I am preparing to improve a building in a cold climate zone. Since it does not seem to be properly insulated, I am reading a book about insulating and weatherizing. That teaches me that I should create an airtight thermal boundary. I have many questions about that, but this one is about the airtight aspect in particular.
Although the exterior of the building has largely been neglected, the interior has new drywall installed almost everywhere. Can that drywall act as an air barrier? I have found an article about the airtight drywall approach, but it is not clear to me whether this will be a feasible approach for a situation like mine, where the drywall has already been installed. Somewhere else, I read that "air does not move through most solid objects (concrete blocks and masonry in general are notable exceptions)", so since drywall is not mentioned as exception, that seems to confirm that it should work fine.
In order for a drywall to act as the air barrier, does it need to be a special kind of drywall? Or does it need to be treated with special paint at all? Does anybody have experience with the airtight drywall approach?