I did that in the last weeks. I live in Germany and we have a flat that is not very modern but has reasonable insulation. What I learned that under these conditions, with two people in a 30 square meters room, CO2 rises so quickly that one has to do intermittent, full ventilation (about 3 to 5 minutes, in Germany it's called "Stosslüften") for about every hour, to keep CO2 levels below 1500 ppm. During the night in a smaller room, one can get easily levels of 3000 ppm. That's not dangerous, but it is not recommended for places where people have to work in a concentrated manner, like offices, or schools, for example. The recommended limit by the federal office for environment is 1000 ppm.

And that build-up of the stuff is a big difference to where we were living before (Edinburgh) where that kind of ventilation was basically unnecessary because no window closed that hermetical (well we lived in one of those wonderful 140 year-old houses in New Town).

In the end, with good insulation, it becomes increasingly important to have some active ventilation - it is also more economical, because it saves on heating. And that is in fact becoming more widespread with new family houses.