A Debris Hut is the simplest and warmest survival
shelter possible that doesn't utilize fire for warmth.
To build a debris hut, you basically need one long log,
a bunch of smaller sticks and branches, and lots of debris (leaves, plants
etc). The debris forms the insulating layer that keeps the heat (your body
heat) inside the structure, and therefore keeps you warm.
The main log is placed up against a support of some
sort, in this case a standing tree trunk.
The smaller branches are then placed along this log,
forming an inverted "V", somewhat like the classic tent shape.
A doorway is formed at one end. In this photo the door
is at the left.