In late 2016 I bought a CO2 monitor to check the air quality inside our house. On the basis of this monitoring, which revealed poor ventilation, we decided not to seal up our house in order to save energy. We chose rather to live in a fresh-air house keeping our windows open throughout the year. We coupled this with keeping warm by heating ourselves, not the air in our house. Very comfortable; very energy efficient. At that time I produced a Microsoft Sway which described my monitoring and decision making process.
Since that time I’ve heard little mention of indoor CO2 monitoring in the mainstream media but in recent weeks it has suddenly become a topic of great interest. Indoor air quality experts are now talking seriously about using indoor CO2 levels as an indicator for the likelihood of Covid 19 spread. High levels of indoor CO2 = low rates of fresh air exchange in a room = potential high viral load in the air people are breathing.
The moral of the story appears to be simple: monitor the CO2 in your living areas and ventilate well. In my view breathing good air, and minimising the spread of Covid, is more important than saving energy [though I believe in our house we have managed to both improve our air quality and lower our energy use].