Most outdoors people understand what a base layer is and the function it performs in keeping you warm and dry. However, when it comes to a thermal mid layer there is still a lot of confusion about its purpose and what one exactly is.
PURPOSE OF A THERMAL MID LAYER
The primary role of a thermal mid layer is to add extra insulation to your layering system without being overly bulky. It is usually worn over your base layers but some garments, like our Factor 2 mid layer range, can also be worn directly next to the skin.
Its secondary role is to carry on transporting moisture away from your skin via your base layer to the next layer of clothing. If your mid layer has poor wicking properties then you are going to get damp very quickly when working hard. This can lead to rapid heat loss when you slow down or stop.
TYPES OF THERMAL MID LAYERS
Personally i think a lot of companies wrongly label garments as thermal mid layers, such as micro fleece jackets and lightweight down jackets. For a start they are far too baggy to be true mid layers and are often poor at managing moisture. A lot of the time it is a clumsy attempt by a brands marketing team to adhere to the layering system – base layer, mid layer, outer layer – when their range is not complete or when their range is too large.
Our classification of a mid layer is pretty simple. It should be snug fitting, offer good moisture management, and be insulating. Basically it is a thicker version of a base layer. When we were developing our award winning Factor 2 mid layer range, we took these parameters and designed a bespoke fabric around them. Nearly three decades later our Factor 2 is still one of the best thermal mid layers you can buy.
WHEN SHOULD A THERMAL MID LAYER BE WORN?
Deciding if wearing a thermal mid layer is necessary is dependent on the weather conditions, the activity you are intending to perform, and ultimately your own personnel tolerance to the cold. If in doubt, always err on the side of caution, as you can always remove a layer.
For a relatively sedentary activity such as fishing where little heat is generated from movement, you are going to want to wear as much insulation as you can to ensure you trap and retain body heat.
On the other hand, if you are a proficient skier, then you are going to have an aerobic workout even in very cold temperatures, so overheating may be a problem wearing a full layering system. In this situation we would recommend wearing a base layer top and bottom with just a mid layer top to protect your core. If you find you are getting too warm then you can always remove a layer relatively easily.
For extreme cold conditions, such as polar expeditions, then a good base layer and thermal mid layer are a necessity, and a re just the start of your layering system.