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Roof suspended from above, such as from ridgeline of tarp. Cord adjusted in length using cordlock.
Roof elevated using pole inserted in polecup inside bugbivy (same pole as used to hold up foot end of tarp, details here), with cord attached to any heavy object (rock, food bag, etc).
Further discussion of bugbivy here.
Weighs about 255 grams (9 oz), including stuff sack, when constructed according to the design below, which is for a man 71" tall (180 cm). Easy to modify design for larger or smaller users. If modifying design, allow plenty of girth at the top, so as to make it easy to get in and out of the bivy.
Experience shows that lightweight fabrics placed against the ground eventually develop holes. Also, even without holes, silicone coated ripstop is known to be of limited waterproofness when subjected to pressure. Thus water which flows under the tarp will seep through the bottom of the bivy either through holes or due to the weight of the body; hence it is best to avoid camping in areas with potential for ponding water. If protection from ponding water is necessary, then the bivy should be constructed with a floor of polyurethane-coated ripstop with an extra heavy coating of urethane (5 oz/sqyd, after coating, "storm-proof" ripstop), but this will more than double the weight. Another possibility is heavy silicone-coated ripstop, which weighs 1.9 oz/sqyd before coating and about 2.4 oz/sqyd after coating, and which will add about 3 oz to the bivy weight as compared with using 1.4 oz/sqyd (after coating) silnylon. Compared to the the 1.4oz/sqyd silicone-coated ripstop, the heavier silicone-coated ripstop is much stronger, though it can still develop holes. It also has somewhat higher hydrostatic pressure, though still far less than heavy urethane-coated ripstop. Silicone-coated ripstop dries faster than urethane-coated ripstop, especially since most urethane-coated ripstop is coated only on one side, and the uncoated side is used as the ground side, and this uncoated side absorbs water. Silicone-coated ripstop is thus more resistant to mildew than urethane-coated ripstop. Also, urethane coatings are known to peel away after much use, especially if left damp for any length of time, whereas silicone-coated ripstop doesn't have this problem. All things considered, the best approach is to use the lightweight silicone-coated ripstop so as to save weight, accept the fact that this lightweight fabric will develop holes over time (tears in the no-see-um netting, which can be repaired in the field using duct tape, will normally force replacement of the bugbivy long before there is a large number of holes in the floor), and avoid camping on dished ground where ponding water might be a problem.
Roof at head end elevated either by suspending from tarp ridgeline or other overhead attachment point, or using a pole inside bugbivy. However, even when head end of roof is elevated, netting may still touch upper body. To protect arms from mosquitoes biting through netting, wear long-sleeve shirt of tightly woven fabric which mosquitoes cannot bite through, such as supplex nylon. To protect top and back of head (especially if balding), wear sleep hat of tightly woven fabric (sleep hat sewing details here). Another possibility is a thick knitted hat, since while mosquito mouth parts can easily poke through either netting or thick knitted fabric alone, they cannot reach through the combination of these two fabrics.
Another way to defeat mosquitoes or ticks that might attempt to bite through the netting, is to treat the netting with permethrin. Note that monofilament polyester (standard no-see-um netting in the outdoor fabrics stores is monofilament polyester) does not wick and does not absorb moisture and thus the permethrin treatment will not be as effective as it would be with nylon or cotton or multi-filament polyester netting. However, some permethrin should cling even to monofilament polyester netting. It would be better to treat the netting before sewing, since the permethrin will serve no purpose against the silnylon and will tend to stink when the silnylon gets wet (at least until it wears off). Permethrin is destroyed by both sunlight and oxygen, and so the treatment will only last a few months, at most. I don't bother treating my bugbivy with permethrin.
To raise foot end of bugbivy in very warm weather, just shove sleeping quilt or bag into foot end. Other than in very warm weather, quilt or bag will lie over legs, and thus elevating foot end will not be necessary.
My first bugbivy was based on Ray Jardine's original zipperless net-tent design from Beyond Backpacking (the forerunner to the current Trail Life). I found Jardine's design complicated to sew, a nuisance to setup in the field, and cumbersome to get into and out of due to the small door opening. So I simplified the design and made the door opening much larger. The bottoms of the doors on both Jardine's and my zipperless designs were simple flaps held down by the ground pad and other objects. These closures were effective at keeping flying bugs out, but not so effective for crawling bugs, such as ants, spiders, beetles (all of which I encountered when using my zipperless bugbivy) and possibly ticks (which I didn't encounter but which concerned me).
So I then changed to a design with a #3 zipper running partway down the ridgeseam. This design was very effective at keeping both flying and crawling bugs out. However, the zipper failed in two ways. First, it froze shut in cold weather due to condensation from my breath. I was able to unfreeze the zipper using body warmth and thereafter made it a rule not to use the zipper if I expected freezing temperatures (not a problem since there are few bugs when it is cold). Second, the zipper wore to the point where it would not stay shut. I fixed this in the field using the old trick of squeezing the back of the slider slightly with the pliers on a leatherman multitool (which I then carried in my repair kit but which I no longer carry). But this got me to thinking that I could not really rely on the zipper and so I switched back to a zipperless design, but with a drawcord closure for the door rather than just a flap of netting. The drawcord closure is just as effective as a zipper at keeping bugs out, and much more reliable. My current design, with drawcord closure, is also somewhat easier to get into and out of than the design with zipper on the ridgeseam.
Some people use conventional bivy sacks with tarps for protection from both wind-driven rain and bugs. However, conventional bivy sacks are uncomfortable sweatboxes in the summer and late spring, at least until the sun goes down and things cool off. These are also the seasons with the most bugs. Many conventional bivy sacks provoke feelings of claustrophia, due to being smaller in girth than my bugbivy. Those without zippers tend to be much harder to get into and out of than my bugbivy, due to the small opening. Those with zippers have the potential for zipper failure.
As of 2012, Oware produces a "hot-weather bugbivy" which is similar to mine but has a number of design flaws: (a) uncoated nylon rather than coated silnylon for the floor, for some crazy reasons explained on the website which I'm not going to waste space discussing; (b) front too narrow, which will make it difficult to get in and out and also interfere with sitting cross-legged on the ground pad in the morning without also sitting on the netting, which will tear the netting; (c) no polecup, and hence no way to raise netting when camping on bare rock; (d) no flap to plug gap in opening when drawcord is cinched tight; (e) only one cordlock on hanging cord rather than two, with one as a spare. It might be possible to get them to build a bugbivy according to my specs. Then again, most of these gear manufacturers have a not-invented-here syndrome and refuse to consider anyone else's design.
There are a number of other commercial bugbivys available (most of which have zippers):