The Igloo effect refers to the phenomenon that allows igloos and similar structures to remain warm on the inside, despite being made of snow and located in frigid environments.
The effect is primarily due to two factors: insulation and body heat.
An igloo constructed with snow blocks will thermally insulate its interior from its exterior.
It is known that, whilst snow itself has a temperature below the freezing point of water 0 °C (32 °F), it has excellent thermally insulative properties as it consists of air pockets trapped between ice crystals.
Also, smaller igloos require less heating than larger ones to achieve the same temperature level.