the mid layer
The insulating intermediate layer typically comprises a shirt, fleece, pile jacket or similar. If the temperature exceeds 10-15 °C, and there is no rain and only a light breeze, there is no real reason to wear more than underwear and a fleece. In windier conditions, a windproof/breathable fleece would be the more comfortable option. The advantage of a fleece is that it breathes better than a conventional outer shell, which is designed to withstand even the heaviest rain. The fit of your fleece is incredibly important. A soft chunky fleece may feel great when you try it on in the shop, but if it is too thick, it will be too bulky to wear under a jacket. A fleece is undoubtedly the most popular choice for the insulating intermediate layer of clothing, as it dries significantly faster than wool. If you need to be cost-conscious, a fleece is one item you can economise on.