[whafro]

Directionally, I agree with this, but it's important to be clear that it varies by jurisdiction. A blower door score of 3.0 ACH50 (air changes per hour at 50 pascals) is code in some states, while a less-performant 5.0 ACH50 is code in others. Neither of these are terribly "air tight" and you can generally get to 1.5 or even 1.0 without too much additional effort.

But my 1920s house – built very sturdily – scored a 19.5 ACH50 when we moved in, and after a not-insignificant amount of weekend work is now at a 10.33, well off from modern code even in lax areas.

[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.

[modo_mario]

How does that happen tho? I'm thinking about the gaps in my own that's only slightly younger and the only gaps i can think of are at the door and maybe there's a gap at one corner of the roof but i can easily fix. Nothing gets trough that brick and mortar.

[whafro]

First, there are many gaps in pretty much every home. Junctions between drywall/plaster joints, windows and window frames, every electrical switch/outlet box, details in the attic and roof, the rim joists and sill plates, and yes, even any exposed masonry.

Secondly, exposed masonry does often warrant being air sealed:

https://www.greenbuildingadvisor.com/article/insulating-old-...

https://www.jlconline.com/how-to/insulation/air-sealing-a-br...