By Oswaldo Rivera,

Down is still one of the most efficient materials for insulating jackets in the mountains. It has been known for many years that the down of ducks and geese traps warm air and insulates against the cold outside air. So neither Finch, nor Bauer, nor any other name can lay claim to such a discovery.

What is Fill Power? 

Fill Power is a value that refers to the thermal capacity of the feather, i.e. its property to create an insulating barrier of air between the inner (warm) body and the outer (cold) environment. It is a value expressed in cubic inches, also called CUINs (CUbic INches) or cubic inches per ounce.  To obtain this value, a calculation method is used to evaluate the swelling capacity of one ounce (30 grams) of feather contained in a container under controlled conditions of temperature (20ºC) and humidity (60%). The higher the Fill Power, i.e. the more volume an ounce of that type of feather expands, the better quality it will be as it will be able to provide more insulation for the same weight.

Test to calculate the Fill Power

What is the amount of fill?

In all insulating, technical mountaineering garments, a value in grams of fill is added. This not only indicates the amount of material used but also provides a reference for selecting the best range of use for a garment. Logic dictates that the garment with the best fill power and the highest amount of fill will also be the warmest. However, in practice, this is not always the case.

Remember that down functions as a net that insulates the body, trapping warm air. If that net cannot expand to its ideal volume within the baffle (the section of the garment containing the down), the net collapses or at least does not inflate as it should. In the development of OS2O garments we make multiple tests to achieve a perfect balance between Fill Power, the amount of fill, and the size of the baffle, always depending on the final use of a garment.

Different values of Fill Power (FP)

Are 200 grams of 500 FP fill better than 100 grams of 850 FP fill? It all depends on the use we give to the garment: a smaller amount of filling results in a more compressible and lighter garment; therefore, in order not to compromise its heat capacity, it is necessary to use a better quality feather (higher Fill Power). Garments with a lot of filler are usually more robust, they do not need to provide maximum warmth at minimum weight, and therefore a feather with a lower Fill Power can be used. Note that in this field it will compete with other synthetic fill garments, but that is another story.