Staying warm is a critical factor in your ability to remain comfortable and enjoy a full day in the mountains regardless of which activity you are doing. However, getting your insulation just right is tricky, as too much of the wrong sort can lead to you being improperly protected for the conditions you are in.

We at the Ski Club are here to ensure you can stay warm in whatever conditions you are venturing out, from the depths of winter to bright sunny days, and to help you make the most of every moment in the mountains.

Long gone are the days of string vests – well, almost – and heavy woollen sweaters to keep you warm. Modern technology has given forth to a range of layers that can effectively regulate the temperature protection provided as well as provide exceptional breathability to vent moisture vapour from evaporating sweat.

Key to this is your layering system; between two and four layers combine to provide complete weather protection, with each layer providing its own function. Most of your warmth stems from your third layer, known as a mid-layer, although all layers can provide some warmth protection. Good examples of this include using a heavier weight merino base layer or fleece – this is often ideal if you are going somewhere cold and are standing still, such as on an expedition to see the northern lights or if you are belaying for a climbing partner, and will be working with other warmth-providing layers.

Modern mid-layers provide warmth using two different materials. In this guide, we explore the major differences between them, and whether one is more suitable for different conditions or activities than the other.

Natural Down

Down feathers from ducks, or more commonly in the outdoor industry, geese, provide exceptional warmth for exceptionally little weight. Often seen as the gold standard for cold weather jackets, there are, however, a number of limitations when using down, and it is important to shop around to ensure you are getting the right down jacket for yourself.

Down feathers provide superb, unbeatable warmth, although the quality of the down is hugely important to ensure you get the right combination of warmth and weight. Good quality down has a high down-to-feather ratio and as a result, a high “fill power” - this is defined as the amount of space, normally in cubic millimetres, 100g of down will occupy. The more space occupied by the material, the more air can be trapped in the down clusters, and the warmer the jacket will be. This therefore means that you can get away with less, higher quality down, and therefore a smaller, lighter jacket, and remain as warm or warmer than in a bigger, heavier jacket using poor quality down.

There are two main drawbacks to using down, which go a long way to defining the uses and roles for down jackets. Firstly, whilst being superbly warm, down lacks the ability to breath and self-regulate as well as synthetic insulation and can cause over-heating. Therefore, you should aim to use thinner, lightweight pieces when active, such as skiing, or heavier pieces for when you are heading somewhere really cold and not moving.

Secondly, down must be kept plump and fluffed, retaining the air pockets that provide its exceptional heating powers. To this end, the face fabrics used are often extremely delicate, and must be managed well to protect them. Whilst normally provided with a DWR (Durable Water Repellent) coating, due to the delicate nature of the fabric this will provide only a small amount of water resistance. Wind resistance will also be minimal as a result. The bigger issue is that when down gets wet, it clumps together, collapsing the air pockets and massively reducing its heat retention abilities – however, you need to trade off protecting the down with not using too heavy a face fabric.

Down jackets should therefore be used either as a mid-layer, where you can place a shell over the top to protect against the wind, rain and snow, or as a heavier outer jacket in extremely cold conditions i.e. where any precipitation falls as snow, which will put less pressure on what water resistance there is. Summit or belay jackets are variants of this latter use – huge down jackets that you throw on over everything when you are not active in extremely cold conditions, such as stopping at the summit of a peak or belaying for a climbing partner. These will be overpowering when worn for activities, such as skiing, and won’t fit below your ski jacket!

Sources of Down

Down is the product of animals. Down used in the outdoor industry is almost always from eastern Europe, where Polish and Hungarian geese produce down of exceptional quality. All down must be plucked from the birds – only eider down is collected without plucking but is improper for outdoor activities. Programmes such as the Responsible Down Standard (RDS) ensure that the birds used in this process are treated humanely and have not been subjected to unnecessary harm, especially ensuring there is no live-plucking and the whole animal is used, such as part of the food industry.

Synthetic Insulation

Synthetic insulation has come a long way since simple Polartech Fleece. Modern insulation is lightweight but powerful, with superb breathability and exceptional self-regulation abilities. Using a single thread of polyester to weave mats of insulation, in a similar principle to loft insulation, creates an effective multi-use product.

Despite not being as warm as down insulation, synthetic insulation is far more versatile, and can retain its performance when wet or when compressed for an extended period of time, such as stuffed in the bottom of a sack.

You can also use a much heavier face fabric over the top, resulting in far better wind and weather protection that with down jackets. This means that many synthetic insulation mid-layers can also double up as a soft shell in appropriate conditions.


Rab are a British brand, based in Sheffield, that have been using down for their entire existence, and are extremely well know for their sleeping bags and down jackets. Their microlight piece has been a keystone of their product line, and of the entire mid-layer market, for many years, balancing exceptional warmth with a light, fitted mid-layer design and a useful Pertex water resistant face fabric.

At the other end of the scale, the Rab Positron is a great example of a summit jacket – a huge amount of high quality, 800 fill power down packs a real punch, designed to be thrown over everything when stopping at high elevations.

Other good examples include the Arc’teryc Cerium range, including the SL and LT mid-layers, and SV which pushes more towards a summit/belay piece than the former two. The Mountain Equipment K7 is similar to the Rab Positron, using 800 fill power down with a 90:10 down-to-feather ratio for ultimate protection for alpinists.

For synthetic, a great place to start is the Arc’teryx Atom, an icon of modern mid-layer design. The LT model is the perfect weight to serve as your go to insulation for a huge range of conditions, and packs brilliant weather protection to act as a soft shell too.

Ortovox have twisted the rules on synthetic insulation, using natural wool fibres instead of polyester to manufacture their insulation, thereby creating a far more sustainable product. Their SWISSWOOL insulation is able to wick sweat away better than traditional synthetic insulation owing to the unique structure of wool fibres.

Finally, heading the other way, The North Face have created a synthetic version of down, known as Thermoball. Whilst lacking some of the warmth associated with natural down, Thermoball still represents exceptional versatility, with a tough face fabric, and exceptionally lightweight insulation.

Got any questions about kit, equipment, or staying warm and dry in the mountains? Get in touch with the Information & Advice team on 0208 410 2009 or at [email protected] to get your questions answered.