[Cthulhu_]

It's a tradeoff, because good ventilation - ideally just open a window - also means heat is leaking out of the house, which costs money (and co2 emissions) to restore.

There's cyclic systems, but I live in a neighbourhood where some houses were equipped with it; on the proper setting, it was too loud so people turned it down, then people got sick from high CO2 levels in their house.

[BizarroLand]

There are ERV and HRV systems that will cycle fresh air in while exchanging the heat and moisture from the outgoing air to the incoming air.

It's a neat technology but the jury is still out on it.

Some people praise them, other people revile them, and they seem to be either too bulky or too cumbersome or too expensive or too inefficient for a DIY retrofit project.