Oh, I didn’t say it was comfortable. It was just an illustration that even the unfinished house was capable of creating a 40-degree-plus temperature difference in midwinter.
What is the difference between “fine” and “comfortable”?
It does seem that temperature regulation on several cloudy days would be difficult without some supplementary heat.
It was fine until I got out from under the covers. 
once I was up, it was chilly. But once I opened the door and felt the difference between +6 and -30 (it has warmed up outside, and a brilliant sun was shining), I knew how well the place could capture heat.
That’s where the thermal mass comes in. Because there is so much heat stored in it, the interior of the house changes temperature slowly. It can compensate for rapid changes.
The worst-case scenario is three or four months without sunlight during cold weather. This would deplete the heat store, and it would take a long time to recharge. For this reason, supplemental heat can be used. Fortunately, our local climate is usually sunniest when it’s coldest.
This technique needs to be used with more than just an eye on the local climate.
We’ve [thread=584284]closed on a house. [/thread] Thanks.