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The GO Outdoors Guide To Insulation

Insulation is included in any garment which you need to keep you warm. Most typically seen in padded jackets and gilets, insulation is  also used in footwear and clothing, such as lined walking trousers, or wellington boots and ski shoes, as well as sleeping bags. 

Thermal insulation is designed to keep you warm by trapping a layer of warm air around the body. In terms of water vapour control, it needs to allow body moisture to pass quickly from base layer to outer (ie. it needs to be breathable). 

The most popular types of insulation are either fleece, or down.

There is always a fine balancing act between the levels of breathability you can achieve with an insulated garment and the amount of warmth you need.

For walking the dog in cold conditions you may not need much breathability, because your heart rate will be lower. However for an Alpine expedition, you need breathability so you don't overheat, which in turn can cause sweat to be produced, which if not wicked away, causes dampness and discomfort.    

Breathability allows hot air to circulate, simultaneously allowing sweat to be released to the top layers of the fibres and absorbed by the environment. 

Different conditions require different levels of insulation, and different levels of breathability. For a sleeping bag, where you will be stationary with no exertion to produce your own body heat, you will need the maximum amount of insulation, and breathability will be a lesser concern.

However, for mid layers such as fleeces worn under or over varying layers, you will need to be both insulated, but also able to release sweat. 

To read in more depth about what breathability is and how it affects you, see our Guide To Breathability with the Expert Advice Section. 

Insulation- What Level To Choose

Internal temperature is affected by many factors, from your own levels of natural insulation (such as your body fat percentage), the air’s moisture levels, altitude, your own metabolism, and even your digestion and energy levels. Here are some facts on internal insulation and how it varies. 

  • Older people tend to feel the cold more than young people. Females have a different sense of the cold to males, usually less body fat equates to feeling colder. Infrequent campers may be more susceptible to the cold compared to hardened mountaineers.
  • Poor nourishment via food or hydration can lead to poor homeostasis management (the ability of your body to react to environmental changes and keep you at a healthy temperature). 
  • Larger-built people will feel the cold less than those with a slender build.


Natural or Synthetic Insulation- Which is best?

Synthetic Insulation

  • Performs well when wet 
  • Good value for money 
  • Less care required in cleaning and storage
  • Poor weight to warmth ratio when compared to down insulation
  • Bulkier than natural, high quality down


  • Extremely warm for its weight 
  • Extremely compressible 
  • Performs badly when exposed to dampness/wetness
  • More expensive than synthetic insulation
  • Care required when cleaning and storing for optimum performance


Down Insulation

Down Insulation- Best for periods of low activity/inactivity in cold temperatures

Down is the fine layer of fluffy feathers underneath the normal feathers on water fowl, usually geese or ducks. 

Younger birds tend to be covered in down at birth, whereas older birds have a layer of a down.

 Younger birds' down is not as full and lofting as older birds', so cheaper down products tend to be made from the down of younger birds, whereas more expensive down garments will have been made with the down of older birds for improved quality. 

Eastern European down is generally the highest quality down.

The difference between feathers and down:

Feathers are divided into two distinct styles. 

There are fluffy and light ‘tendrils’ with a very small inner nucleus which are often seen on newborn birds; and a typical feather that falls from a bird, with a long spine within the main body, known as a countour feather. 

Down is not the countour feathers of the animal, but the soft and light tendrils, with no spine. An insulated garment is typically a mix of these contour feathers, as well as the down tendrils. 

Feathers are unable on their own to retain loft, which is why there must be certain percentages of down to balance a percentage of feathers in any garment classed as being a ‘down insulator’.

This is where percentages such as 80/20 come from. This indicates the garment contains 80% down, and 20% feathers.

How Down Insulates:

When down is compressed the filaments of the fibres create small air pockets that trap in air, and therefore heat. This is seen naturally within animals who need insulation.


How Down Is Collected:

Down and contour feathers are separated during the manufacturing stage when a fan pushes all the components of the down upwards with a cylinder. 

Due to having a spine and extra weight, feathers fall faster, so are able to be separated from the pure tendrils. These tendrils are delicate and ‘fluff like’ with minimal weight and high lofting capabilities. 

At the manufacturing process, down is then sorted, washed, and sterilized. It is the sorting that determines the fill power which is stated as a number such as 500 or 600. This rating is based on how many cubic inches it displaces in a given area, or simply, the space the down takes up.

The difference in tendrils:

The longer the tendrils, the more surface area it spans, giving it a higher fill power. 

The higher the fill power, the higher the insulating capabilities, and the lower in weight the garment can be.


Fill Ratings in Down

The American market uses a different rating system to the UK/EU. The general rule for conversion is 600 US fill power is in fact, 500 EU fill. (Generally take off 100 of fill power for an EU/UK conversion.)


Measuring the Insulation Properties of Down:

The higher the fill power of the down, the longer the tendrils are, meaning it occupies more space and volume. This means you need less of it to stay as warm.

 Higher fill powers are also lighter in weight. Most insulated items containing natural down contain 80-85% down and 15-20% of feathers. Natural down is often a more bulky and heavyweight purchase than a manmade synthetic alternative and is therefore more costly.

  • Down fill keeps its warmth with ease
  • Down fill does lack some of the breathability and opportunity for freedom of movement. 
  • Down tends to be more expensive, but is highly durable.
  • Down needs to be kept dry otherwise the feathers clump together. This can be dried off, but the fill power will never recover totally if down gets completely drenched. Quality is usually expressed as a percentage, which shows the ratio of down to feathers. 
  • The fill power is in a rating style, and this will indicate how well the down retains its loft, or simply, how efficient it is. The higher the percentage and fill number, the better, lighter and warmer the garment.
  • Down is easily compressed.

