The underquilt is what keeps your back warm at night. In a hammock you need something under you to stay warm, so I made a synthetic underquilt because the insulation is easier to work with. It also helps with moisture, but that's beyond the scope of this right now.

It has multiple tie-outs that I attach mitten-hooks and shock-cord to, so they can be easily adjusted or removed. They attach to almost identical tie-outs along the edges of my hammock, making this a very versatile system I can fine-tune as my needs change.