Why don't Igloos melt from inside?

An Igloo is a dome-shaped house, typically built from blocks of solid snow. 

 The igloo is a temporary winter shelter built by Alaskan and Greenland  Inuits,  commonly while traveling on hunting expeditions.

Inside of Igloo these Inuit people burn oil lamps, which, coupled 

with the emissions of body heat, can bring the interior temperature up to around 10 to 15°C.
Even with this relatively high temperature, far above freezing point, it is sometimes wondered why the insides of igloos do not melt.
 In fact, the inside of the igloo does melt, but not to a large extent. When it does melt slightly,  the water quickly re-forms as ice on contact with the snow and ice next to it, while the cold outside air and the thickness of the blocks of snow which, like ice, is an excellent insulator ensure that they quickly refreeze.
At the top of the igloo is a hole to allow ventilation. This is also a key reason why igloos don't melt. The hottest air and smoke rise up through the hole.
The entire igloo does eventually melt, but not until the outside temperature rises with the change of seasons.