Lets get this out of the way: I won’t make you one of these. Working with these fabrics and with stretchy Climashield is not something I find fun. This design is straightforward and quick to make, so create your own ugly.
Ever since my first Wilderness Classic nearly a decade ago I’ve been turning this idea over; what is the lightest and fastest way to get a bit of sleep in the midst of a fast wilderness trip? Curling up around a fire would seem to be the easiest answer, and has the advantage of self-selecting for only the most vitally needed sleep (read; you get cold and wake up). The problems are the questionable quality of rest, and the potentially considerable time put into making a fire under unideal conditions. Adding a tarp or bivy sort of addresses the second issue, but not the first. In the last decade truly UL sleep items have become common enough that most peoples answer to this question has been to just bring a standard backpacking kit, or at least a light bag, tarp, and minimalist pad. These systems can be in the 2 pound range, but usually come in between 3 and 4 all told (stakes, etc). Not much weight, but not a tiny amount either.
The functional intermediary between these has long seemed to be a light synthetic bag come insulated bivy sack. Enough insulation to maintain ~4 hours of warmth around freezing, and a waterproof/breathable shell with minimal seams, that sort of thing that would allow you to flop under a half ideal spruce and stay protected enough in the just the bag. Synthetic insulation, as sub 10 oz down fills tend to be overly sensitive to moisture accumulation. This winter a friend bugged me enough that I finally overcome my reluctance and made two such bags. In the next few months we’ll truly find out how they perform in the field.
I used 10D WPB for the shell, .66 oz/yard taffeta for the liner, and 3.6 oz (120 grams/meter) Apex. The former is the obvious choice, being essentially alone at that weight. The taffeta has a nice feel and is calendered, with synthetic insulation I reckoned that eeking out every little bit of warmth with low CFM fabrics all around was a good call, with no functional downside. I went with safety orange for use in signalling aircraft. I certainly could have used lighter insulation, but past experienced suggested 120 g/m was the lightest that would still be useable in all but the most specialized situations. I made the neck cinch out of 30D ripstop, as anything lighter doesn’t let the cord run so smoothly, and in time abrades along the opening.
Using the (raw cut) dimensions in the above photo, finished weight was just over 16 ounces. The fit is narrow, on purpose, but long enough to mostly go over the head of someone a 6 feet tall.
The main design challenge was avoiding any exposed seams in the top of the shell, as I really didn’t want to get into sealing anything. To fix the top of the insulation to the bag without doing this, I stitched the liner, shell, and cinch tunnel together (left photo) and then folded the shell out of the way, slid the insulation in, and sewed through the interior seam, insulation, and liner fabric (right photo). Apex is stretchy enough that you can be imprecise here with no problem.
After this, stitch around the side and bottom edges, then put the footbox together.
The footbox is a point down triangle. The photo show it inside out (left) and then right side out (right) in both cases with the top of the bag facing up. What you can’t see well is that the top of the footbox is longest, making the two seams run backwards, with the footbox overhanging them. My expectation is that anything short of serious, sustained rain will not wet this out.
The ~1 pound weight it what I wanted out of this. It approaches down bags of comparable warmth, and should exceed them in damp conditions over a few days. Packed size is another matter. Squeezing air out of the bag is not a simple thing, and without tons of compression it wants to stay as a roughly basketball-sized lump. It will go smaller, but in the game of ounces the pack space this demands is less than ideal.
Field report to follow this summer.
Bedrock & Paradox 
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