A man and woman sitting over a camping stove wearing insulated jacketsA man and woman sitting over a camping stove wearing insulated jackets

Insulated Clothing Buying Guide

Insulated clothing is a fundamental part of the modern winter layering system. We offer a huge range including everything from simple insulated vests to the warmest down jackets for polar expeditions and alpine mountaineering.

Over the years classic down has remained popular with people in the outdoors while several brands have developed hydrophobic down. The results are impressive; water resistant down survives considerably longer in wet conditions and dries far quicker than untreated down which is ideal in our damp UK climate but can also be more reliable on more exotic excursions. Synthetic insulation technology has also seen huge improvements in creating a more efficient warmth to weight ratio, but it can be tricky trying to find the best one to suit your needs.

This guide explains the pros and cons of down and synthetic insulation for different activities and weather conditions. It also explains more about why we get cold, how insulation works and how we measure the effectiveness of insulating materials. Of course, choosing the right jacket depends on plenty of other factors (such as the fit, features, brand and aesthetics) so a knowledge of insulating materials is just the first step in choosing the right garment for your intended activity.

Down vs. Synthetic Insulation


collection of down jacketcollection of down jacket

Down is a natural material from geese and ducks. It's not the long feathers but the 'fluff' beneath.

The Benefits of Down

  • Fantastic warmth for weight, unbeaten by any synthetic
  • Excellent compressibility allows small pack sizes
  • Highly resilient so lofts back quickly when released from compression
  • Garments drape well and feel luxurious

The Drawbacks of Down

  • Moisture is the enemy; wet down loses all its insulating properties
  • Damp down will take a long time to dry
  • Good quality down is expensive relative to synthetic alternatives


collection of synthetic jacketscollection of synthetic jackets

Synthetic insulations are made by PrimaLoft, Polartec and other fabric manufacturers, while most outdoor brands have their ‘own-brand’ alternatives.

The Benefits of Synthetic

  • These materials maintain good levels of warmth even when wet
  • Synthetics dry out more quickly than down garments
  • They offer great value relative to down clothing
  • Easier to maintain than down

The Drawbacks of Synthetic

  • Synthetics are fundamentally less warm than down
  • They won't compress as small as down
  • Extra bulk also makes garments heavier and less comfortable to wear

Clearly both down and synthetic materials have their place. Synthetic insulation makes sense for the damp cold of British winters, while down is better suited to the dry cold of high alpine environments. That's not to say that you should never wear down garments in Britain! Down is still the best performance material, but it is worth being aware of its limitations and avoid getting it wet if possible – or you can look at treated hydrophobic down.

Water Resistant Down

Water resistant down jackets (for instance Rab or TNF ProDown) answer many of the problems associated with normal down and are an excellent choice for a damp climate like ours. However they are still only resistant to moisture – they're not fully waterproof!

In recent years several outdoor brands have introduced water resistant down. There are various ways of going about this. Rab have partnered with Nikwax and effectively pre-treated their down with Nikwax Down Proof, which is already a successful and widely used after-care product. Patagonia have gone high-tech with their 'Encapsil' process. This is a plasma treatment that modifies the molecular structure of each down fibre to make it hydrophobic, while also adding strength. The down fibres can maintain more stable air spaces between each other and consequently loft to an incredible 1000 fill power. While Encapsil is a silicone-based chemistry, other treatments will be more like the fluorocarbon-based Durable Water Repellant (DWR) treatment on a waterproof jacket.

Whichever one you choose, it's important to remember that insulated jackets are primarily designed to keep you warm, not neccessarily dry. If you are looking for a waterproof jacket that is warm, be sure to check the outer fabric of the jacket is waterproof and the seams sealed.

Why We Get Cold

Conduction, convection, evaporation and radiation all conspire to make you lose warmth very quickly in cold conditions.


Skin loses heat to anything it touches. The body loses heat around 25 times quicker in water than air – hence why staying dry on the mountains is so important.


Air flowing past the skin can pull heat away from the body. For this reason many insulated jackets have a windproof outer to prevent air travelling through the garment.


One of the main ways we stay cool is to sweat. Walking uphill can cause sweating, but if you then stop at the top (and particularly if wind can hit your skin) it will quickly cause excessive cooling and make you cold.


Heat generated by the body is radiated into the atmosphere. A process called vasodilation causes blood to flow close to the skin’s surface during aerobic activity, but cold weather can carry too much heat away when you stop.

How We React to the Cold

The human body has an optimum internal operating temperature of around 37° Celsius. Luckily the body has several ways of maintaining that nice 37° average in cool temperatures. Shivering creates heat as a by-product of muscle movement. Blood vessels near to the skin can constrict to reduce the amount of warm blood flowing close to the surface, keeping it close to the core to preserve heat. Hairs rise to trap warm air next to the skin. These automated responses would be sufficient to keep us alive on their own in cool temperatures.

