Lived in one for 4 days in the Finse area of Norway a couple of years ago
We did get the temperature up to 4 degrees with 4 burners going, but the roof started to melt.
Advice on living in snowholes, IMHO is;
Use a good sleeping mat.
Use a good sleeping bag.
Use a bivvi bag to keep your sleeping bag dry.
Don't breath into your bivvi bag, or sleeping bag, your breath will condense and wet it.
Ventilation holes are a must! best case scenario is you get a screaming headache, worst case you don't wake up.
Eat lots, drink lots of warm drinks, you need the calories to keep warm.
Use a candle to heat the interior, and to monitor the ventilation. When the candle goes out, open the ventilation holes up!
Our so called expert chose the frozen lake surface to build the snowcave on, bad move as there was nowhere for the cold air to sink away from us, so we had a fairly cold time of it. We should have taken our cue from the Norwegian Mountain Rescue who were building theirs further up the mountain.
I reckon we had about a metre of snow above us for the roof and that supported our weight, and the weight of the Norwegian Mountain Rescue guys skiing over the top of it.
The last couple of nights we had it blew a storm up outside and noone wanted to go outside to get snow to melt for brews and cooking, so we just started to cut lumps out of the walls. It got quite roomy after a while!
Did I mention it was cold?