While you may think it's the down inside a jacket that keeps you warm the reality is that it's actually nothing more revolutionary than air and the secret to staying warm is the way in which down traps this air to create a layer of warmth around the wearer.
A bird's down is a layer of fine feathers that lies underneath the exterior feathers of the bird and is made up of two distinct types; down feathers and down clusters. The down feathers are much as you would imagine, though very small, with a spine down the middle and soft filaments spreading out from the spine while the down clusters are round with the fine filaments forming a loose sphere.
While Goose down feathers are soft, when they're packed into a jacket or sleeping bag they have a tendency over time to start aligning the spines, or quills, resulting in a flattening of the fill and less gaps to hold the air. Down clusters, on the other hand, like to return to their natural spherical shape which trap air more efficiently. The first essential in choosing a down product, then, is to keep the proportion of down cluster as high as possible and the proportion of down feather as low as possible. The proportion of cluster to feather is commonly written as 95/5, 90/10, 80/20 etc.
Unfortunately not all down is the same and cluster sizes can vary greatly. The geese that are the source for lower quality down are primarily bred for food and are only 4 months old whens slaughtered, and while this down can be carefully sorted, washed, and blended, it will be limited in how well it can loft. The best quality down comes from mature birds that are kept for breeding purposes. A much smaller number of these birds are raised and while lower quality down can often be sorted mechanically this better quality down is shed naturally in the spring and has to be collected by hand The lower quantity available and the labour intensive manner of collection puts this down at a premium price.
Down insulation is rated by fill power, which is expressed as the number of cubic inches displaced by a given ounce of down (in3/oz).The higher the fill power the higher the down will loft and the more insulating air pockets it will have to trap warm air and create a barrier against the cold. To measure the fill power of down an ounce of down, usually blow dried for maximum loft, is put into a graduated cylinder, and a small weight is placed on top. The volume below the weight is measured in cubic inches and this number is the fill power. Put simply the higher the fill power rating the less down you need to achieve a similar level of insulation. The warmth is a combination of the amount of air pockets which is determined by the loft of the down and the thickness or amount of loft..
Image: Down and Feather Company
Natural or synthetic?
While down is a fantastic natural insulator it's not perfect. When down gets wet the spheres collapse, air gaps close, and the clusters become an amorphous lump with no insulating properties whatsoever. While cold, dry, climates like the Himalayas and Arctic are ideal environments for down the wetter, more temperate, climate of the UK and Northern Europe is far from ideal and for 400 years we've learned to live with this limitation. Over recent years, however, man has attempted to mimic the natural properties of down in a succession of synthetic materials. Synthetic options, such as Primaloft, give most of the benfits of down but retain a high proportion of their insulation even when wet. Offsetting the wet weather performance, however, synthetics have tradionally been heavier and bulkier than down although the gaps between natural and synthetics has been closing over the years. Developments such as Primaloft 1 now come very close to the characteristics of down in terms of softness and weight to warmth ratio.
Over the last few years a third option has arrived on the market - water repellent down. By treating the down with a DWR like process companies like Berghaus and Sirra Designs have managed to make the natural down filling hydrophobic. While not 100% perfect hydrophobic down will retain around 90% of its insulation even when soaked through, and without the weight and "softness" hit of typical synthetic fillings. Available either on its own or as a hybrid combination where hydrodown and synthetics are used together in the same garment, hydrophobic down looks set to take an increasing market share over coming years.
In short sysnthetics and hydrophobic down make the best choices for the UK's winter weather, while nothing beats a good natural down filling in drier climates.
Design and Construction
Down jackets are usually constructed by using a series of compartments called baffles. These compartments,prevent the down from migrating around the jacket while simultaneously allowing the manufacturer to use the optimal amount of filling to promote maximum loft in each individual part of the product. Larger individual compartments will allow the down to ‘bunch’ more, leaving you with colder and warmer patches.
There are two main types of construction using baffles; Box Wall and Sewn-Through. With sewn-through designs the jacket or sleeping bag's filling is formed into channels by simply sewing through from the inner to outer fabric layers, whereas with Box Wall construction each baffle has side walls. Sewn-Through construction is generally used in cheaper, lower specification, products as cold spots can develop where the stitching compresses the down.
As important as the down itself, it's worth paying attention to the fabrics either side of the down. Because down, and synthetics, rely on trapping air to create a layer of insulation it's vital that the outer fabric should be windproof - if not then the cold air from outside will swamp the down's air pockets and prevent a warm layer building up. Equally important is the coating on the outer fabric, which should be water repellent to prevent the known issues of down getting damp. The outer shells of down jackets are generally water resistant and highly windproof but this does depend on the type of fabric used. Many top-end jackets have specially developed fabrics which balance the jacket’s water resistance with its abrasion resistance to improve wear and tear.
It's also important to look at possible routes for cold air to enter the jacket, starting with zips. By their very nature zips are air-permeable and cold will find its way in wherever possible, so zips should preferably have a protective, insulated, baffle mounted on either the outside or inside of the zip. The specifics of how the hood fits, how the hem and cuffs adjust and the fit are all covered in Part II of the guide, but special attention should be paid to these areas as each provides a possible route for cold air to enter.
