Sleeping bag construction and features

Down sleeping bags appear to be quite simple products when viewed from the outside, or even the inside. However, this belies the fact they can be technically quite complex and sophisticated. This page gives a guideline to various baffle constructions, features and terminology used.

Baffles

Baffles are the chambers between the sleeping bag outside shell, and internal lining. The principle of these is to hold the down in position as uniformly as possible.

The simplest form of baffles is called the “stitched-through” method. This system is where the shell and lining are stitched together to create the chambers into which the down is inserted.

Heat loss

This system is really only used on very lightweight products as it only allows a limited volume for the down to loft in. This method also allows heat to leak through the area around the seams where down cannot fill the corners. This method is of limited relevance to alpine or expedition sleeping bags.

To create a suitable chamber that allows the down to loft, and to minimize heat loss, it is necessary to have side-walls that are between the sleeping bag shell and lining. The generic method is called the “box-wall” construction, and this forms the basis of the four major kinds of baffles found in sleeping bags. They are shown below in order of increasing thermal efficiency.

The idea behind the box-wall baffles is to hold the down as uniformly as possible. However, in the standard box-wall design, heat can still escape along the walls as the down does not fill the corners so well. The slant wall and trapezoid designs, which are virtually identical, create an overlapping effect and reduce heat-loss. V-tubes are the logical extension to these and as such are generally recognised to be best method. This construction does carry a small weight penalty as there are more baffles, and thus uses more fabric (albeit extremely light fabric). Many sleeping bag designs employ a combination of constructions.

There are additionally a small number of designs such as vertical V-tube, chevron and radial baffle systems, but these really concern the orientation of the baffles in plan view, and not the orientation of the side-walls.

Tubes

The above are examples of single layer baffle systems, and represent the construction used by the vast majority of producers. There are also double baffle systems, but these add considerable weight and are only found in sleeping bags for exceptionally cold conditions (where the large amount of down also needs extra baffles for greater control of down distribution).

Side baffles

Sleeping bags basically have two halves – the top, above the sleeper, and the bottom, below (and compressed by) the sleeper. The top and bottom halves are normally separated – by the zip on one side, and a side-baffle on the other. Some sleeping bags do not have a side-baffle – deliberately so the down can be shifted from the top to the bottom or vice-versa.

The side baffle can be a source of heat loss as down does not always sit in this area well. Consequently, in warmer and more sophisticated designs, an actual chamber (V-tube side baffle) is positions for the full length of the bag. The diagram below shows a cross-section of baffle-less, standard side-baffle, and V-tune side baffle designs.

Side baffle

Differential fill

Many manufacturers argue that as much of the down in the bottom half of a sleeping bag is compressed by the user’s body, then it is not insulating effectively. Consequently, they put more down into the top half than the bottom half, most commonly in a ratio of 55-60% in the top, and 40-45% in the bottom.

Differential cut

The differential cut refers to the circumference of the external shell of a sleeping bag being greater than that of the internal lining. This is necessary to create volume between the two layers in order to allow the down to loft.

Zip baffles

Zips are heavy. They are also a source of heat loss. To reduce this, zip baffles have to be added – yet more weight. Obviously, the most efficient and lightest sleeping bag design would have no zips. However, the fact that virtually all sleeping bags have zips (although examples of zip-less ones do exist) illustrates the commercial fact that zips make sleeping bags easier to use (getting in and out) and more versatile (regulating the temperature in warmer conditions by opening the zip from the top, or the bottom for bags with double sliders).

The length of a zip thus becomes a balance between weight and versatility. A full-length zip means a sleeping bag can be opened right out. A half-length zip is the preserve of specialist designs – lightweight and uncompromising. Many manufacturers opt for something in-between.

In cold conditions, a lot of body warmth can be lost through the zip area, and for this reason baffles are added to insulate the zip area. In lighter weight bags, single baffles are the norm. For bags destined for use in colder climates, then the use of two (overlapping) baffles becomes necessary.

Hood design and neck/shoulder baffles

The greatest area of heat loss is from the head. In cold conditions, it becomes necessary to be able to close the hood around the head. This is usually done with a simple adjustable drawcord. Sleeping bags designed for very cold areas usually have more sophisticated, ergonomic hood designs to efficiently envelop the head.

Similarly, it is possible for a lot of body warmth to escape from the inside of the bag up past the neck and shoulders. Most sleeping bags will have a neck/shoulder baffle, again with an adjustable drawcord, to close any possible channels. Some bags will allow this baffle to be moved out of the way in warmer conditions.

Foot-box design

The foot-box is an area given a lot of attention by many manufacturers. Fundamentally, it is a matter of creating sufficient space for the feet in a variety of sleeping positions, but balancing this against not creating too much empty space that needs to be warmed up. In our opinion, if this is not an area with excessive over-design, then it certainly is over-hyped with a plethora of marketing names for different shapes or designs. Overall, this is an area that makes very little difference to the performance of a sleeping bag.