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Dry down gives considerably more insulating power per unit weight than the best synthetic insulations, even after accounting for the additional fabric required for baffles in down gear. Down also compresses better than synthetics, and thus down would appear to be a better choice for insulation, assuming cost is not an issue. However, down has several problems.
First problem is mildew. If down is left damp for several days, it will mildew, and avoiding such dampness in sleeping gear is almost impossible in continually wet conditions. Down clothing, especially down vests, can be dried out while sleeping, and thus is fairly easy to keep mildew-free. But sleep gear tends to absorb moisture during the night due to condensation, especially in the foot area. If weather is such that the sleep gear cannot be sun-dried, mildew will set in after a few days. I have had several bad experiences with down sleep gear mildewing. The mildew is very hard to remove, due to difficulty with laundering down. This problem with mildew is the main reason I no longer use down. I have never had a problem with mildew with my continuous-fiber quilts, even though I never bother to dry them out during the day.
Second problem is related to the first, since need to dry a down bag/quilt greatly increases chances of ripping shell fabric. For example, bag/quilt might be draped over rocks during the day and then blow onto adjacent thorns when wind picks up, or draped over a thorny shrub before it is realized that shrub is thorny. Rips in down bags/quilts will prove disastrous, since down will leak out immediately and be lost. Whereas continuous-fiber polyester bag/quilt, because it seldom needs drying, is less likely to be ripped in the first place and then if it does get ripped, there is no loss of insulation and the rip can simply be sewed up in the field.
Third problem is that down collapses and loses all insulating power when it gets thoroughly wet. Again, this is more a problem with sleep gear than clothing. If down clothing gets wet, it can usually be dried by sleeping in it, even in continually wet conditions, especially if down compartments are small, as they often are in clothing. But drying down sleep gear in continually wet conditions is nearly impossible. I have never gotten down sleep gear soaked, but I have gotten a synthetic sleep quilt soaked. (I foolishly slept in a depression, because it was the only comfortable spot to camp, depression seemed shallow, and I thought rain unlikely. When rain arrived, and heavy rain at that, depression filled with water and became a small lake. Luckily, temperature wasn't particularly cold when this happened.) I was able to dry out this synthetic sleep quilt in the field. Had I been using down, I would have had major problems. Getting the quilt soaked was clearly my fault, but everyone makes mistakes now and then. Gear that allows recovery from mistakes is better than gear that doesn't, which is another reason I have sworn off the use of down in sleep gear.
Down comes from waterfowl, and thus it appears strange that down doesn't work when wet. Explanation is that down as it exists in living form on geese is quite different from down used as insulation in clothing and sleeping gear. First, down in geese is coated with oil, which tends to protect it from moisture. This oil is washed off when down is prepared for use in clothing and sleeping gear. Second, and more importantly, each individual down fiber on the bodies of geese is attached to a muscle, and thus is held in place and can be erected to add loft, whereas down in clothing and sleeping gear is loose and thus can shift position and clump together. It is this second factor, down's lack of stability, which is the real reason why down performs so poorly when wet. By contrast, continuous fiber polyester insulation (Polarguard, Climashield) is very stable, because it consists of strands of silicone-coated polyester fiber laid on top of one another, with the strands locked in place by stitching so they can't move. Furthermore, the silicone-coated polyester fibers themselves do not absorb moisture, though moisture will stick between the fibers due to capillary action. Continuous-fiber polyester insulation thus loses insulating power when wet primarily because the water between the strands of fiber conducts heat better than the air it displaces, rather than because the insulation loses loft or shifts in position or clumps. Water between polyester fibers that doesn't run off due to gravity can be burned off by body heat during the night. By contrast, down tends to clump when it gets wet, so that huge gaps are left with no down, and body heat escapes through these gaps rather than being harnessed to dry out the down.
