Chris, You are right to be attracted to the “aesthetic” of just throwing down your bed on the ground and crawling in. I find the nuisance of erecting any shelter to be very discouraging to me to use a shelter at all. I hate putting up tents! I hate stakes! With just the bivy, you are actually sleeping under the stars! which is a whole other experience than sleeping under opaque fabric, more the experience I am looking for in backpacking.
I always backpack with a bivy, a Montbell Breeze Dri-tech, long-wide, 8.1 ounces (https://www.montbell.us/products/disp.php?p_id=1121330). The couple of recent times I have tried again to go without I said “never again.” Not SUL, but I’m not a trail pounder.
I don’t have any problems with my WPB bivy use except that if would be nice if the fabric would allow complete elimination of condensation (I DON’T want it to be air-permeable, as I have heard Epic fabric is) and never produce a clammy feeling (but in cold weather I more often than not WANT the extra humidity). But that’s a subject everyone is accustomed to talking about regarding rainwear as well. And, as a detailed discussion somewhere on BPL pointed out, pretty much nothing will stop dew accumulation when conditions are right.
Understand that my perspective for the following comments is my predominant experience of 3-season solo off-trail hiking in the desert and mountains of the southwest plus the Sierras in summer.
1. Certainly a WPB bivy sack is a good idea as a backup to a leaky tent/leaky floor or to protect against dripping condensation and ice, and as a reassuring item in the event of an unexpected rain while sleeping out. I stuff my sleeping bag into my pack, with or occasionally without a stuffsack. This keeps it ready for easy instant deployment as a ready bed, but just as importantly is surefire protection from rain in a leaky pack. But one caveat: don’t think of a bivy as a replacement for a tent/tarp. Once you spend one night in one out in the rain, feeling that cold water running directly over and under you sapping your strength while dealing with condensation and contemplating the need to get dressed and pack up without the shelter of a tent, you will never want to do it again.
2. But I think of my bivy even more as wind protection than water protection. With a bivy you can be reasonably sheltered in very windy conditions that would make it very difficult to erect any kind of tent or tarp, conditions in which just trying to open and spread out a tent or tarp will aggravate you and, worse case, risk having your tent or tarp impaled upon or shredded by adjacent bushes or trees. Some will say if it’s that windy you’ll want to be in a tent for warmth, but I I don’t find that to be often the case in warmer seasons. I very very often use my bivy simply as wind protection, lying on top of or casually under sleeping bag, in very windy but warmish conditions. Without it, it would have to be full-on full-warm inside the zipped up sleeping bag—very uncomfortable. Sure, a light so-called “DWR” uncoated nylon might be most comfortable in SOME conditions, but would not provide the warmth of the full-on windstopping feature of coated WPB, or nor would it provide any genuinely useful water protection. Tradeoffs.
3. The extra weight is not 100% penalty. It becomes part of your warmth system: It adds substantial warmth, so you carry an accordingly lighter sleeping bag. Yes, you could get an equal amount of warmth from a lesser weight of down or Apex, but those don’t offer versatility or wind protection or rain protection or incidental bug protection or the opportunity to contain your shed clothes, misc gear items you want to keep close in a big wind. And, importantly, a bivy keeps my sleeping bag clean and a lot of the dust out of my nose.
4. In addition to keeping scorpions and ants out of my bag, a bivy provides mosquito protection for the many nights I encounter in which they tend to come out mostly in the early evening and leave in the middle of the night. I shelter in my bivy, in or out of my sleeping bag, either completely in the bivy or, most often, partly under an additional length of fabric I now always carry, which I call the “drape”. The great utility of this light, very breathable but bite-proof piece of fabric could be a subject for its own thread, but suffice to say here that it is my go-to protection against mosquitos, not a tent.
• The Montbell I use is just a sack:
— No netting because there is no practical way to keep the netting away from your face, at which point the mosquitos will bite you. See my comments above about the “drape.
— Most importantly it is all made of WPB fabric—I would urge you to never buy or build a bivy with a non-WPB fabric on the “bottom”. I’ve done that twice and it is a disaster. If you are like me, within less than an hour of falling asleep the “bottom” is likely to wind up on top and then the condensation begins to accumulate.
—No zipper, just a drawstring. It is so easy to slip in and out of this decidedly baggy sack that it seems absurd to carry extra zipper weight and have the extra wind leakiness of a zipper and then to worry about having to keep the zipper flap pointed earthward in the event of rain; see comments above about “bottoms up.”
—It has a sort of “hood,” i.e., it’s built longer on the “bottom” than the top. This is somewhat useful IF you put the “hood” over your head and NOT under it the way everyone but me seems to use a mummy bag. If you were to be out in a rain this would let you breathe easily but stay dry. But mostly I use it as a warming hood while I lie on my elbows to eat or read a book, while still having good ventilation.