That temperature drop sounds very unlikely. If you live in a well insulated home then you must have ventilation too, and that alone should replace the air inside multiple times each day.
I have measured this while being away for a week during winter.
While there is ventilation in the building, it's off while noone is at home.
Although the scenario in question was for a complete blackout where the ventillation would be off as anyway, your are right that people inside would need to open windows every now and then which would increase heat loss.
While there is ventilation in the building, it's off while noone is at home.
Although the scenario in question was for a complete blackout where the ventillation would be off as anyway, your are right that people inside would need to open windows every now and then which would increase heat loss.