Communicating footwear thermal property

Describing a simple method developed by SATRA of expressing thermal properties of footwear.

by Tom Bayes

Measuring the true thermal property for whole footwear is a very complex issue, which can be achieved by using SATRA TM436:2010 – ‘Determination of whole shoe thermal insulation value and cold rating’. However, it can be a challenge to communicate the results to the retailer and customer in a way that is easily understood. SATRA has developed an easier way of expressing these results.

Thermal insulation is measured using the ‘R’ value. This ‘R value’ is the heat loss (in watts) divided by the area (in m²) for every degree (either in kelvin or Celsius) gradient. This number particularly suits the footwear sector and footwear materials, as they are in sheet form and usually in several differing layers. The R value accounts for all the methods of heat transfer. Of course, most of this transfer will be in the form of conductivity but, in footwear, it can also often include convection (air exchange) and radiation. Thus, the combined insulation value of several differing layers – which may use various methods or mechanisms – can be measured to produce a number for the combined system.

This is analogous with building materials, where the total R value for a room or building can be calculated while incorporating elements including internal and external walls, windows, doors and even openings such as fireplaces and vents. With footwear, the total R value for the product will include not just the insulation, but also air exchange such as through purposely included ventilation ports or the leg opening. It will also provide a value that is inclusive of the sole unit, which can have vastly different thermal properties to the upper.

The measured heat loss itself can be divided by the total area of the insulation. This gives the heat loss in watts for the area per degree of difference of temperature gradient. Like a building or a room, it can be assumed that the footwear space enclosed by the insulation contains a heat source (in our case, the foot). Knowing about how much heat energy is emitted by the foot and the temperature difference across the insulation, the total potential heat loss can be calculated.

Coping with the cold

When it is colder outside the footwear than inside it, the heat loss increases as the gradient increases. There are two solutions to this. One is to increase the insulation to reduce the heat loss, and the second solution is to increase the heat output of the foot, which is achieved by increased activity of the wearer. Activity increases heat output of the human body, and less insulation is required for the wearer to remain comfortable. This is the foundation of the SATRA cold rating: a prediction of a comfortable outside temperature based on the measurement of the R value, the estimated activity level and what represents a comfortable in-shoe temperature.

Although we are all individuals and have differing concepts of comfort, there is a happy medium, referred to as a ‘comfort zone’. Humans are sensitive to heat loss. Too much heat loss and we start to ‘feel cold’, although this is not actually getting cold.

In cases where the body temperature does drop, even small fluctuations (single degrees in body temperature) can have severe health consequences. Hypothermia, when the body temperature drops too low, will lead to impaired brain function and judgement, often in a situation where sensible decisions are needed. The subsequent bad decisions made as a result often compound the situation to an inevitable end. The body has a sophisticated warning mechanism – when we touch something that is heat conductive, it feels cold, even if the object is close to body temperature. Humans interpret this as discomfort.

Heat is also a problem

Conversely, being too warm is just as dangerous. An increase in body temperature by as little as two degrees can lead to a seizure and the loss of rationality. When the body needs to lose heat, the main mechanism is perspiration. The body exudes moisture and the subsequent evaporation – the latent heat of evaporation – carries away an enormous amount of heat energy. This allows humans to maintain a body temperature in environments where the actual outside temperature far exceeds our own. In addition, we need to lose heat when engaging in high activity levels.

Consider a ‘thought’ experiment. A group of spectators watching a marathon running event. It is cool, the spectators are wrapped up warm, the runners are in shorts and t-shirts. If we swap the clothing around, the spectators are now grossly uncomfortable dramatically stamping their feet and rubbing their hands to keep warm, while the runners are all suffering from heat exhaustion and soaking wet through excessive perspiration. This is exactly what the SATRA TM436 test method and the cold rating is designed to predict – the correct amount of insulation and thermal comfort for the activity, with metabolic rate being taken into account.

