Brace yourselves; this'll probably be a long one.
I have a Hennessy ULB Asym and have been reading far too much on here and a few other sites about insulating the bottom. I'll be giving it its first use in a week on a seven or eight night hike on the AT in CT/MA. The nighttime weather up there has been in the 50's lately.
I've been experimenting with various pads the last week or so. I'd finally settled on a cheap 3/8" CCF pad from Target that was 27" wide. I trimmed it to fit and it was pretty good, but then I read Sheldon's post yesterday and got scared again of sweat/condensation because of the pad.
Since starting this hammock thing, I've thought an underquilt seemed like the best thing, being very warm and on the outside, so that once you climbed in, you wouldn't have to think about it or mess with it. I don't want to use down, though, and it seemed like a complicated thing to make myself, having to worry about being sealed correctly, not compressing the insulation, etc.
A buddy gave me a $75 REI gift card last night, so I went up there today to look for something to make an underquilt with. I was about to buy an REI short Polar Pod, then found this North Face Allegheny:
It uses TNF's proprietary HOT insulation and is rated to 40 degrees. It's just a rectangular bag with a drawstring at the neck and a full zip. Best of all, it's drab green and black. Weight is 2lbs, 9 oz. I would have gotten the regular, but they only had the long size, so I got that. Cost was $70. Insulation looks to be 1.5" high.
I was going to just use the top half of it and put drawstrings at the head and foot, but then found these plans specific to the UL Asym:
I unfolded the bag into a big rectangle, then cut it to the dimensions listed. I was able to preserve enough of the top drawcord channel to use and then sewed a silnylon sleeve onto the foot. I sewed tie-out loops to the six corners. On the top and bottom drawcords, I tied one end to the tie-out loop at that corner and the other I fastened a large-ish cord lock to use as a stop.
The only change I made to the plans was to cut darts at the two side corners. The darts are ten inches deep and nine inches wide and do a perfect job of keeping the sides snug.
I cut the shape out, sewed the cut edges closed, then sewed a hem by just double folding them.
It fastens to the tarp with 550 cord at the head end, elastic at the foot (so it'll slide to the side and back when you get in) and connects directly at the side to the hammock side tie-outs. All quilt lines attach to the hammock attachment points with very small hooks. The head and foot lines each attach to a sliding 550 loop on the tree lines.
It took about five hours to make (including dinner and two episodes of Family Guy). The result is much nicer than I expected. The fit is very good and it seems like there's plenty of room to be insulated in a variety of positions. Finished weight is 24 oz. including all lines and a stuff sack. The volume is lower than my sleeping pad, so I think it'll pack up nicer (plus, I've always hated packing a non-Z-Rest pad into my pack).
Not quite as warm as a JRB, and 4 oz. heavier, but $150 cheaper and synthetic for those of us into that. I also really like the green/black thing. I think if it works as well as I hope on my upcoming trip, I'll probably wind up making a nicer one from scratch from a Ray-Way quilt kit. I could probably make it for the same weight, but warmer.
Anyway, that's the end of my fascinating tale. Here are some pictures. There's one with it on my empty hammock, one with my girlfriend in it (now she's two kinds of hot), one of it packed into its stuff sack, and the last one of it laid out on the ground.