Sleeping Bag Construction
Sleeping Bag Construction
WHAT ARE BAFFLES
“Baffles” is the name given to the chambers in which the insulating material is contained in a sleeping bag. The better the design the better the performance of the sleeping bag.
The design of baffles is important because it determines how well the insulating material is held in place. There is nothing worse than a sleeping bag whose insulation has shifted creating a thick wad in one place and little or nothing in another part of the sleeping bag.
Stitching can be another problem, particularly in the case of cheap sleeping bags. When baffles are created simply by stitching through to create a “quilt” effect, each such stitch is uninsulated which creates lines through which body heat escapes. This is called “stitched-through” construction. The baffles which are created in this way have no sidewalls.
One way to avoid this heat loss problem is to use two layers of stitched through insulation with the stitches in the second layer offset relative to those in the first layer. Unfortunately, this doubles the weight of the sleeping bag.
More Advanced Designs
Most sleeping bags are constructed using a single layer of insulation.
The only single-layer constructions which do not create cold spots along seams are the “V”, “trapezoid” and “boxwall” designs. The baffles in each of these designs do have sidewalls to keep the insulation in place. There are no “lines of cold” as a uniform thickness of insulation is present throughout the sleeping bag.
The simplest is the boxwall design. The baffle sidewalls are vertical, hence the baffle shape is box-like.
Several types of boxwall designs exist, including rectangular, diamond shape and curved. They all have one thing in common, however, which is vertical baffle sidewalls.
A more complicated single layer baffle system is the V system.
Here, each baffle sidewall is inclined at an angle and joins its neighbour to form a series of “Vs”.
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” gives twice as many baffles as boxwall 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 sidewall fabric.
Differential Fill means that the quantity of down in the base is less than the quantity in the top of the sleeping bag. At Criterion we employ a 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.
Note – The Criterion Lady sleeping bags are shorter but contain the same amount of down as in the corresponding men’s sleeping bags. The down ‘saved’ from the reduced length has been redistributed to the base and foot area of the ladies’ sleeping bags, which has altered the Differential Fill ratio to about 55:45.
‘Brick’, otherwise known as ‘offset double boxwall’ involves two boxwall layers.
The two boxwall 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 Criterion Expedition sleeping bags to provide the high levels of insulation that are needed in conditions of extreme cold. Bases are boxwall (see below : “Boxwall construction is best for the base !”).
Other considerations – sleeping bag base
Boxwall construction is best for the base !
For optimum insulation the down should be of uniform thickness over the whole area of the sleeping bag. On the top side the trapezoid construction gives better performance than boxwall.
In the sleeping bag base, however, the insulation value of the down is much reduced because it is compacted by the weight of your body preventing loft. The effect of this compaction far exceeds any disadvantage from using a boxwall construction rather than trapezoid. Furthermore, your body weight prevents displacement of the down – which is another reason why trapezoidal is unnecessary for the base.
Boxwall is lighter than trapezoid, which makes it a better option for the base.
Criterion sleeping bags utilize boxwall construction in the base.
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 boxwall; lighter than “V” construction.
Apart from the Expedition range and our two lightest models, the Ultralight 200 and Quantum 200, all of the other Criterion sleeping bags utilize trapezoid construction in their top sides. Bases are boxwall (see below : “Boxwall construction is best for the base !”).
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.