How does an igloo stay warm inside?

They do melt if you use them improperly. An igloo is used to protect from the wind, precipitation, animals, and EXTREME cold. Used properly, the interior will not get above the freeing point. If a fire will be lit inside for warmth or cooking, a roof vent will be opened in the very top, not just to let the smoke out but also to keep the heat from building up. They may not be warm enough to sit around in shorts and watch t.v. all night, but just below freezing is still a heck of a lot warmer than gale force winds full of snow at 60 below .