Down

Down quality

The quality of the down in a sleeping bag makes a huge difference to the performance of the bag, in terms of warmth, weight, durability and compactness. However, down quality is a very difficult thing to measure, as in a finished bag the down can’t be seen, and often very little information is provided about its quality. Some manufacturers simply state whether goose or duck down is used: hardly sufficient. The species of the down makes little difference compared to other, more important characteristics.

What is loft?

Down forms the lightweight undercoat of aquatic birds’ plumage, insulating them in extreme cold. The plumes and clusters in good quality down create hundreds of thousands of tiny air pockets, where air is trapped and warmed by body heat.

Loft is a measure of the ‘fluffiness’ of down – or, to be more scientific, the volume taken up by a given weight of down. The higher the loft (or fill power), the more room it takes up and the more efficiently it traps air. For a given weight of down, the higher the loft, the larger volume it will fill, meaning a smaller weight of down is required to fill each sleeping bag baffle.

The test for measuring loft entails placing a given weight of down in a cylinder with a specially calibrated weight on top. The weight is allowed to fall slowly on to the down for a period of time. The position at which the weight stops, supported solely by the down is the loft: the higher this reading, the loftier the down and the better its insulating power. The loft is calculated before the bag is even made, using new, clean down. As time passes, age and cleanliness will have an impact on the down’s loft.

Composition

The composition of down fill is described by the ratio of down to feather. To measure the down/feather mix, a sample is separated into four parts: down plumes, down fibre, small feather and ‘other’, with each represented as a percentage (for example 90 per cent down, ten per cent small feather). Generally speaking there is a correlation between a high percentage of down and high loft; however, this is not always the case.

Species

The best down comes from large, mature, free-range birds from cold climates as the down clusters are larger. The extra spring and surface area of these clusters not only provides more loft and warmth, but also means that it tends to last longer. Until recently it was accepted that in terms of performance, down of the same quality (loft, composition etc) from ducks and geese was the same. However, recent research shows the best down is formed by the biggest clusters, which come from larger, more mature birds. As geese are normally larger birds, they generally produce better quality down as is tends to be stronger, last longer and have more filling. Goose down also generally has higher lofts than duck down, but not always. However, the difference is not large: you may be talking about a bag lasting for 19 years instead of 20 so it really is not a major consideration.

Fill weight

Many manufacturers sell bags based on the amount of fill they contain. For example, a bag may have a fill weight of 700 grams of 750 loft goose down. This means that the bag should contain 700 grams of down, which has 750 loft and comes from a goose. (But is it independently tested? There’s the question…)

Fill ratio

This is the proportion of down placed in the top and bottom of the bag. Most people move in their sleep, so loading the top of the bag with lots of extra down isn’t necessarily effective. A common fill ratio is around 55/45, meaning 55 per cent of the fill is in the top of the bag where it provides maximum warmth.

Synthetic

Short-staple fills such as PrimaLoft® are the predominate choice for synthetic bags. These are made up of short strands of fine-denier filaments that are densely packed to minimise heat loss. This makes the bags feel soft and flexible, much like a down bag, and allows for great compressibility. However, they are a bit less durable than continuous-filament fills.

Continuous-filament fills such as Climashield® use a thicker, continuous filament that is lofty, strong and durable. They have a stiffer feel and are less compressible than short-staple bags.

In addition to their fill, some synthetic bags also have different fill layers, creating extra loft with minimal weight and bulk. Common approaches are to use a thermal barrier to reflect heat back into the bag (a principle similar to a space blanket), or using a loose layer of fill to trap pockets of warm air.