Insulation Guide | What is Insulation?


When it comes to sportswear, kits and outdoor clothing, the word ‘insulation’ is flung around with abandon, but do we really know exactly what it is and what it entails? How does insulation affect clothes’ performance and what sort of property is it? To answer these questions, we need to first look at the terminology and understand insulation from a scientific point of view. Our guide will take you through the ins and outs of clothing insulation to help you grasp the facts behind this nifty process and inform your choices when shopping around for outdoor gear.

The Definition of Insulation

Simply put, clothing insulation is the thermal insulation provided by the clothing. In a purely physical sense, insulation is the process that reduces the transfer of heat energy between two objects of differing temperatures. Under normal conditions, a hot object that comes into contact with a cold object will lose heat to the cold object, as there’s always a natural tendency for thermal equilibrium.

Insulation greatly reduces the effects of this heat transfer, normally for some kind of external benefit. Therefore, the mechanism of insulation works by conserving heat energy within an object and maintaining its temperature at a constant. Insulation does not create heat; instead, it traps pre-existing levels of heat to prevent its loss. Although the main purpose of clothing is to protect the wearer from cold, the fact that insulated clothing also shields from excessive warmth is often overlooked.

In normal circumstances, however, insulated clothing is mostly intended to keep us warm and fend off the biting cold. These are the main channels through which we lose heat in the cold:


With a normal base temperature of 37°C, our skin loses heat to almost everything it touches. That’s why a chair becomes toasty after you’ve sat on it for a while! Our bodies lose heat 25 times quicker in water than in air – this is why it’s crucial to remain dry in the outdoors and especially on mountains.


Air flowing against the skin can also steal heat from the body. This is why many insulated jackets also incorporate a windproof outer layer to stop air from infiltrating the garment and making you cold.


Sweating is the body’s way of maintaining a stable temperature if you’re undertaking intensive exercise (such as walking uphill). Sweating cools the skin if it gets overheated, but if you’re trekking up a mountain and stop at the top, conditions will be colder and the cold sweat will stick to your skin, causing excessive cooling.


Heat generated by the body is also lost to the atmosphere through radiation. During intensive exercise, blood flow increases and occurs closer to the skin – a process known as vasodilation. This causes the body to radiate – and therefore lose – more heat into the surrounding air.

Insulated Clothing

Insulation in clothing works by trapping tiny pockets of warm air close to the skin. Air – a gaseous atmosphere – is quite a poor conductor of heat, and is therefore ideal as an insulating element. The air contained in an insulated clothing item will block heat from escaping from the body, and the more trapped air there is, the higher the insulating factor.


The benefits of insulated clothing are most apparent in the outdoors world. Come rain, wind, snow or hail, insulated garments are the best type of clothing to keep you warm and maintain an optimal core body temperature. Unlike layering techniques, which are meant to allow you more flexibility in the temperature range you wish to operate in at a given time, the advantage of insulated clothing is that it’s perfect for temperature stability – at zero effort.

Due to the numerous air pockets embedded within, insulated garments also have the welcome fringe benefit of padding. If you’re being active on a challenging terrain, the cushioning that insulated garments provide can prove useful for mitigating discomfort or injury from impacts and other interferences.


The breathability of insulated garments, however, is a problem. The more layers of clothes you wear, the thicker they are and the more air trapped in them, the more difficult it is for sweat to evaporate. This presents quite a challenge when the insulated clothing is part of a package designed to see you through a mountaineering expedition or high-intensity skiing trip.

If you’re being really active in very cold weather, your body will generate more heat and apart from evaporation screeching to a halt, most insulated clothing will quickly become overloaded. The air pockets prevent the clothes from breathing, as these properties are in direct opposition. Even if the weather is cold, exercising in proper insulated clothing will start to make you feel uncomfortable after a while.

Modern technology, in all its wonderful complexities, is catching up: each year, garments that are both highly insulating and still retain a degree of built-in breathability are becoming more frequent on the market. Generally speaking, this is achieved by combining high-loft fibres with a low-density knit. The secret behind this arrangement lies in the stability of the fibre. The fibres are durable, and combined with other special forms of fabric, allow minimal flows of air into the garment. This causes convective cooling inside, without cooling you down too much.


Insulated clothing provides varying levels of insulation. Down (natural) and synthetic insulation are measured differently due to their different qualities. ‘Loft’ is the amount of space the down will take up in the garment – directly related to how warm the material will keep you. The more space the down fills, the more air it can trap. Consequently, the higher the loft, the better the insulation and the greater the compressibility for a given weight of down.

The International Down and Feather Bureau (IDFB) regulates the industry and sets universal standards for measuring the insulating factor of down. The fill power is measured by putting a set mass of down into a cylinder, lowering a disc onto the down to compress it and finally, releasing it. The volume of the space the down expands to fill is the fill power. The IDFB’s tests use a Lorch cylinder with 30g of steam-conditioned down. A 700 fill power rating means that 30g of down expand to fill 700 cubic inches.

Here’s a breakdown of the down insulation quality spectrum:

500 fill power – standard quality

600 fill power – good quality

700 fill power – high quality

800 fill power – extremely high quality

900 fill power – very rare

1000 fill power – only ever used in one extremely specialised jacket

Aside from fill power, the amount of down also influences the insulation capacity of the clothing item. Surely a 700 fill power jacket will be significantly warmer than a 500 fill power one with the same weight of down, but a jacket containing 300g of 500 fill power will still feel warmer than one having 30g of 700 fill power.

Down vs Synthetic

When considering the insulating properties of clothes, the major choice you’ll have to make is between down and synthetic filler. Down is the soft undercoating of waterfowl beneath the feathers – a light ‘fluff’ that has an incredibly favourable weight to warmth ratio. Its excellent compressibility allows small pack sizes and is also highly resilient, lofting back instantly after compression. Down garments drape very well and will have you feeling wrapped up lusciously.

On the other hand, synthetic filler is made by fabric manufacturers from artificial material that is designed to mimic the functions of down. The singular benefit of synthetic insulation is that it won’t be marred by moisture, whereas clothing made from down will lose its insulating power if the garment gets wet. Synthetic filler is much easier to maintain than down, and far more cost-effective. They also dry out more quickly than down garments although the extra bulk due to lower compressibility makes them heavier and less comfortable to wear.

For the damp chill of British winters, synthetic insulation is probably the more suitable option, and you can find jackets and trousers that do the insulating job well. Down garments are best left for the dry cold of high Alpine environments that won’t spoil your costly clothes.

Remember that insulation is an important factor to deciphering technical outdoor clothing specifications and you should always keep these all these considerations in mind. Ultimately, you’d be missing out on a whole lot of special features that you can carry around with you if you acquire your next kit piece bearing our pointers.

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