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All that said, there is a kind of bivy that offers significant advantages in weight over most other shelter options, and that is the “bivy sack,” sometimes called a waterproof sleeping bag cover.
Companies like Oware and Mountain Laurel Designs make bare-bones fully waterproof-breathable bivy sacks weighing around 12-13 ounces, but many people make their own. These are true mountaineers’ bivouacs, and as such, they still suck for general camping.
An exception to all this talk of “general camping” is desert camping, where heavy rain is a non-issue. The crampedness is easier to put up with when you’re never “stuck” in your tent. Also, stealth camping might be a situation where there are other factors at play.
Many desert campers take the bivy sack idea one step further… there is now a kind of bivy sack where the top is a non-waterproof, DWR material. This mitigates disadvantage #2, internal condensation. There are non-waterproof (but still water resistant) top materials that are much more breathable than something like Gore-tex, and this is where bivy sacks get really lightweight (5-6 oz range).
These are also very easy to DIY, and many good gear companies make them, but if you want to see how it’s done, Google the Titanium Goat bivy (pic related).
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