Don't get me wrong, this walls and roof with lots of insulation and windows orientated for the sun makes so much sense for passive heating, and in general insulation seems like the most obvious way to save energy.
At night it has been getting to 17° outside, with it getting in the mid to high 30s during the day. But we have been hovering from 65-69° inside despite the much larger outdoor temperature swings.
To only swing a few degrees means a huge thermal mass that charges during the day from the sun and discharges at night with little temperature change and huge insulation along with say body heat and cooking etc. Not that it's not possible but only varying 4 degrees F with no heating system other than the sun during the day seems like a stretch, will be happy to be proven wrong.
I'm in the process of building a house - it has windows and is nearly fully insulated, but there is nothing done inside yet (the floor is sand). In September we had a cold patch of 10C maximum for a few days, and on the last day I went to see how warm it was inside the house. The masonry on the south wall was 17C, and on the north 15C - and that was with some windows open upstairs. Masonry takes an awfully long time to respond to temperature changes.
My house has a lot of thermal mass so the sun will shine all day on the block walls and heat it up and it takes a while to cool back down at night. It does smooth out the temp swings, but bottom line my air conditioner must remove the BTU's that make it into the house, and it's a lot.
Obviously having much more insulation helps, and ideally you would want the thermal mass inside the insulation, so insulation on the outside of blocks. Then in a cold climate you need the irradiance from the sun to go through windows and heat the interior having the interior thermal mass absorb the heat while minimizing temperature rise.