[nyokodo]

> Neither of these are terribly "air tight"

Newer homes in the US often have a hole in the basement explicitly to make them not airtight. This is presumably to avoid CO2 buildup.

[whafro]

Sure, though it's often not simply a hole, but an active ventilation system to ensure fresh air makes it into the structure. Ideally, this is a heat recovery ventilator that uses the heat in stale air to preheat incoming fresh air.

Below about 3.0ACH50, you generally explore active ventilation, which is why 3ACH50 is code, since it doesn't require that extra machinery.

[handrous]

Moisture is the bigger reason for that, I'd expect.

[downut]

Or radon.