Use 7/16 OSB as air barrier in HVAC channel – GreenBuildingAdvisor

OSB makes a decent air barrier, but why not just use conventional ductwork formed from sheet steel? The smooth inner surface of steel ductwork is a big plus for air ducts for various reasons (less resistance to airflow, less accumulation of dirt over time, etc.). If you want to use a rigid material, I’d try something with a smooth surface like masonite, then put the smooth surface towards the interior. Note that any wood product will be much more prone to air sealing issues than steel ductwork is.

BTW, you may also have issues with fire code here. Air handling spaces often require the use of nonflammable materials.

Bill