The best snow to use for building an igloo is snow which has been blown by wind, which can serve to compact and interlock the ice crystals. An igloo that is built correctly in a spiral from the inside it will support the weight of a person standing on the roof.
To heat an igloo a soapstone lamp was used causing the interior to melt slightly and refreeze - strengthening the igloo. The temperature in an igloo can raise up to around 0°C when it’s –40°C or from 2°C up to 10–20°C with use of the soapstone lamp and body heat.
The igloo is truly an ingenious design and a great testament to the Inuit people.
Source: CBC, The Canadian Encyclopedia, Wikipedia