The principle behind an igloo is hidden in the material it's constructed out of. Igloos are normally built from compressed snow, which is sawn into blocks, and then these blocks are stacked around a hole, which is dug out after the blocks have been set. Solid ice is a poor insulator, when compared to compressed snow. The snow has many many more air pockets per cubic
foot, and is also lighter. Also, igloos do not have flat bottoms. The inside of the igloo is tiered, or terraced, the uppermost level being where the people sleep, the middle is where the fire is and the work takes place, and the bottom level actually is a "cold sump". The principle is that all the coldest air from inside the igloo runs downward off the terraces and collects in the bottom, thus allowing the upper portions to stay warmer.
The entrance for the igloo is usually at the bottom, and includes at least one right angle, which keeps the high winds from blowing straight into the igloo and chilling the residents or blowing out the fire. They also all have a small hole on the top that keeps the smoke from building up inside the igloo. All of these factors take advantage of underlying physics, and the temperature inside an igloo is likely to be 20 degrees or so, while the outside temperature in northern regions can drop down to -50 degree Fahrenheit during the daytime. 20 degrees may not be what some consider to be comfortable, but a 70 degree difference is certainly welcome somewhere so cold.
Frank DiBonaventuro, B.S., Air Force officer, Tinker AFB, OK.