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How to choose a mid layer

The mid layer, whether it's a polar fleece or an insulated jacket, is fundamental to the outdoor layering system. It provides warmth. We'll explain the different types and which to choose for each occasion.

The importance of the mid-layer


Within the layering system the main purpose of the mid layer is to provide warmth. This means it is an essential layer in the Winter and cold weather.

The creation of the layer system coincides with the invention of fleece fabric. It’s not a coincidence: to find a warm lightweight fabric that is quick drying, breathable and isn’t bulky revolutionised mountain clothing and inspired this system. Up to that point, wool mid layers were used (jumpers) which were often heavy and bulky, or the outer layer was a thick coat with lining.



This material, generally made from PET, was developed in Malden Mills (now Polartec) as a substitute for wool. The owners decided not to patent the material, and as a result many manufacturers and qualities can be found. However, reputable outdoor brands for technical clothing tend to use Polartec to manufacture their fleeces. This is due to both their high quality and also because they produce it using recycled materials and is therefore considered an ecological product.

Classic fleeces

Normally they are made from different grammage: 100, 200, 300, or 400. The higher the number, the thicker and warmer the fabric is, but also less soft and flexible.

Columbia Fast Trek

They have been the kings of mid-layers for several decades; however, the higher grammages have been used less in recent times due to the appearance of new materials that have allowed new combinations in the layer system (except for micro-fleeces -100- which are still commonly used). These new combinations and garments are more versatile and give morefreedom of movement.

They are still used in activities such as hiking and urban use, but in mountaineering and other technical activities other types of garments are now preferred.

Micro-fleeces and new materials

The 100 grade fleeces, which are very thin, also known as micro-fleeces, are still used in technical activities. Lighter, more flexible and highly breathable. They allow freedom of movement and the weight/warmth ratio is very high.

In recent times they are accompanied by other materials which, although originally conceived as thick top layers (like the classic Powerstretch), were soon adopted to manufacture thin mid layers. As they are warm, quick drying, wick away moisture, and above all allow freedom of movement. We can name PowerGrid or PowerDry by Polartec inside this category.

Arc'Teryx Delta LT

So why has this change taken place? What has provoked that in technical or athletic activities, thick fleeces have been displaced by new materials that in reality provide less warmth.

Basically, the appearance of modern lightweight down, and fibre filled jackets.

Montura Stretch Pile Mix


At the moment, multi purpose lightweight jackets with synthetic or down fillings are in common use. Their design allows them to be used under a top layer or can be used as an external warm jacket, without a third layer. They are also excellent as an auxiliary pieceas their packed volume inside a pack is minimal. This is a huge advantage; they can easily be carried in a backpack or a mountaineer/climber can keep it hung from their harness and use it when belaying, when they aren’t moving. The advantage they have over fleeces, apart from being compact is that they are lighter and more supple while providing the same warmth capacity and their volume is much lower. All of this make them a better alternative and more versatile than a fleece.

In reality, there are some whose design is more adequate as a top layer (although they can be used as a mid layer), and others that are designed to be used under other garments.

The North Face Thermoball Jacket. Fibre that imitates down

So why has the arrival of these jackets modified the combinations used in the layer system, decreasing the use of traditional thick fleeces? This is mainly because with the change, in cold situations we can practically talk about a double mid layer. We can wear a thermal t-shirt (base layer), with a micro-fleece or some kind of garment such as a PowerGrid, and a fibre jacket. In situations with rain or snow it can be complemented with a waterproof breathable top layer, and in conditions of extreme cold the micro-fleece or similar can be used too.

In other words: we change the thickness and volume of our classic second layer for two garments that, combined, occupy the same volume, but are much more versatile in their possible uses.

What advantages do these jackets have in this system?

This type of jacket is extremely supple, provides freedom of movement and warmth capacity and also provide protection from the wind. Thanks to this, even though it is a layer to provide warmth and is complemented with a top layer, it allows us to use an insulated as an external layer. This could be denominated as “two and a half layer”: base layer, two warm layers that provide protection from the wind but not the rain.

Arc'Teryx Atom LT Hoody

The system is similar to softshells, but much more comfortable.

What are the drawbacks to this system?

  • If we use them in the traditional way, substituting the second layer, breathability is lost: they are less breathable than a traditional fleece. Although they manage the moisture sent from the base layer, fleeces perform better in this sense. That is why they are still used, above all in less cold situations or low intensity activities.
  • If we use 2nd and 3rd layers, they are perfect for aerobic activities such as ski touring etc, but they are less resistant to abrasion than softshells, which is a handicap for mountaineering.

Ventrix system by The North Face

One of the problems in using a fibre or down insulated jacket is that they are not as breathable and do not manage moisture as well as classic fleeces. To resolve this problem, The North Face has developed the Ventrix system.

How does it work? It allows heat autoregulation depending on whether we are moving or not.

It is a jacket with fibre filling, similar to modern athletic designs which can be used as a mid or top layer. But inside there is a fundamental difference: all the internal fibre is full of micro perforations, similar to knife slits.

When we are not moving, the perforations remain closed, just like any other jacket.

However when we move and the fibre is stretched and contracted, it opens the micro perforations allowing heat and humidity to escape. This regulates the internal temperature.


The original mid layers had good warmth capacity but they did not protect us from the wind and they were not durable to abrasion from rocks for example.

Furthermore, these layers didn’t have resistance to moisture, meaning that as soon as it started to rain or snow we had to put on a shell.

In many occasions (for example, drizzle in mild conditions), the shell increased sweat and discomfort. In situations like light rain or dry snow, the high protection of a hard shell was unnecessary, with all the disadvantages of lack of freedom of movement and breathability that they bring.

First wind resistant garments were created, but they were not the best solution due to their rigidity and lack of breathability.

So investigations began to create materials that allowed a comfortable durable and elastic mid layer. It had to provide moisture wicking properties, that could be used along with a base layer -without a shell- in circumstances that at that time required the use of a hard shell (snow, light rain, wind, situations with activity where there is abrasion such as mountaineering or climbing).

Softshells were born. It was an excellent addition to the system, and is still in common use. Mountaineers also use it as a 2.5 layer system, with a base layer, light second layer, and a softshell.

There are different designs. Some of them imitate mid-layers, others are more like shells. These in reality, in Winter scenarios with snow, function like a top layer. In the two photos in this section you can see the difference: the same Arc’teryx softshell in “mid-layer” and “shell” versions.

Outdoor Research Ferrosi

A commonly used combination is a softshell with an ultralight shell jacket, which can be easily carried in the backpack and used when necessary.

Explore the range of mid-layers in Barrabes

In Barrabes we have a wide range of mid-layers in various styles. Be sure to choose the right one for you in terms of weight and packability, warmth capacity, breathability, protection from the wind and rain and resistance to abrasion.

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