Suppose metabolism is at the rate of 3000 kilocalories/day, but that metabolism is lower while sleeping than while active, so that perhaps 500 kilocalories are burned during 8 hours of sleeping. Some of this heat energy will be lost via respiration, and the remainder will be lost via radiation, conduction, and evaporation of insensible perspiration from the skin. Insensible perspiration from the skin (as opposed to the moist tissues of the respiratory system) is typically a minor factor. So let us assume that 300 of the 500 kilocalories is lost via radiation and conduction from the body exclusive of the respiratory system. This is sufficient heat energy to evaporate half a liter of freezing water from a damp quilt over the course of a night. This explains the advantage of continuous fiber polyester insulation over down (at least when the down chambers are large), in environments which tend to produce condensation within clothes or sleep gear. Other than in the foot area, amount of heat being given off by the body should be more than sufficient to re-evaporate condensation (as well as dry off damp clothes), and thus keep insulation dry, provided this heat can be harnessed for this purpose.
Consider first continuous fiber polyester. Water vapor which condenses in insulation above torso remains above torso, in the path of heat being radiated and conducted from torso. Eventually, condensed moisture will pick up enough heat energy to re-evaporate, and thus insulation stays dry. Now consider down. Moisture condensing above torso causes down there to clump and and fall to the side of the quilt or sleeping bag, so there is no longer any insulation above the torso and body heat leaks out rapidly and is lost rather than being productively used to re-evaporate condensed moisture. It is possible to shake the quilt so that the damp down is located above the torso, but each such shaking causes a loss of heat energy in the form of warm air escaping (bellows effect). Also, it is difficult to sleep if the quilt needs to be shaken every so often to keep down in place above body rather than at the sides. Problem of down clumping and falling to the sides does not exist in the geese from which down is derived, because in geese each individual down feather is anchored to the skin. When down attached to a goose's skin gets damp, it fails to insulate the same as down in clothes or sleep gear, but—very important—it doesn't clump or shift, and thus will soon dry due to body heat. Likewise, problems with down are greatly reduced when the down compartments are very small, so that damp down can't shift very far. This is why down clothing that is not too thick and has small down compartments works quite well in moist environments. That is, it is primarily the LOOSENESS of down in bulky clothes and sleep gear, rather than down's other qualities, which causes problems in moist environments, and conversely it is the STABILITY of continuous fiber polyester which allows this insulation work so well in these same environments.
Correct way to use down for sleep gear is as follows. First, use a quilt rather than a sleeping bag. Down does not break down due to repeated compression into a stuff sack, but it will break down from grinding under hips and shoulders, especially for side sleepers. Second, underside of quilt should be silnylon or other waterproof fabric, thus creating a vapor barrier which eliminates condensation issues. Third, about 24" of top side of quilt near head should also be silnylon, to prevent fouling of the down from body oils, saliva, underarm perspiration, etc. Down chambers will need to be longitudinal (head to foot) rather than horizontal (side to side) for this to work, since otherwise, chambers near head would be waterproof on both top and bottom and thus down inside could never dry out from water that did somehow get in. Fourth, quilt made to these specifications should be reserved for temperatures well below freezing, since vapor barrier on underside of quilt makes it impossible to dry clothes by sleeping in them, which is a significant disadvantage in rainy conditions (rain mostly occurs when temps above freezing during day). Note that vapor barrier greatly increase effective warmth of any quilt, down or synthetic, because of reduction in evaporative heat loss, thus reducing need for a very thick quilt in very cold conditions. Vapor barrier also allows breathing under the down quilt, which is another way to greatly increase effective warmth, by reducing respiratory heat loss. (Though be careful not to cut off fresh air supply and suffocate.) Breathing under a synthetic quilt is okay even without a vapor barrier, because (as discussed above) condensation is not a major problem with synthetic quilts, other than for temperatures well below freezing. I routinely breathe under my Climashield quilts for added warmth when temperatures are slightly below freezing.
Following Ray Jardine's recommendation, I switched from sleeping bag to quilt, and haven't regretted it. All my synthetic quilts were sewn by myself. I also experimented with a custom-made down quilt from Nunatak, but discarded it when it became heavily mildewed during a long period of continuous rain.
Jardine's original quilt design did not have a hood. Later, he added a flap to the quilt, which he calls a "gorget" and which is a sort of half-assed substitute for a hood. I sewed a quilt with this gorget and found that it worked okay when I was lying on my side, but was very uncomfortable when lying on my back. So then I came up with my own design for a hooded quilt, as described here.
