes above 23,000 feet, they were ‘far too hot at the low altitudes’.

In a letter to SATRA dated 22nd October 1954, Sir Edmund Hillary commended the SATRA boot, explaining that it was ‘a great success’, and thanking SATRA for generously donating the footwear to the expedition.

Climbing to the highest point on the planet – an attainment that wrote a new page in the history books – was the sum of multiple contributing factors. SATRA designed and made footwear that was fit for purpose in that it both took the climbers to the top, and it also protected them against extreme cold temperatures. These boots prevented the 1953 climbers from suffering frostbite (the very first Everest team to avoid it), and paved the way for the development of modern high performance footwear.

Publishing Data

This article was originally published on page 10 of the October 2015 issue of SATRA Bulletin.

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