Polyethylene
A standard method of doubling up on control layers - but it has to be done correctly and carefully.

If it's already being used as the vapor barrier on the warm side, some attention to detail could allow polyethylene to double as an air barrier too. Therefore cost is reduced.

Ideally, air barriers should be installed on the 'cool' side. However, if used on the warm side, the ideal way to ensure that polyethylene is effective as an air barrier requires the sheeting to be sandwiched between two layers of drywall, meaning added cost.