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 mid layer, essential for winter mountain activities. Photo: Rab Equipment

The Outdoor Layering System

Any mountaineer or person who carries out outdoor activities in unfavorable conditions needs protection from the elements (low temperatures, wind, rain, snow.)

That is to say: they need to be warm and dry. And the latter is not only achieved by preventing rain, snow, etc. from getting in, but also by ensuring that our sweat get out and does not keep us soaked.

Looking for a solution to this problem, the three-layer system was created:

  • A third outer layer that is waterproof (to prevent rain, wind and snow from passing inside) and breathable (so that moisture caused by sweat escapes to the outside).
  • A second mid layer whose mission is to provide warmth
  • And a first base layer against to the skin whose mission is to wick moisture away from the body, and in some cases, cooperate with heat retention with the second layer.

The Importance of the Mid Layer

Since its main mission in the system is to retain valuable body heat, the mid layer is essential in winter and cold conditions.

The creation of the layer system was due to the invention of polar fiber. It is no coincidence: finding a very light fabric, with superior heat capacity, quick drying, low volume and good breathability, revolutionized outdoor clothing.

Until then, either warm wool inner layers (sweaters) were worn, which - in some cases - could be very bulky and heavy, or the outer layer had thick padding. This last option was especially not very versatile.

After their invention, for several decades, polar fleeces were the kings of the mid layer. But this, as we will see, has changed.


1. Polar Fleece

This material, generally made of PET, was developed by Malden Mills (now Polartec) as a substitute for wool. Holders of the patent rights decided not to exercise them, so it can be found from multiple manufacturers and qualities.

However, outside of daily use, technical mountain equipment brands tend to rely almost entirely on Polartec fiber for their polar garments, both because of its enormous quality and because it is manufactured from recycled cans and plastic containers, and It is considered a material of great ecological benefit.

1.1 Classic Fleeces

It is normally manufactured in different weights: 100, 200, 300, 400 (100 grams, 200 grams, etc.). The higher the number, the thicket and more heat capacity.

Its virtues are innumerable, with an exceptional heat/weight ratio.

They have been the kings of the mid layers for several decades; However, its more technical use has been very restricted due to the appearance of new materials that have allowed new combinations of the layering system. These new combinations and garments, mainly with insulation, are more agile, and the truth is that today it is difficult to see a classic thick fleece in technical mountain activity.

The North Face 100 Glacier FZ for women
The North Face 100 Glacier FZ for women
Due to their comfort, they continue to be used massively for down time, hiking, trekking, everyday life, etc., but in mountaineering, mountaineering, etc., this other type of garment is increasingly preferred, which we will see later.

With two exceptions: the thinner fleeces already seen (also called microfleeces, as a low-volume warm inner layer, extremely light and agile), and the Powerstrech.

1.2 Elastic fleeces, Powerstrech

They are elastic fleece. Very comfortable and agile. The thinnest ones were born as a thick base layer, and are very effective.

The most classic material is Polartec Powerstretch, highlighting in recent times the PowerGrid or Power Dry from Polartec, although many brands have their own, of similar quality and performance.

Rab Nexus Jacket, thin and elastic fleece with hood, very technical, model for men
Rab Nexus Jacket, thin and elastic fleece with hood, very technical, men's model

<Polar fleece for men in Barrabes

2. Insulated mid layer: lightweight jackets with down or synthetic insulation

And what has caused the polar fleece to be displaced in technical and athletic activities?

Mainly, the appearance of modern lightweight jackets with synthetic or down insulation, whose use is widely spread at the moment. They are very effective and, above all, very versatile.

Their design allows them to be used under the outer layer, but they are also used as an outer heating jacket, without an outer layer, since they have excellent properties: lightweight, very warm, with some protection from wind and precipitation, and very breathable.

They are also excellent as an auxiliary element: their compressed volume inside their bag is minimal, a good advantage when carrying them in a backpack, for a climber or mountaineer to carry the bag hanging on their harness to use in meetings, etc.

One way of using this type of garment that is increasingly used is the so-called "belay jacket" or "Chamonix style". Basically it consists of these garments being carried in the backpack and, to avoid getting cold if we need more warmth at a stop while belaying, in very cold times, they are placed on top of everything, even the waterproof and breathable third layer if we are wearing it. It is a very effective and quick system to regulate our thermal needs during activity.

In reality, there are some whose design is more suitable for an outer layer (although it can be used as a mid layer), and others specially designed to go under other clothing (although they work well as an outer layer).

