I've been using an Exped Downmat 7 M for several years now. First in tents and snow holes and now in my DD hammock.
As we all know, down is a great insulator but useless when it gets wet and nearly so when compressed (under your body).
The downmat avoids both these problems by having a waterproof shell and, when inflated, lofted down that cannot be crushed because of the air trapped in with the down.
This solution works both on the ground and in my hammock. For best results I put the downmat and my Marmot Never Winter inside a NATO goretex bivi bag and put the whole lot in the hammock.
Have kept me warm and dry in temperatures as low as -18C (or 0F if you like awkward temperature scales) and in driving sideways rain.