Generally the more down in a jacket (fill weight), the warmer it is going to be but this assumes the down is the same quality. However, down quality can vary significantly and is measured by its fill power. A higher fill power down will keep you warmer than a lower fill power. To make matters worse there are two widely recognised methods of measuring Fill Power - the 'US' Method and the 'EU' Method. Both methods work on the same methodology but the 'US' Method gives a slightly higher figure than the 'EU' method for the same quality down - so an 800+ Fill Power (EU Method) is approximately equivalent to an 850+ Fill Power (US Method).
Wherever we quote a Fill Power for a specific garment (or sleeping bag) we specify that the measurement is either 'EU' or 'US' method - so that you can be sure of the facts. Because the difference is small, and for simplicity, when writing in general - such as for this article - we won't make the distinction.
A general rule to follow:
Down is the most efficient insulation for clothing and sleeping bags; it is much warmer for a given weight than other insulators and it is long lasting. Over the years, there have been some issues surrounding the ethical nature of down products and how they are produced.
The most ethically sourced down is that gathered from nests, however this is a time consuming process which is not feasible for production processes. Down is also recovered 'ethically' as a bi-product of the meat industry when birds are killed for food, this is the most common source of ethical down.
The down used in outdoor gear is a natural byproduct of the food industry; the waterfowl which provide the warm insulation have been farmed or raised for food, not their feathers. By using the plumage for natural insulation, it helps to reduce the amount of agricultural waste.
According to the IDFB (International Down and Feather Bureau), down is the environmentally friendly choice for outdoor gear, as it:
To find out more about ethical down and the brands that use it, click here.