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. Our range includes everything from simple insulated vests to the warmest down jackets. These are for polar expeditions and alpine mountaineering.

Over the years, classic down has stayed popular with people in the outdoors. But, several brands have made hydrophobic down. The results are impressive. Water resistant down survives much longer in wet conditions and dries far faster than untreated down. This is ideal in our damp UK climate. It can also be more reliable on exotic trips. Synthetic insulation tech has also improved a lot. It now has a better warmth to weight ratio. But, it can be tricky to find the best one for you.

This guide explains the good and bad sides of down and synthetic insulation. It also explains more about why we get cold, how insulation works and how we measure the effectiveness of insulating materials. Choosing the right jacket depends on many factors, such as the fit, features, brand, and looks. Knowing insulating materials is just the first step in choosing the right jacket for your 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 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

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

PrimaLoft, Polartec, and other fabric manufacturers make synthetic insulations. Most outdoor brands have their own 'own-brand' versions.

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

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

Synthetic insulation is good for the damp cold of British winters. Down is better for the dry cold of high alpine places. That's not to say that you should never wear down garments in Britain. Down is still the best at performance, but be aware of its limits. Avoid getting it wet, if you can. Or, you can look at treated hydrophobic down.

Water Resistant Down

Water resistant down jackets, such as Rab or TNF ProDown, fix many problems with normal down. They are great for our damp climate. 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 pre-treated their down with Nikwax Down Proof. This product is already successful and widely used. Patagonia have gone high-tech with their 'Encapsil' process. This is a plasma treatment. It changes the molecular structure of each down fibre to make it water-repelling. It also adds strength. The down fibres can maintain more stable air spaces between each other and consequently loft to an incredible 1000 fill power. Encapsil is silicone-based. Other treatments will be more like the Durable Water Repellant (DWR) on a waterproof jacket.

It's important to remember that insulated jackets are for warmth, not dryness. 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 make you lose warmth fast in the cold.


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, especially if wind can hit your skin, it will quickly cause excessive cooling and make you cold.


The body radiates heat into the atmosphere. A process called vasodilation brings blood close to the skin during aerobic activity. But, cold weather can take away too much heat 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 the skin can constrict. This reduces the flow of warm blood near the surface. It keeps the blood close to the core to preserve heat. Hairs rise to trap warm air next to the skin. These automated responses would be enough to keep us alive on their own in cool temperatures.

Unfortunately, many places we want to explore are too cold. This includes the summit of Everest, the North Pole, and even 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 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 many tiny pockets of air close to the skin. Air is a poor conductor of heat. So, the air trapped in an insulating garment reduces the amount of heat that can escape. The more air trapped, the warmer it will be.

Breathability of Down & Insulated Jackets

The more you wear, the harder it is for sweat to evaporate. Heavily insulated jackets are not meant to be 'breathable'. Moisture can't move easily through many 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. A traditional insulated jacket is classic for belays. You leave it off while climbing (and exerting energy). Then, you put it on at a belay to keep the heat you’ve built up. But, 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 design offers good warmth. It also has 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 tough enough to withstand use and abuse. Open-weave, air-permeable face fabrics combine with them. The air-permeable face fabric allows small amounts of air to enter the jacket and cause cooling. But, it won't cool you too much.

Polartec Alpha is meant to work over both 'stop' and 'go' phases. It keeps you more comfortable over a wider range of conditions. 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 relates to the warmth that the down will provide you, as it can trap more air the more space it fills. Higher loft means better insulation and greater compressibility for a given weight of down. Fill Power represents this.

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 placed 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. Each test gave slightly different results. Both have now been replaced by the International Down and Feather Board (IDFB) Part 10 standard. It is 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 say for sure that 800 fill down will be warmer than 600 fill down of the same weight. This is true no matter the test method. But, as always, 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. Only a small number of birds kept for breeding purposes will only achieve maturity. They collect their down 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 duties very seriously, and work to source down ethically.

Take A Look At Our Other Buying Guides

Waterproof Fabrics

Knowing the main features and benefits of each different waterproof fabric is just the first step in choosing the right garment for your intended activity.

Read More


Layering Systems

A basic layering system has three important functions: to wick sweat away from the skin; to trap body heat; and to protect you from the weather.

Read More


Waterproof Trousers

Waterproof trousers come in a huge array of different styles, each one best suited to different activities in the outdoors.

Read More