IS IT COLDER INSIDE OR OUTSIDE A SNOWCAVE?

My son did a science fair project on snow caves this year.  He built one up about 7 feet tall and 20 feet wide and dug a space out for two people to lay in.  I thought it would always be warmer inside the cave, but there is a point, even with people inside that it starts acting more like a refrigerator and is cooler inside than out.  

First we tested it with two young women sleeping inside overnight.  That night there was just a tent material front door on the cave.  These are the results:

The next experiment was with a 9 hour candle lantern inside.  It tipped over in the middle of the night and went out.  Once again, it just had a thin tent material door on the opening.  These are the results:

The next night we tried again with the 9 hour candle lantern.  This time it did not go out until 6:30 am.

Then we covered the door with a summer sleeping bag to make the cave more airtight and we tried it with no one inside and no heat source inside.  The temperature spiked during the day, but the inside temperature was steady.

The next test was with a candle and the door covered with the sleeping bag.  The candle went out at 6am, but only kept the cave a couple of degrees warmer than without it.   And it was still colder in the cave than outside.

Two boys laid inside the cave and watched part of a movie on a tablet with the sleeping bag over the door.  Two bodies only raised the temperature by about 6 degrees.  It was still colder inside the cave than outside.

Bear in mind that a snow cave protects you from wind and rain and snow, but all other things being equal, it is only warmer in the cave if it is below 23 degrees outside and you have covered the door with a sleeping bag.

He took first place in the 6th grade and 2nd place overall.  Good job, Buddy!