Synthetic Insulation

Synthetic Insulation- Best for periods of exertion in cold temperatures

Man made, synthetic insulation is made using polyfibres, the lab made equivalent of natural down. 

Synthetics are used to attempt to replicate the warming and heating effects of down, without the bulk, and with the chance to achieve higher levels of breathability, all at a lower cost. 

Although heat is circulated with ease, synthetic insulation does suffer from heat loss at a quicker rate than a natural down jacket, making it more suitable for times when your body is still producing its own heat, for example during periods of exertion. 

  • Synthetic Insulation provides plenty of circulated heat with more freedom of movement. However synthetic insulation loses heat quicker than a traditional down fabric. 
  • Synthetic insulation performs better when wet unlike natural down
  • Synthetic insulation is cheaper and easier to care for than natural down 
  • Synthetic insulation is heavier and bulkier, meaning you will need more pack weight to accommodate it.

Examples of Synthetic Insulation

Thinsulate Insulation

Thinsulate is typically used to increase the warmth between the outer layer and the inner liner of a garment. Thinsulate is lighter and slimmer than natural insulators.

Thinsulate traps in air molecules within the garment or shoe and then circulates this air. 

Created with a fine knit, Thinsulate traps in these air molecules close to the skin, allowing warm air to circulate and insulate effectively.


Primaloft Insulation

Initially designed as the first alternative for natural down by the army, Primaloft is designed to be a tough synthetic insulator. Primaloft will absorb at most roughly 1 percent of its weight in water, whilst the tiny woven fibres allow it to trap in insulation. 

Primaloft is made from a tightly woven microfiber structure, helping the body retain warmth and conserve energy. 

PrimaLoft can be used as a thermal insulator in gloves, walking boots and shoes as well as gloves, jackets and sleeping bags.


Polarguard Insulation

Polarguard is a hollow core fibre that is able to insulate like a down fabric. Used in outdoor wear and sleeping bags, Polarguard is a synthetic insulator with a cross section construction made from triangles, that prevent it from flattening, keeping it’s high loft even after long periods of use.


Used by Columbia, Thermolite is a synthetic insulator that uses the warmth and softness of a down insulator, with a hollow core fibre fabric construction. This is based on natural down insulation, which is created from natural air pockets. 

Thermolite provides warmth at low weights. The key behind Thermolite’s insulating capabilities is from the large surface area, which helps moisture to evaporate, keeping you wamer.


Garment Construction To Aid Insulation

Baffle Walls:

Baffle walls are the way an insulated item (usually a jacket) is constructed in ‘panels’. Baffle walls can be small or large. Narrow baffles allow the down to stay in place and prevent it moving, trapping in more air pockets and retaining higher levels of warmth.

Stitch Through Construction:

This is a construction where the stitching of an insulated item is both vertical and horizontal, also known as ‘crosswise’. . This method of stitching has the result of making the down sit in small pockets, similar to the natural air pockets found on an animal’s skin. 

It also prevents the down from slipping within the garment, where it’s effectiveness is diminished. The closer the stitching, the less ‘bulky’ the jacket will look, but the less warmth it will retain. 

2 Layer Offset:

 2 Layer Offset is used in sleeping bags as an alternative to stitch through construction, which can leave cold spots in the fabric, which is more of an issue in static use, such as sleeping, where less body heat is produced by the user. 

2 layer offset garments are made so that the seams are sewn in the middle with insulation both above and below them. The insulation layer is then split into 2 layers, which provides coverage in the cold spots for additional warmth.


Sleeping Bag Insulation

Tents are designed to be breathable and weatherproof, and not insulated, so to stay warm in the outdoors you need a sleeping bag to provide the bulk of your insulation. 

Sleeping bags work by trapping air within the bag itself. Your body is the heat generator for this insulation system and is able to create extra heat for warming up the environment around you. 

There are many technical bags with features that aim to help retain this extra heat and therefore reduce the work for your body.

 Baffles that tighten above the shoulders also stop heat from escaping and cold from entering, acting just like a hood.


Seasonal Insulation Ratings- Sleeping Bags

Sleeping bags are rated for different conditions and temperatures.

  • One season bags are designed for summer (June to August) use only. (UK) 
  • Two season bags cover late spring to early autumn when sub-zero temperatures are rare. (UK) 
  • Three season bags, with a 0° comfort rating, are for early spring to late autumn use and can be used on mild winter nights (UK) 
  • Four season bags have a comfort rating of -5° and below and are for winter use.(UK)
  • ‘Five season’ or ‘Expedition’ bags are for high mountain and other extreme use in temperatures down to -40°.

Insulation from the Shape of the Sleeping Bag

The shape of the bag is important for minimising the air movement around your body while it is inside the sleeping bag. The smaller the volume of air the less work your body has to do.

 The easiest way for manufacturers to reduce this space is to taper the bag, known as mummy shape. The hood is also one of the best ways to stop warm air being lost from your head and to keep the circulation of warm air going around inside the sleeping bag.

 In cold or winter environments this is an essential feature. (Sleeping bags that don't have this feature are designed for warmer temperatures, usually indoors, in your caravan or for summer climates.) 

Read more in our Sleeping Bag Guide.