Tricks to add warmth if it is not enough at 15 or 20:
1: a space blanket between hammock and UQ, and/or vapor barrior clothing ( just make sure you understand how to use these and practice some, otherwise stay away)
2: even a thin CCF pad added on the coldest nights can add 10F easy- maybe not as comfy but better than shivering
3: sleep in fleece layers or inside a synthetic bag
4: hot water bottle ( don't spill it! )
5: lots of head insulation to make up for the lack of a mummy bags hood
For I consider that the sufferings of this present time are not worthy to be compared with the glory that is to be revealed to us....that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God. For we know that the whole creation groans and suffers the pains of childbirth together until now.