The coldest I have slept in a hammock is -14C.

I *regularly* sleep in a hammock overtop of snow, and in lows that hit down around -5C.

I use a 27 inch wide blue foam pad inside the hammock.
I then rig a silnylon poncho (an integral designs silponch) with a shockcord underneath the hammock, as a windblock and insulation holder.

Normally, for me to get down around freezing.

If I expect lower temperatures, I add 2 folded mylar space blankets to add a bit more air gap between my poncho and my hammock bottom.

I have then done down to -10C by adding a hammock shaped piece of poly batting insulation from wal-mart.

Two weeks ago I was down to -14C in marten hills, alberta, using the hammock, with a silponcho, the pad in the hammock, and an exped wallcreeper pl primaloft sleeping bag (0c rated) unfolded in between the hammock and the poncho. (I was not actually expecting it to be that cold, but I did fine, and slept good. I wouldn't want to go a lot colder in that rig though!)

In those cases, I had a good down bag in the hammock, and was wearing fleece, baselayer, toque and a merino wool top or two. in the -14C case I also wore a 300wt fleece top.