Thermal Insulation: Theory vs. Practice

2017 / January / 29 / Behind the Scenes /

ake a look at the Hanwag website and you’ll see that some of our Winter and Alpin models have temperature details for their thermal insulation. We asked Johann Friedl, footwear designer at Hanwag, to explain what this information tells us…

Hanwag footwear designer Johann Friedl answers my questions:

Take the Abisko for example. According to the Hanwag website, it provides thermal insulation down to minus 45°C. Is this right then?
Well, not exactly. You see people are not machines. These temperature details are provided by the Test and Research Institute Pirmasens (Prüf- und Forschungsinstitut – PFI) according to DIN EN 20877. In general, people feel the cold and are sensitive to temperature in different ways. It depends on a number of factors. For example, have you been on the move the whole day in difficult terrain and are feeling exhausted, or have you just come out of a warm mountain hut after a big meal and are not feeling at all tired? In practice, perceived temperature may differ.

So why measure thermal insulation at all?
Because retailers and customers need a benchmark. Otherwise, they won’t know where to start. Such reference values are important, even if they vary slightly. We measure thermal insulation so that we know exactly how long a boot retains body temperature. These reference values themselves are real and do not vary significantly. They also make it possible to compare one model to another.How is thermal insulation measured according to EN 20877?
Two temperature sensors are placed in the footwear. One on the insock – the flat component similar in shape and size to the insole, which is stuck to the insole and one on the lining at the ankle. The shoe is then filled with steel ball bearings and sealed with a polystyrene cap. It’s then measured at room temperature. Once the results are ready, the shoe is then placed in an airconditioning cabinet. The temperature is then gradually lowered down to minus 40 or sometimes even minus 50 degrees – after the ball bearings were at body temperature. The temperature in the airconditioning cabinet is compared to the temperature of the ball bearings inside the shoe. This allows us to test exactly how long the shoe retains body temperature. It shows us how changes to our insulation improve its performance, or where we need to make further improvements.Does this also take sweaty feet into account? Does moisture not affect insulation?
Yes, of course. Moisture has a big impact on how we perceive temperature. This applies in particular if you are at higher altitude in cold conditions. In this case, you want your feet to remain as dry as possible. The footwear test methods for thermal insulation are not able to simulate sweaty feet. However, the test allows for all scenarios. The sweaty feet factor is taken into account. There are agreed values. Don’t forget though, there are other factors that influence foot climate, for example your choice of socks. Wearing the wrong socks might cause your feet to sweat profusely. This can adversely affect their ability to regulate temperature.