Underlayment is structural. I am guessing but air leaking sheathing would mean actual voids and I would consider that defective. You may mean vapor barrier and no sheathing is a vapor barrier except green ones that has plastic coat on it.
I would be interested on air passing sheathing if it was engineered as such for lighter weight, but historically anything made to reduce weight has not been great as structural material. Also not cheap.
I would be interested on air passing sheathing if it was engineered as such for lighter weight, but historically anything made to reduce weight has not been great as structural material. Also not cheap.