Before last night, the coldest temperature I've been able to take in a hammock was about 45*F. Any colder than this and I'd have to abandon camp at about 4:00am for the house (I sleep out most nights, it's more peaceful).
How did I get from 45 to 28? Good question.
Here's the system that kept me barely comfy at 45*F:
hammock: Hennessy Ultralight Backpacker pre-asym
under insulation: two Z-rest pads
over insulation: 30*F bag or quilt (I used different bags on different nights).
worn: jeans, Tshirt and Polo, Moonstone Cirrus pullover, fleece watch cap
Occasionally, I'd get the urge to rig an underquilt from an existing bag, but it doesn't work so well on a Hennessy without the proper tieouts.
Here's the system that I used last night, and that kept me warm at 28*F:
hammock: homemade 8 feet long speer-style hammock
Under/Over insulation: Marmot Never Summer 0*F bag.
worn: jeans, Tshirt and Polo, Moonstone Cirrus pullover, fleece watch cap.
Without an integral bug net, I was able to zip the sleeping bag around the outside of the hammock, and while it was a little tight, I was definately warm...Too warm, in fact, at some points. I suppose you could say that I cheated a little, but since the sleeping bag only went up to my chest, I laid on top of a down jacket, which also insulated my neck and sides of my face. Still, not too shabby for 4.25 pounds.
I figure with a little DIY ingenuity, I can get the weight down some.
The best news of all, though, is that I don't think I hit near the bottom of this setup's comfort temp. Sure, it's a 0*F bag, but in a hammock, I think it might keep me warm into the teens, or perhaps the single digits.
thought I'd share.