Friday, 02 May 2014 22:46

Down time - A buyers guide to down Part I

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Insulation - The basics:

When it comes to selecting an insulated jacket it makes sense to have a look how insulation it works and with just a basic grasp of the essential principles you'll be able to make sense of the bewildering array of fill powers and fillings that face you. 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.

Down Feather

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.


Fill Power

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..


Fill Power Jars


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 12 months 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.


Down Jacket Construction Diagram


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.

Note: This article was restored from the archives. It's published creation date is inaccurate.