[weberer]

>The good news is that I stopped worrying about making my house "tight" for the sake of energy efficiency. I keep some windows cracked all year and don't worry about how tight the door seals are.

You could also install an energy recovery ventilation (ERV) system.

[Cthulhu_]

Houses where I live were fitted with those. People turned them down to the lowest settings because they were loud. Then people developed health issues due to high CO2 and they had to refit the systems.