|Fabric and construction|
Fabrics that are ‘downproof by construction’ are ideal as they don’t use coatings which can reduce breathability: instead, the very tight weave prevents down from escaping. (Small amounts of fill will escape, usually through the seams, but not enough to affect the performance of the bag.)
Down sleeping bags designed for use in damp conditions need a water-resistant outer fabric as if the fill gets wet, its performance is greatly reduced. However, the fabric still needs to be durable, comfortable and highly breathable so moisture can escape.
Fabrics come in different weights, measured by the fineness of the threads that make up the weave. The finer the thread, the lower the denier and the lighter, more silky and expensive the fabric. (For example, calico uses thick threads and would have a high denier, while parachutes feel silky due to the thin threads used in this low-denier fabric.) High-performance sleeping bags are generally made from lightweight, low-denier materials that feel luxurious against the skin but add to the bag’s price.
The most important job of the shell is to keep the fill in position. This is done through a complex series of compartments within the sleeping bag called baffles: without these, the fill could move away from high points on the body.
The baffles are created by sewing mesh barriers between a sleeping bag’s inner and outer, dividing this space into a series of compartments to be filled. The mesh fabric allows air to circulate effectively, as well as allowing the bag to compress. This fabric often incorporates stretch, which allows fill to loft and prevents baffles ‘blowing out’ when placed under stress.
‘Offset baffles’ are often used in sleeping bag construction. Although this sounds complex, it basically means the baffle walls do not attach to the sleeping bag inner and outer at right angles. Instead, this angle is slightly less or more than 90 degrees, reducing down migration and allowing greater loft.
Down is subject to the laws of gravity, shifting from a high point to a low one whenever it can. Conventional sleeping bag designs have horizontal baffles, running straight across the bag. A sleeping body inside the bag turns a flat paddock into a mountainous range, and the fill spends the rest of the night trying to slip from the peaks down on to the plains. As the night progresses, this migration of fill can cause cold spots along the highest points from your neck to your waist.
A common way of preventing this movement of down is by using vertical baffles in the chest area of a sleeping bag. The down cannot go anywhere, ensuring it stays over the chest where it is needed most.
However, with vertical baffles the down cannot be moved at all; with other baffle shapes it can be shaken from the top to the bottom to suit weather conditions, increasing the bag’s versatility. There are other baffle shapes that minimise the involuntary movement of down while allowing it to be shaken from top to bottom when required.