What is down insulation and how does it work? Our tech guide explains the difference between duck and goose down, fill power and how hydrophobic down works.
How does down insulation work?
Down is the fluffy next-to-skin coating found on the underside of birds. It is one of the most effective methods of providing insulation in clothing. Down is not the same as feathers. Feathers form the stiff outer plumage of a bird and aid in flight and waterproofing but provide very little insulation. Down clusters trap pockets of air, which in turn trap warmth from your body, giving you the comfort and insulation you need.
Down clusters trap pockets of air, which in turn trap warmth from your body, giving you the comfort and insulation you need.
What is Fill Power?
In simple terms, Fill Power measures the loft of the down clusters. The more loft the clusters achieve, the more air is trapped and the more insulation is achieved. The higher the Fill Power number, the warmer the wearer can be, assuming the same quality and weight of a given amount of down. Higher Fill Power ratings can also mean weight savings. With high Fill Power numbers (such as 850 fill power) less down is required for a given level of insulation, so the garment can be lighter for the same amount of warmth as heavier, lower Fill Power down garments.
What is a good Fill Power number for outdoor clothing?
A Fill Power number below 750 (which may be written as ‘750FP’) does not, in our opinion, indicate a high enough rating for serious technical outdoor clothing. Such a garment is likely to be either not warm enough or to be too heavy due to the need to fill it with excessive amounts of down to compensate for the low Fill Power. We use an extremely high quality 850 FP in the majority of our down jackets, with 750FP being used only in our down gilet and an incredible 900FP also being used in the Ragnarök waterproof down jacket. We state clearly which Fill Power we have used on the product pages of our website. Claims of Fill Power above 900 should be treated with caution.
Duck down vs goose down?
Goose down is generally superior to duck down at higher Fill Powers due to it having larger down clusters (it’s a bigger animal) able to trap more air. Those Fill Power numbers are an important measure when choosing your down products. Goose down comes from older, more mature birds than duck down and is also more resilient and longer lasting than duck down. Jöttnar down products use goose down and not duck down.
How does hydrophobic down work?
Regular down’s biggest weakness is its reaction to excess moisture. Expose down to rain and it will clump, losing its loft and warmth. Hydrophobic down resists this degradation by applying special polymer treatments to the down clusters. This increases the down’s surface tension, causing water to bead up and roll off the cluster. Loft is maintained and the wearer can have less fear of poor performance through garment insulation failure caused by clumping, or failure of the garment to dry out quickly. Depending on the application process used, hydrophobic down can resist water up to 35x longer than untreated down. It also dries out much quicker. It shows no sign of the polymer treatment breaking down even after 30 washes (a far higher number than most down jackets will be washed in their lifetime). Jöttnar was the first UK brand to use hydrophobic down in its products.
Regular down’s biggest weakness is its reaction to excess moisture. Expose down to rain and it will clump, losing its loft and warmth. Hydrophobic down resists this degradation by applying special polymer treatments to the down clusters.
Does the hydrophobic treatment contain chemicals harmful to the environment?
Some hydrophobic treatments can be harmful. We use DownTek™ ZeroPFC™, a perfluorocarbon-free water repellent down that uses an innovative, nature-inspired approach to achieving water repellency. Instead of perfluorocarbons, DownTek™ ZeroPFC™ uses lipids - an idea derived from nature - to coat the down and render it highly water-repellent. DownTek™ ZeroPFC™ uses a proprietary application process, which employs the use of nano-level lipids instead of a bath process – meaning no by-products in our water-recovery systems. You can read more about DownTek™ ZeroPFC™ here.
What does the ratio number mean on down products?
You may see numbers such as 90/10 or 85/15 assigned to down products. These indicate the respective percentages of down to feather in the insulating mix, with the first number being the down percentage, and the second number the feather percentage. The higher the down number, the better quality the mix and the warmer you will be. Feathers make themselves known by appearing as tiny quills which can push through fabrics (this is easily dealt with, but annoying). It is not possible to completely remove feathers from down during processing. But that doesn’t mean it’s not worth trying. We use only a 93/7 mix, the highest that can be guaranteed.
With high Fill Power numbers (such as 850 fill power) less down is required for a given level of insulation, so the garment can be lighter for the same amount of warmth.