Any high performance Builder will tell you that if there is any magic in building to super high standards it is in the Air Sealing! The Passive House Institute sets a benchmark for buildings to approximately 0.6 ACH50 for minimum certification. What does this mean? It means that when measured with pressurized testing equipment, the building being tested needs to have almost no holes in the exterior walls, floor or roof. These holes, even if they are small ones, let through energy & water making the interior space less comfortable, and less healthy. Even super small cracks and openings let in dust that is itself super small. These particulates build up over time since they can't get back out through the same holes they came in though (think about how you could get salt back into a shaker through the same holes it comes out of).
There is quite a bit of literature on the many gallons (yes GALLONS) of water that can accumulate through even the smallest holes in an assembly. This is the main way that assemblies can have issues with decay and mold or mildew growth, and eventually cause failures.
ACH50 means Air Changes per Hour at 50 pascals. Just like you can blow up a balloon with air pressure, a building can be pressurized with a computerized fan. The computer can record the volume of air that is being moved through the fan, and this volume of air has to come from someplace, right? So that volume is relative to the openings in the assembly. The more openings, cracks, or holes in the assembly, the more air goes through the fan at a set amount of pressure. The less air that goes through the fan, the fewer the openings....pretty simple really.
The 0.6 part of the requirement is the minimum that the old system would allow, but the issue was that if you had a large building the amount of allowed openings (no structure built in the real world can have no cracks, of any kind) is large and the smaller the building is the small the amount of openings allowed. The obvious issue with this was that it then becomes easier to build bigger and harder to build small. The exact opposite of building to an environmental ideal!
The solution to this is that the requirement needs to be linked to the square footage of the house in a more direct way. So the new standard is a bit more complicated, but more fair too. <0.05 cubic feet per minute/sqft. of shell area @ 50 Pa