With one inch extruded polystyrene foam board insulation between the drywall and the roof members the r value increases to r 25 with no thermal bridges.
Insolating the attic directly on the roof joints.
Do not use plastic sheeting in this case.
Essentially this system offers excellent insulation value because wood conducts heat and cold.
If you re insulating the roof directly to insulate the roof itself you either use spray foam or retrofit with it with rigid foam board.
They are the easiest to insulate because the insulation is installed between the framing members or joints of the floor according to the university of tennessee extension.
Insulating an unfinished attic can be a simple diy project.
The technically correct approach depends on a number of factors.
Place the plastic sheeting on top of the rolled insulation.
If the plastic sheeting is in pieces overlap the pieces by 8 inches and staple it in place directly to the roof rafters and the wall.
An unfinished attic is an attic that is not considered part of the living space.
If there is a floor you will need to remove it to properly insulate the roof rafters.
Whether or not there is an inner roof lining and the total thickness of the roof structure.