Mid layers

The role of the mid layer – often referred to as the insulating layer – is to retain the body heat you generate and keep you warm. This works by trapping warm air in thousands of tiny air pockets. A mid layer can be any item of clothing worn over your base layer and under your outer shell, such as a synthetic gilet, micro-fleece hoodie or down insulated jacket. Down jackets provide a superior warmth-to-weight ratio, but they are expensive. Synthetic jackets provide better value and improved insulation when wet, while fleeces are generally heavier and cheaper but deliver far better breathability.

Choosing the right mid layer for you will depend on the type of conditions and activities on your schedule. In the height of winter, thicker and warmer layers – such as a puffy down jacket – are likely to be required.

In milder conditions, a lightweight microfleece with less insulation may suffice. Or, for improved versatility, it might be wise to carry two or more mid layers of varying fabrics and thicknesses.

On top of insulating you, a mid layer also needs to assist with the movement of moisture from your body to the outside of your clothing, in conjunction with the base layer. Lightweight fleeces are generally better at this, because they are naturally permeable. Synthetic and down jackets are so good at retaining warmth that it’s actually easy to overheat, therefore they are perhaps better suited to times when you’re still – in camp or at a summit – rather than when you’re working hard and require better breathability.