An Igloo is an efficient shelter made of snow/ice blocks. At first thought, the snow is cold so how could it keep one warm. The true value is in the insulation. The design creates a small insulating shelter. The cold weather stays out, the body heat stays in. This is part of the reason they are built small, like tents. The small volume is easier to heat by residual body temperature alone. The more people in the structure, the quicker and warmer it will become.
The human body averages 98.6 degrees Fahrenheit (37°C), the reason for hypothermia is the environment is sapping too much of that heat energy away from the body. By creating an igloo or similar structure, it is possible to prevent the body heat from moving too far away. As the body releases heat energy, it becomes trapped in the igloo by the thick snow walls, and builds up. The interior will not reach 98.6, but it is possible to raise it to a much more comfortable temperature than the frigid outside air.
One thing to remember with igloos is that they are cut down into the snow pack, not built on top of it. This allows a smaller surface area presented to the outer environment. Also, the orientation of the door must be taken into consideration. Allowing the cold wind to drive right in through the door is not a good idea.
Further, the snow itself will still be cold to the touch. Proper clothing is essential, the igloo just helps to maintain an environment with a higher ambient air temperature, thereby slowing the rate of heat loss from the human body. Any structure performs the same function, the igloo works with body heat because of its small size compared to the size of the occupants. Obviously, if the igloo is built too large for the number of occupants, it will not be possible to heat with residual heat alone.