3-layer system for trail running

The thermal layer is often a first layer with long sleeves designed to retain heat while wicking away moisture (e.g., a t-shirt made of merino wool or brushed polyester).

The insulating layer, on the other hand, is more often worn as a second layer (midlayer) and provides stronger insulation, notably through its thickness or material (light fleece, softshell, etc.). It is useful as soon as temperatures drop significantly.

As soon as you go out in exposed terrain, in the mountains or on a long outing, yes. Even if the weather seems mild, a lightweight waterproof jacket compressed at the bottom of the bag can make all the difference in case of a sudden change. It is also a required item in many races.

Here are the most common pitfalls:

• Overdressing at the start: you risk overheating from the very first climb. Start lightly clad, especially if you know you will warm up quickly, as sweating too much can chill you later.
• Mixing incompatible fabrics: a first layer made of cotton, for example, will hold sweat and chill you.
• Not taking off your waterproof jacket soon enough: even if breathable, a waterproof jacket partly blocks heat from escaping. If the weather warms up or the effort intensifies, you can quickly end up soaked with your own sweat.
• Wearing a mid-layer that is too warm for a hard effort: you may have to remove it within the first few minutes.

No. The system is a logical framework, not an encouragement to overconsumption. If you are a beginner, start with:

• a first breathable layer of good quality,
• a good waterproof jacket that you can keep for a long time.

You will then add on according to your needs, your practice, and the season.