Baffled by Baffles?
'Baffles' is the term given to the chambers created between the outer shell and the inner lining. These chambers are used to hold the insulating material uniformly thoughout the volume of the sleeping bag, and prevent the insulation clumping together in one area and compromising the performance of the sleeping bag.
The baffle types listed below start with the most basic 'stitch-through' type, increasing in thermal efficiency up to the uncompromising build quality of 'offset double box wall' or 'brick' baffles.
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.
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.
Advanced Baffle Construction Techniques
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 five major kinds of baffles found in sleeping bags. They are shown below in order of increasing thermal efficiency.
The simplest of the "box wall" variations, the basic box wall has vertical side walls to keep the insulation in place, and has no "lines of cold". A uniform thickness of insulation is present throughout the sleeping bag, greatly improving performance compared to "stitch-through" construction.
Slant Box Wall
A slanted wall construction creates a more difficult path for heat to escape along the relatively low-density insulation area against the side wall. This improves the thermal efficiency of the bag even more than box wall construction.
Trapezoidal Box Wall
This is a popular design for the high quality sleeping bags of many manufacturers, as it provides a good compromise between warmth and weight - warmer than box wall, yet lighter than V-tube construction.
Advanced shingle insulation system offers exceptional heat retention, optimal loft and superb cushioning properties.
Thin layers of insulation combining aspects of shingle and blanket construction.
Here, each baffle sidewall is inclined at an angle and joins its neighbour to form a series of "V" baffles.
For a given size of bag and for a constant width of baffle, the greatest number of baffles is obtained using the"V" design. V-tubes give twice as many baffles as box wall construction, given equal measurements. For a fixed quantity of down, the more baffles there are the less chance of the down being displaced - hence "V" is better than boxwall as regards warmth. Its disadvantages are weight and cost, since in order to create twice the number of baffles requires additional (albeit very light) side wall fabric.
Offset Double Box Wall
The two box wall layers, one on top of the other, have their baffle walls offset relative to each other, which creates an appearance similar to that of a brick wall. Brick construction is used in sleeping bags which contain a large amount of insulation material.
Brick construction is utilised for the top side in the Cumulus Expedition sleeping bags to provide the high levels of insulation that are needed in conditions of extreme cold.
Perpendicular Double Box Wall
Taking the double box wall one step further, this baffle configuration uses and outer layer of horizontal baffles, and an inner layer of vertical baffles to minimise down shifting when rolling from side to side.
This type of baffle construction is used by Marmot in its extreme mountaineering range.
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-tube side baffle designs.
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 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.
Differential Cut and Fill
This is a manufacturing technique by which the outer surface of the top side of the sleeping bag is wider than lower (inside) surface. This helps the down to expand outwards from the centrally located body position thus facilitating optimum lofting.
Differential Fill means that the quantity of down in the base is less than the quantity in the top of the sleeping bag. Many manufacturers use a fill ratio of top: base = 3:2 (discounting the hood, neck and zip baffles). Hence the base has 40% of the total insulation instead of the 50% it would have if there was no Differential Fill. The compaction of the base restricts loft and reduces its insulation value so the difference in warmth between 40% and 50% is quite insignificant. On the other hand, having 60% in the top gives a big warmth benefit compared with having 50%, since no compaction means that the extra down traps a fully proportionate extra quantity of air and loft is increased by a fifth.
Quite simply, the design of the foot box must ensure that your feet are kept warm in any position. There are many designs on the market. As a rule of thumb, more baffles will reduce the chance of cold spots but in our experience all of the manufacturers we feature on our Downbags.co.uk design effective foot boxes.