Gone a week in winter - no heat - no freezing - a Testimonial to a bermed home

Testimonial for our bermed home in the Maine foothills - Closed it up on a Monday morning - no furnace - drapes pulled 1/2 way - temperatures ranged from the low 30's to single digits at night - and it snowed about 10-12 inches one day with wind - we returned in the middle of the night on the next Saturday - living room was 63 degrees and the back bedroom was 61.  Can anyone beat that?