Also, do not use a flame in your tarp if there is snow on the ground under it. Snow absorbs CO2, so you can smother and not wake up to avoid it. CO2 is the trigger for realizing that O2 is scarce, and waking a person up to react.
A candle has been used in a quinzee with a snow floor for, well, longer than you and I have been around. You do need vents but I can't see a tarp getting set up tight enough to not allow fresh air in. Cooking inside a snow shelter or tarp is definitely something you don't want to do though.
I will have a completely closed in hammock setup with my 10X11 tarp and doors shut tight. 20* tonight..was wondering if a carefully positioned candle lantern will make much of a dif warming the inside?
oh, you're a brave one!! My luck would have a freak wind coming up and pushing my tarp right into the flame.