What is a Mid Layer? A Buyer's Guide

A flexible mid-layer can be one of the most useful pieces of kit in your expedition armoury. It should regulate your temperature, keeping you warm during periods of low activity but breathable enough to prevent you overheating as you ramp up the exercise. Additional qualities like wind and water resistance will help widen a reliable mid-layer to a dependable, year-round layering option.

In the following guide we will look at the different types of mid layers available and the important role these play in your layering system.

A mid layer is a garment typically worn as a second layer over a base layer for additional insulation in low temperatures. The term “mid layer” can actually encompass a range of quite different garments from fleeces, to soft shell or insulated jackets.

Each garment type will vary in terms of function, be that the level of insulation, breathability, or even mobility, and it is important to consider the conditions and activity you will be partaking in before deciding which type of mid layer is best for you.

Remember, the ultimate goal of a good mid layer is to keep you well insulated without causing you to overheat during periods of high activity. It’s also worth noting that, depending on the duration and nature of your outdoor activity, your mid layer is the one you are most likely to be regularly removing and putting back on, so there are stowability considerations to be had.

FLEECES

Fleece fabric technology has come a long way in recent years, with today’s fleeces offering improved warmth, weight, durability and weatherproofing on their forebears.

Available in varying degrees of thickness, fleeces have historically been seen as the go-to for low temperature, low activity pursuits. However, that’s where ThruDark’s range of high performance fleeces stands apart from the rest. Utilising the latest in synthetic fleece fabric innovation from Polartec®, these are designed for high activity pursuits in the harshest of environments.

Soft shell jackets are stretchy, mobile layers often constructed from a fleece fabric or brushed polyester. Their emphasis is on breathability and they are designed to offer a limited level of water and wind resistance. Typically speaking, you would opt for a soft shell layer instead of a fleece for more active pursuits that require mobility and durability such as climbing.

That’s right - you can wear an insulated jacket as a mid layer. Insulated jackets do not work by generating heat, but by trapping the heat generated by your body. Such a jacket will typically comprise of a water-resistant and/or rip-stop outer layer with insulation then stuffed within box compartments or “baffles” between the outer and inner layers. This insulation is usually made up of either “down” or “synthetic” fill. For further information on insulated jackets - check out our relevant guides below. 

• Insulated Jacket Buying Guide
• Down Insulation vs Synthetic Insulation: Which Is Best?

For the purposes of this guide, an insulated jacket may serve as a mid layer when worn beneath a waterproof hard shell. Layering beneath a waterproof hard shell will bolster the insulation provided by the jacket with a final weatherproof barrier, protecting you from wind and rain. This is just about the most robust layering system you can employ on a trip and is best exercised in situations of low activity and extreme, adverse environmental conditions.

As mentioned above, a mid layer is designed to enhance insulation and enable movement — therefore comfort is key, meaning fitment is crucial. Too loose and the mid layer will not trap heat as it should, too tight and this will reduce your range of movement.

Something we have also touched on is weight and stowability. The weight of a mid layer will range from bulkier and heavier fleeces and insulated jackets to lighter soft shell jackets. When choosing which is best for you, firstly consider the nature of activity and environmental conditions, but also the duration and nature of your expedition. Think about how much you need to carry and whether you can manage with the additional weight of a mid layer as and when this inevitably requires storing in your luggage.

And lastly, exactly why is breathability important? Profuse sweating is generally uncomfortable it can have an adverse effect on garment wear and tear. However, most importantly, sweating too greatly in a cold environment can cause your body temperature to drop too harshly, which can lead to complications such as hypothermia. That’s why adequate moisture management is vital for a number of reasons.