There are many seam types used for strength, simplicity, looks
or feel, or to solve fraying problems of cut fabrics. Almost anything
works well on fabric edges that are woven, knit, fused by hot
cutting, or bonded with durable coating. Nothing works durably
on cut edges of uncoated woven filament nylon or polyester or
even if coated with poor bonding coating (which is needed to keep
high tear strength). Fabric edges which can fray from wear must
be protected by folding under or covering with tape. That works
well on fabrics with rough textured thread such as cotton, Cordura,
Nomex, or acrylic awning fabric, but will eventually fail on slippery
nylon or polyester filament woven taffeta or ripstop taffeta.
The ONLY way to make durable seams in those is to bond the fabric
edges with hot cut fusing (hot knife or laser) or bond edges with
coating. The urethane coatings used on tent flys (and on our older
tents) do a good job holding seams together until long after coating
has worn to the point of not being waterproof. The more durable
slippery silicone coatings dont lock threads near as well
(thus the very high tear strength) so edges must be fused or well
Its easy to design and make seams that will take any normal in use loads, but very difficult to predict gross over loads of mishandling. Thus if you want your gear to last, dont abuse it.
SIMPLE EDGE SEAM: ideal for lightly loaded exterior seams. Easiest to seam seal (single line on exterior only). Down proof, soft & flexible, about 70% of fabric strength. Double stitch may be used for security but does not increase strength.
SIMPLE FLAT LAP: for flat construction requiring highest strength. Single stitch used where it will be seam sealed between the lap forming an adhesive bond as strong as the fabric. Double stitch achieves up to 95% of fabric strength. Difficult to seal due to thru stitches. Thread exposed to wear. Sails are often zigzag stitched both to hold edges flat and to make seams easy to rip out for shape adjustment, but zigzag will not wear near as long as straight stitch. If edges are tucked under to hide them, as is necessary on fabric that frays, it is called a flat felled seam. Automatic folders makes it one of the easiest to sew, and thus seen on almost all mass produced tents.
INSERT SEAM: variation on simple flat lap used to attach an edge in middle of a panel, such as baffles in sleeping bags.
TUCK STITCH: a variation on insert seam that hides the thread on outside, makes a stiff, lumpy seam and puts exterior fabric loads directly on thread.
EXTERIOR EDGES: Folded in, makes a neat balanced seam with minimum bulk.
ROLLED: for hiding edges that are likely to fray. Other methods: BOUND, and
SERGED (zigzag over edge) often used on knits and cheap clothing.
Sometimes youll read that the mark of good construction is use of flat felled seams. Then ask, how come you dont see flat felled seams in highly loaded items like sails or parachutes? How come you DO see flat felled seams on the cheapest imported and heavy roadside tents? The use of flat felled seams only proves edges are hidden, which isnt good, may be bad.
Designing a Down filled sleeping bag from an engineering viewpoint, its obvious you need to limit the spread from inner to outer covers to the thickness the Down can expand to, and need to prevent the Down from shifting when expanded in use. Vertical strips of fabric, called baffles, will accomplish both objectives. Free expansion of the Down could curve the fabric surfaces out between baffles by an amount determined by thickness and width between baffles, but in real life the weight of Down and fabric flattens the inside to conform with the occupant. Thus the thickness varies only about 1/2 as much as calculated for free expansion.
If the volume of Down equals the free expansion volume of the
bag there is no space for the Down to shift to, so it stays in
position. Any lengthwise shift between inner and outer cover will
tilt the baffles and thus compress the Down slightly.
If the free volume in the bag is more than volume of fill, then the Down can easily shift off to lower side area leaving a thin cold top. That will happen if the baffles are too wide for the fill, or are spaced far apart.
Unfortunately many bag designs evolved from sewn thru quilts:
to eliminate cold seam lines someone put a quilt over a quilt
with the seams offset. To eliminate the extra layer of fabric
in the middle someone sewed a middle layer alternately from inside
to outside: viewed from side the inner baffle layer formed a series
of wide V shapes, so was called V baffles. That worked but still
had weight of excess fabric. As with the vertical baffles, the
outer surface will arch up between seams while the inner surface
conforms to the occupant. Any lengthwise shift between covers
compresses the Down much more than for vertical baffles. In the
50s Roy Holubar was producing excellent V baffled
bags, and did a lot of promotion on the idea that the thin edges
of each tube was overlapped by the thick middle of
adjacent tube (ignoring the obvious that the baffle was just angled
thru a uniform thickness of Down). To make a lighter weight lower
cost bag he went to baffle strips instead of full interior Vd
layer, and to stick with his overlapped tube argument
he put the baffles on a slant, like 1/2 of a V. That would have
been fine if materials were rigid and as non moveable as drawings
on a sheet of paper. As with the V baffle, shift of covers in
ONE direction over compressed the Down. But when covers shifted
the other way it became grossly over length vertical baffles,
letting the Down easily fall off to one side. Disaster?? Only
to the user, not salesmen. On display in stores bags are hung
vertically, in a way that holds covers in alignment. With slant
baffles even a 50% underfill looks good: the empty areas
of one tube are covered by filled area of next.
One big advantage of Down over all other insulators is that insulation CAN be adjusted by shifting it. On warm nights, compress the Down into the ends of cross tubes with gentle directed pats, thinning the top (this is difficult in V baffled bags). If you feel cold later, simply pat the sides freeing the Down to expand into middle for more insulation
Conclusion: vertical direct tension baffles are best, providing the same uniformity of thickness as V baffles at same spacing, but with less weight, less sensitivity to motion, and easier thickness adjustment.
Most baffles go across the bag from side to side. At times someone makes bags with baffles going the length of bag. Its easier to make that way and easier to contour a bag around you. But packing tends to cause large shifts, so it takes lots of effort to get it uniform before use. If you shift Down for warmer conditions its almost impossible to fluff it back from inside the bag.