Our personal record is 34 degrees with a bare shell and we did not have any significant comfort issues. We used a 4″ memory foam mattress we found on overstock.com and did not have the headliner or any active heating system at the time, but we woke to significant condensation (which the headliner addresses). Our plan is to use 12V/USB heated blankets if we were to camp below freezing for more than a few nights in a row.