Installed an fairly overprovisioned heat recovery ventilation system. What effectively means a lot of pipes to move the air in and out of the rooms. Nothing fancy: supply in the bedrooms, exhaust in the bathroom, etc, just a little more air volume moving. Heat recovery system (a fancy word for a aluminum heat exchange) recovers the heat going out of the house by warming the incoming air. Keeps mostly the CO2 levels way below 800 ppm, normally around 600 ppm.