Patagonia Ms Nano Puff Jacket for men
Patagonia Ms Nano Puff Jacket for men
They can be filled with down or synthetic insulation. Each one has its advantages and disadvantages; To know what they are, and which one best suits your needs, we recommend you read the article ”Down-Insulated Clothing vs. Synthetic-Insulated Clothing: Differences, Pros and Cons”. It is very interesting, and will also help you understand this type of clothing in depth, and the 2.5 system.
Rab Microlight Alpine Jacket W, third/second layer filled with down for women
Rab Microlight Alpine Jacket W, third/second layer filled with down for women
The advantages of down or synthetic insulated mid- or outer- layer jackets

  • More versatile.
  • Lighter and more ductile than a thick lining.
  • At the same heat capacity, its volume is smaller (although it depends on the quality of the fiber or down)
  • Very packable, little volume in the bag.


  • Lower resistance. A fleece is almost indestructible. Insulated jackets, although they may have durable exteriors, are more prone to tears, etc.
  • Higher price than polar fleeces
Arc'teryx Atom LT Hoody, men's model And why has the arrival of these jackets modified the combinations of the layering system, and caused the use of traditional thick fleeces to decrease?

Mainly because, at the moment, in situations of intense cold, we could practically talk about “double second layer”, or 2.5 layer system.

The 2.5-layer system

Consists in:

  • A technical base layer (1st layer)
  • With a microfleece or some garment with PowerGrid or similar.
  • And fiber-filled jacket.

It is very versatile. In snow or rain situations, the system is completed with a third waterproof or breathable layer, in intense cold situations the microfleece or similar can be thicker, and in less cold situations we can only wear the jacket with the 1st layer.

That is to say: we change the thickness and volume of our classic second layer for two garments that, combined, occupy the same volume, but give much more agility and more possibilities of use.

As we say, among this type of garments we can find some thicker ones, and others very athletic.

Kari Traa Tirill Jacket W, stretch-filled hybrid technical jacket for women
Kari Traa Tirill Jacket W, hybrid stretch-padded technical jacket for women
What advantages do these jackets and this system have?

  • They combine the great ductility, agility and heat capacity that we have already mentioned with good wind resistance
  • In this way, despite being the warming layer and being complemented when necessary with a third layer on top, it allows us to use the padded jacket as an outer layer, in something that we could call “two and a half layers”: 1st layer, and double heat layer, which also provides protection against wind, but not heavy rain (they usually have greater or lesser resistance to water).
  • It is a system similar to that of softshells - which we will talk about later - but with greater comfort.

    As we say, many of these jackets have a DWR treatment, which although it cannot withstand heavy rain, it withstands humidity, etc.

    Their main drawback could be found in their durability: if we use them as a mid or outer layer, they are perfect for aerobic sports, such as ski touring, etc., but they are less resistant to friction than softshells, which is a handicap for mountaineering.

    3. Softshells

    Although somewhat replaced by insulated jackets, softshells still have their audience and their use.

    They are mid and outer layers that are tear-resistant, windproof, and have good capacity to withstand rain and snow. A mid layer that is resistant and elastic, and with medium/high protection against humidity. It avoided having to use the outer layer in not very unfavorable humid situations, a less resistant and breathable layer.

    For example, in light rain with little cold, an outer shell layer increases our sweating and discomfort. And in light rain, or slightly wet snow, the high protection that the outer layer offers is not needed, at the cost of some breathability.

    They are really between the mid and outer layer. They continue to be used when comfort and some protection are needed, but an outer layer can penalize us due to its lower breathability and lower durability.

    Some closer to the mid layer, others closer to the outer. In situations of snow and non-liquid precipitation, they replace it without problems (many ski jackets are softshells). They are all windproof, and less breathable than the previous types of mid layers, but more than a shell.

    One of its great advantages is its great durability compared to other types of garments, which is why they are usually used for mountaineering, in situations where abrasion on rock is common.

    And all this we have said is referring to winter and the harshest conditions, but where softshells are unbeatable is in three-season situations. These are thinner garments for when we need some protection against the elements, and resistance.

    These garments are widely used for rock climbing, but they are also essential for mountains, trekking, three-season hiking, when we need something warm that does not overheat us. In addition, being water resistant, they will withstand light rain for a period of time.

    To understand it quickly: thin softshells are the mid-season cardigan of the mountains.

    Mountain Equipment Arrow Jacket
    Mountain Equipment Arrow Jacket

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