In addition, if perspiration is promoted because of having too much insulation, this is not ultimately in favour of our comfort. Our garments will become damp. As water is a very good conductor of heat and dry air is a very good insulator, when we alter our activity, the moisture will impact our comfort, thus leading to chilling.

The SATRA artificial foot

In order to measure thermal properties of the footwear, we employ SATRA TM436, which is conducted on the SATRA STM 567 Endofoot machine. This incorporates a heated foot placed in an airflow to simulate the level of wind chill factor experienced when walking. The air movement is vital as, without it, a boundary layer of warmer air will build up around the sample and thus the result will be affected.

The temperature of the artificial foot can be controlled extremely accurately and, subsequently, the amount of electrical energy used to maintain the temperature can be measured. The heating mechanism works in a similar way to an immersion heater and so it is very efficient. The amount of electrical energy required to maintain the temperature of the foot represents the loss of heat energy from the foot. The heat loss is restricted by the insulation afforded by the footwear being tested. The surface area of the foot is accurately known, and so the R value for the insulation can therefore be determined.

Energy is measured in joules, and the rate at which the energy is being used (or lost in our case) is measured in watts. A watt is a rate of 1 J per second, analogous with the velocity or ‘speed’ of heat transfer. As the R value increases, this heat transfer process slows down. A good way to illustrate this in a simple way is to imagine a bucket full of water with a hole in it. The water in the bucket represents the joules, which escape though the hole at a set rate representing watts. The rate is determined by the size of the hole, which represents the R value.

The height of the water in the bucket above the hole represents the temperature difference or gradient. It is important to note that the height of the bucket above the ground has no effect, as it is the water level above the hole that generates the gradient. As such, we can also measure the thermal properties of footwear using a more convenient temperature gradient, which does not need to involve extreme temperatures. The R value is the number in which we are most interested.

Having explained the principles of the rigorous science behind SATRA TM436, it cannot have escaped unnoticed that it is rather complicated to explain. Certainly, it does not seem achievable that the science would be readily known by retailers and, even less so, potential consumers. The above discussion is certainly not going to fit on a swing tag and SATRA has recognised that this needs to be addressed.

The FET

FET stands for ‘Footwear Effective Tog’. The ‘tog’ is a measure of insulation and is well documented. In this context, the term ‘tog’ will be a familiar term to consumers who have purchased products such as duvets and sleeping bags, in order to gauge suitability for particular expected environmental temperatures. SATRA is not measuring the tog value. However, we are using the term because of its accepted consumer familiarity and we are scaling the cold rating accordingly for simpler application.

There are many ways of measuring thermal properties, all of which produce different results. ‘Lofted’ products (such as quilts) are not subjected to compression and the thermal advantage of this increases the insulation value. Footwear is quite different from these items of bedding and other garments in that it is close fitting and is often subject to compression. As such, footwear needs to be measured in context.

The insulation of materials will be frequently measured in a manner not suited to use in footwear. Therefore, the values given may not be applicable or achievable in the context of a footwear application. The tog value is an example of this, measured in an entirely different manner out of context with use in a footwear application. SATRA TM436 gives the R value for the whole footwear and then makes a prediction as to what temperature the footwear is suitable for at different levels of activity.

Using SATRA’s FET, the presentation of these results has been dramatically simplified. Essentially, the cold rating is projected in such a way that the result appears as a single number on a scale from 1 to 20. FET takes the science and knowledge behind SATRA TM436 and converts the ‘cold rating’ to a single thermal rating coefficient. As with the cold rating, FET is based around a rating to maintain the internal temperature of the air in the footwear at a comfortable 20°C and represents the footwear’s ability to hold that temperature.

To give an example of how it would be used, heavy work in a freezer environment involves higher activity, and SATRA may advise that such footwear should have a FET rating of ‘10’. The operation of a fork-lift vehicle is far less active and the driver may need footwear with a FET of 15 to maintain a reasonable level of comfort.

SATRA can advise on the correct specification given the activity and ambient temperature of the environment.

Publishing Data

This article was originally published on page 30 of the May 2025 issue of SATRA Bulletin.

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