Unfortunately many of the places we want to explore are just too cold - be it the summit of Everest, the North Pole, or Aviemore high street on a winter’s day! If the body's temperature falls below 35° then hypothermia will set in. At around 30° the situation becomes seriously life-threatening. It would be impossible to endure freezing temperatures without some form of insulated clothing.

How Insulation Works

Insulation doesn't impart new heat – it prevents you from losing the heat you generate.

All insulation works by creating numerous tiny pockets of air close to the skin. Air is a poor conductor of heat, so the air trapped inside an insulating garment reduces the amount of heat that can escape outwards. The more air trapped, the warmer it will be.

Breathability of Down & Insulated Jackets

Broadly speaking, the more you wear, the harder it is for sweat to evaporate. Heavily insulated jackets are not designed to be 'breathable' – moisture (either as a liquid or gas) can't move easily through several layers of fabric and thick filling. Most of the time this isn’t a problem as you just want warmth, not breathability.

What about when you're being active in really cold weather?

When you get moving and create heat, most insulating jackets will quickly become overloaded. They don't breathe so you could get hot and sweaty inside if you're walking uphill, running or climbing. The classic use for a traditional insulated jacket is for belays; you leave it off while climbing (and exerting energy) and then put it on when at a belay to preserve the heat you’ve built up. However, constantly putting on and taking off your jacket throughout the day can be irritating. The ideal is to spend the whole day in one set of clothing, keeping you warm (but not too warm!) all day long.

Polartec think they have the answer with their Alpha synthetic insulation. This is designed to offer good levels of warmth together with enough breathability to keep you moving comfortably on aerobic pursuits in cold weather. If PrimaLoft is a 'static' insulation, Polartec Alpha is 'dynamic'.

polartec alpha logopolartec alpha logo

This technology uses high-loft fibres in a low-density knit. The key lies in fibre stability; they are durable enough to withstand use and abuse while combined with open-weave, even air permeable face fabrics. The air permeable face fabric allows very small amounts of air to enter through the jacket and cause convective cooling, without cooling you down too much.

Hence Polartec Alpha is intended to work over both 'stop' and 'go' phases, keeping you more comfortable over a wider range of conditions – where most traditional synthetics can only cope with the 'stop' phase. It also gets a very respectable 'Clo' score showing that its fundamental warmth levels are comparable to the best of the competition.

Measuring Insulation

Not all down or synthetic materials provide the same levels of insulation.

What is Down Fill Power?

One of the key terms is 'loft' – how much space a set amount of down will take up. This is directly related to how warm the down will keep you, as the more space it fills, the more air it can trap. Higher loft means better insulation and greater compressibility for a given weight of down. This is represented by Fill Power.

insulated jacketinsulated jacket

How Down Fill Power is Measured

To measure down fill power a laboratory puts a set mass of down into a cylinder. A disc is introduced on top to weigh down the sample. The fill rating is the volume of space that the down expands to fill, measured in cubic inches. Historically the USA Cylinder and European Lorch Cylinder tests were both in use, and each gave slightly different results. Both have now been superseded by the International Down and Feather Board (IDFB) Part 10 standard (the 'internationally recommended test standard'). This test uses a Lorch cylinder with 30g of steam-conditioned down. 700 fill power equates to 700 cubic inches per 30g down.

Within the vagaries of testing outlined above, this is roughly what to expect:

  • 500 fill power – standard quality
  • 600 fill power – good quality
  • 700 fill power – high quality
  • 800 fill power – extremely high quality
  • 900 fill power – incredibly rare
  • 1000 fill power – only ever used in one jacket, the Patagonia Encapsil Belay Parka

We can certainly state that a given amount of 800 fill down will be warmer than the same weight of 600 fill down, regardless of test method, but as ever this is only half the story. The amount of down used also makes a crucial difference. A jacket containing 300g of 600 fill power will feel much warmer than one containing just 30g of 800 fill power!

Most down is a by-product of the food industry. Most birds killed for consumption will only live to around 4 months, and their down will be relatively low-quality. Higher fill power down only comes from mature birds. Maturity will only be granted to a small number of birds kept for breeding purposes. Their down is collected by hand during the natural moulting season. The result is that high quality mature down is much more expensive than low-quality down.

Most manufacturers take their responsibilities very seriously and are striving to source down in the most ethical way possible. Mountain Equipment’s Down Codex auditing system is an example of a brand attempting to manage their complex down supply chain in an ethical and sustainable way (www.thedowncodex.co.uk).

Take A Look At Our Other Buying Guides

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