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 scen

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.