One candle in an igloo will cause a minor mount of melt, which glazes over, reflecting any more heat. We did this when children, to see if it were true, and it was. The temperature got close to 60 F inside, and Inuit have told me they get to about 56 degrees, making it consistently warmer than most cabins in the north. read more
The igloo, if correctly built, will support the weight of a person standing on the roof. Also, in the traditional Inuit igloo the heat from the kulliq (stone lamp) caused the interior to melt slightly. This melting and refreezing built up an ice sheet and contributed to the strength of the igloo. read more