Out of curiosity, what is the effect of this thick insulation on indoor CO₂ levels? Do these houses usually have ventilation systems that can compensate?
Yes you need active ventilation systems, because the thermal efficiency is highly dependant on the house being airtight - to the point where efficiency scores are calculated by overpressuring the house (basically taping a huge fan into the front door with all other doors and windows shut) and measuring the pressure loss. When applying insulation, contractors have to meticulously use foam to seal everything up. It's a major part of the work when building a new house and just a few mistakes can seriously affect thermal performance.
Our house (with a ground based heat pump) uses active ventilation with heat exchangers (and pollen filters) and as a result we have better indoor air than we've had in rented flats in all years before.
Usually heat exchanger ventilations. Fresh air comes in and exchanges heat with used air coming out, so you get fresh air but with the indoor temperature.
HRV (Heat Recovery Ventilator) systems are really cool, too. They solve the ages old problem of bringing fresh air into a home without losing all of the temperature and humidity from the internal environment.
They are designed for a number of air changes per hour/day which would be analogous I guess. I can't remember the number but a Google will tell you, it's different depending on the kind of environment (office/home etc)