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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 260 grams (9 oz), including stuff sack, when constructed according to design below, which is for man 71" tall (180 cm). Easy to modify design for larger or smaller users. If modifying design, allow plenty of girth at top, so as to make it easy to get in and out of bugbivy.
1.1 oz ripstop nylon coated is known to be of limited waterproofness when subjected to pressure, plus it will develop holes if placed directly against ground. Thus water which flows under tarp will seep through bottom of bivy. This should not be a problem if using full length ground pad and assuming there is only a small amount of water under tarp (avoid camping on dished ground where rain likely to form a puddle). Durability of silnylon is not a problem, in my experience, since tears in no-see-um netting will require replacement of bugbivy long before floor wears out.
Roof at head end elevated either by suspending from tarp ridgeline or other overhead attachment point, or using pole inside bugbivy. Netting may still touch parts of 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 thick knitted hat, since while mosquito mouth parts can easily poke through either netting or thick knitted fabric alone, they cannot reach through combination of these two fabrics.
Another way to defeat mosquitoes or ticks that might attempt to bite through netting, is to treat netting with permethrin. Note that monofilament polyester (standard no-see-um netting in outdoor fabrics stores is monofilament polyester) does not wick and does not absorb moisture and thus 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 netting before sewing, since permethrin will serve no purpose against silnylon and will tend to stink when silnylon gets wet (at least until it wears off). Permethrin is destroyed by both sunlight and oxygen, and so 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, push 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 (forerunner to current Trail Life). I found Jardine's design complicated to sew, a nuisance to set up, cumbersome to get into and out of due to small door opening. So I simplified design and made door opening much larger. Bottoms of the doors on both Jardine's and my zipperless designs were simple flaps held down by 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 design with a #3 zipper running partway down ridgeseam. This design was very effective at keeping both flying and crawling bugs out. However, zipper failed in two ways. First, it froze shut in cold weather due to condensation from my breath. I was able to unfreeze 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, zipper wore to point where it would not stay shut. I fixed this in the field using old trick of squeezing back of slider slightly with 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 zippers and so switched back to zipperless design, but with drawcord closure rather than just flap of netting. Drawcord closure is just as effective as a zipper at keeping bugs out, and much more reliable. Drawcord closure design also somewhat easier to get into and out of than design with zipper on 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 summer and late spring, at least until 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 floor, for some crazy reasons explained on 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 ground pad in morning without also sitting on netting, which will eventually tear 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 cinched tight; (e) only one cordlock on hanging cord rather than two, with one as a spare.
There are a number of other commercial bugbivys available (most of which have zippers):