Over the years, I have tried various thicknesses of quilts, in an effort to find the right balance between comfort while sleeping, due to plenty of warmth, versus comfort while hiking, due to low weight. Currently, my preference is for sleep comfort rather than hiking comfort. Hiking with a backpack is uncomfortable by its very nature, and 200 grams additional weight due to a thicker quilt makes little difference to me in hiking comfort. Whereas the extra warmth due to that 200 grams makes considerable difference in sleeping comfort in cold conditions.
I have tried Thermarests and similar self-inflating pads, but, unlike most people, I find these less comfortable than closed-cell foam. I am mostly a side sleeper and normally lie with my weight on my rib cage and hips. With the thinner Thermarests, my hips sink all the way through to the ground, so that there is neither insulation nor cushioning, unless the pad is overinflated, but then the pad is much less comfortable than closed-cell because it somehow cuts off circulation in my arms, whereas closed cell foam does not. The thicker Thermarests are heavy and none of the Thermarests serves double duty as internal frame for my backpack, so I would have to use something else for that, adding more weight. Thermarests can be punctured by thorns and splinters and repairing them in the field is a nuisance.
Most closed-cell foam pads are too thin for the sort of packed and rocky ground I often encounter. In particular, in limestone country (such as in eastern Spain or the island of Crete), the ground often consists of nothing but sharp rocks mixed with a little bit of dirt. It is possible to move some of these rocks, but typically this merely uncovers more rocks.
The Ridgerest, for example, is advertised as being 5/8" thick, but this thickness is the result of the corrugations. After these corrugations flatten (which takes about a week in the hip area, in my experience), the pad is only about 1/4" thick, which doesn't provide nearly enough padding for me over sharp rocks. Also, the corrugations make the Ridgerest cumbersome to use as frame for my backpack, since they prevent the pad from unrolling naturally as I stuff the backpack full.
In my experience, the best commercially available pads, at least for those who plan to store the pad inside the backpack, are the cheap pads found in Europe, which consist of 3/8" of foam sandwiched between polyethylene plastic film. The plastic film coating enhances durability and also seems to distribute the weight of hip bones over a larger area, thus improving cushioning. Some of these nominally 3/8" thick European pads are actually thicker and denser than the others, but the difference is so minor that you have to hold the pads side by side and squeeze to distinguish. The thicker and denser pads are considerably more comfortable when sleeping on hard surfaces, for not much more weight. Typical weight for a 3/8" thick pad 20" wide by 70" long is 350 grams. 1/2" thick pads with plastic film coating are less commonly available in Europe, but are particularly comfortable.
Eventually, it occurred to me that I could simply buy sheets of foam and cut to the right size for a ground pad, rather than buying ready-made pads. The best foam I have so far encounted is the 1/2" thick cross-linked polyethylene (XPE) closed cell foam, from Seattle Fabrics. (This foam comes 60" wide, so 2 yards makes 3 pads 20" wide. There is an additional shipping charge for the oversized package.) My current pad is 20" wide by 71" long by 1/2" thick, plus a separate piece to reinforce the hip area of 20" wide by 12" long by 1/2" thick, with total weight of 460 grams. This is thus a full-length pad, since my height is also 71". This XPE foam is particularly dense and durable. It doesn't compress much to begin with, and hence allows me to sleep comfortably even on sharp rocks, and it recovers well from compression (other than directly under my hip socket, which I why added a reinforcement piece there).
I tried using half-length pads to save weight, but as with the tradeoff between heavy and warm quilts versus light and not-so-warm quilts, I came to the conclusion that the additional hiking comfort due to minor weight-savings wasn't worth the loss of sleep comfort. A full-length pad also adds stiffness to my backpack compared with a half-length pad, which translates to improved carrying efficiency, and this may well offset the additional weight of the full-length pad even with respect to hiking comfort.
If the ground pad is not being used as a backpack frame, and if it is a foam pad (as opposed to a Thermarest or similar inflatable pad), then it can be stored outside the backpack. Note that this will allow the pad to be torn on thorns, such as are common in Spain and other Mediterranean countries.