Down jackets, and to some extent synthetics, are perhaps the least researched and most misunderstood garment in the average outdoors wardrobe. Whereas a lot of thought may go into the choice between GoreTex and eVent, lightweight or durable to many people a down jacket is just a down jacket. While part 1 of this guide covers what are essentially the fabrics and materials and how construction varies according to price part 2 looks at the features of down and synthetic insulated jackets in the same way as you'd look at hardshells, and taking activity into account.
High end down jacket with box wall construction and insulated hood
The down jacket hood, encrusted with ice, is an iconic image of adventure and hood design is an important part of selecting the right jacket. Not all down and insulated jackets have hoods because, quite simply, not all insulated jackets need them. Many hill-goers prefer a hat to a hood and jackets designed for use down to around -5 often either dispense with the hood or make it removable. As the temperature rating drops hoods become more universal as the importance of preventing heat loss through the head becomes more important.
If selecting a jacket with a hood it's important to make sure that the design and size of the hood fits the activity you have in mind. If climbing or mountaineering the hood needs to be able to accommodate a helmet, for example, and it makes sense to make sure volume adjustment is both simple and effective. Removable and stowable hoods should attach in such a way as to eliminate gaps where cold air and snow can get in and volume adjusters should be designed without long cords which can whip up into the wearers face.
Hood showing volume adjuster for helmet compatibility
Zips are one of the principle areas where heat loss can occur, particularly the main front zip, so it's essential to look at features which can reduce or prevent this. Baffles can be placed either inside or outside the zip to prevent heat loss through the zip. An internal baffle will generally sit naturally behind the zip, whereas a baffle on the outside will need holding in place using velcro or studs.
Zips should be easy to operate wearing gloves, and mittens for extreme cold rated jackets, and wherever possible shouldn't have long cords which can either be whipped in the wind or catch on nearby objects. As with hardshells zips can also be used to regulate the temperature, either through a two way main zip, or in some top-end jackets, by using pit zips. Pit zips can make an enormous difference to keeping the wearer comfortable through a range of temperatures or when extremely active in cold temperatures but need to be easily accessible to be seful; so check you can reach and open them with gloves on when trying a jacket.
To many users the design and layout of pockets is one of the most important features of an insulated jacket. Although most insulated jackets have both internal and external pockets it's the hand warmer pockets which naturally get the most attention. With lower priced jackets designed for temperatures at and around zero it's not uncommon for handwarmer pockets to be left open but as the temperature rating drops zipped pockets become the norm. Handwarmer pockets will generally have insulation on the outside but no insulation on the inside until you reach the colder rated jackets, but whether single or double insulated the pockets should have a soft-to-touch lining and be easily accessible.
In addition to the handwarmer pocket jackets often have one or more chestpockets and an internal, usually zipped, pocket. At least one chest pocket is advisable, preferably with enough room to take a map and an internal pocket can prove essential in cold temperatures for keeping batteries warm enough to function. Extras to look out for, and becoming more commonplace, include an option to route headphones internally from the chest pocket to the hood and ski pass/gps pockets.
Chest pockets on a Bergans Sauda (note protective storm flaps being held back to show zip entrances)
Hems and cuffs
There's little point in wearing an insulated jacket if cold air is given easy access at the extremities, so hem and drawcord adjustment is essential. While elasticated cuffs can give a good fit the velcro fastening is generally preferable, gicing both a better fit and allowing the wearer to adjust the gap to regulate airflow. As with zips hems and cuffs should be easily adjustable and preferably single handed. With cuffs this is simple using velcro tabs but with hems it usually involves an elastic cord running through plastic adjusters on either side of the jacket. Commonly the elstic will form a loop having passed through a simple plastic grip and it makes sense to check that these loops will either sit naturally inside the jacket or at least not be long enough to catch and snag on objects.
Some jackets come with lycra thumb loops for close fitting
The collar is one of the most important areas of an insulated jacket. With warm air naturally rising the area around the neck provides the easiest escape so needs special attention. Where possible look for a lined collar with a soft facing and a baffle that encircles the wearer's neck. Where hoods are removable, or fixed and stowable, the hood can form the collar but be aware that this means when the hood is use the insulation around the neck has been removed.
The first thing to recognise when selecting a down or insulated jacket is that a down jacket is not just a down jacket. While you can spot one a mile away and a down jacket is instantly recognisable there's a massive range of variation. In the UK the winter temperature rarely drops below -20C and in urban and more southerly parts of the country -10C is a more realistic target rating so with increased insulation requiring more and/or more expensive down selecting a jacket with these in temperatures in mind will save you overspending.
Synthetic and the new waterproof down fillings are more suited to the UK winter climate where down can suffer from getting wet and synthetic jackets are particularly well suited as belay jackets. For urban and campsite environments, however, the DWR coating on outer fabrics is generally more than adequate to last until you can find shelter and a down jacket rated at around -5 is usually fine unless spending prolonged periods sitting around outside.
Put simply the bulk of the price of an insulated jacket comes down to the cost of the insulation and the more extreme the performance the higher the price. There's no point in selecting a jacket with too much insulation for the environment you're going to use it in, in fact it can be counter productive, and there's probably no more expensive way of over-specifying a garment than having too much insulation so rather than jumping in and just buying the "best looking" or "most featured" jacket take the time to look at where you plan using it first and buy accordingly.