An Igloo is a Specialized Building which must be built on Snow. Entering an Igloo will slowly raise your Body Temperature and help fight Hypothermia when exploring the Snow biome. An Igloo is almost impossible to build during any season other than a Coldsnap as Packed Snow will melt even when built into a construction sign post. You Dig the floor in an Igloo to receive Packed Snow.
Specialized Buildings are buildings which generally must be built on Pavement and can be entered and but don't always not count as "inside". Generally these buildings can be Locked.