A lot of people have a windproof fabric that they attach outside/below the underquilt in order to increase the warmth from the quilt with very little weight penalty.I used a partially inflated Exped Downnmat for a long time between the layers of DD Frontline, and found it warm and also that it flattened the lay slightly. Combined with either a Snugpak top quilt or a sleeping bag. When the Downmat eventually started to leak around the valves (after 15 years, so not too miffed), I bought a DD under quilt. This, in my opinion, is superior when temperature is 10C or more, but I do not find it sufficient on its own below that. Other opinions differ, of course. I have been thinking about laying further insulation - a cut down sleeping bag for example - between the under quilt and hammock, but suspect that without being fastened in some manner it may just eject itself in short order.