There was one other addition to the igloo roof - a hole was cut for smoke from any fires that might be lit inside the igloo. We, of course, did not light any fires when we were in the igloo. But, if you were out on the Arctic Ocean and you needed an emergency shelter, the igloo would be your best protection. A fire inside would make it very comfortable. Any smoke from the fire would need an exhaust point - hence the hole in the roof.