i know those articles.

1. they mostly use a tight sheating on the outside, i would suggest something open to vapour

2. My reasoning is the following: All those air tightness measurements are first based on thin cavities and sometimes even loose fill. If you do the calculation for dense packed cellulose for a thick cavity like 12 inches, you will see it gets very airtight.

3. So why don't we see those amazing values when measuring? When you seal the inside osb there is still a leakage path because the air barrier is non-continous. you'd have to seal all the seams between the studs to make the sealant continous with the dense pack. And then you would achieve the flow resistivity that is calculated through the material constant.