Do not use plastic sheeting in this case.
Insulating attic raftes.
Insulation if your attic is going to be converted into livable space then you will need to insulate the roof rafters.
You can buy air ducts that staple to the inside of your roof above the eave vents and fit between the rafters.
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.
The ones i ve used are made of styrofoam and iirc they cost around 2.
You can use rigid foam panels or do as tom silva does with nearly all houses these days.
In unfinished attic spaces insulate between and over the floor joists to seal off living spaces below.
Hire a pro to install spray foam insulation.
Install ventilation baffles into the space between the rafters before you install the insulation usually by stapling into place.
If the air distribution is in the attic space then consider insulating the rafters to move the distribution into the conditioned space.
1a attic access door.
Garages built without attics are also prime candidates for insulation.
You should only insulate your rafters if you have baffles installed to separate the actual rafters themselves from the insulation.
That s because it s easier for the contractor to spray than clean up existing insulation deal with any can lights or venting etc.
If you re not do the attic floor.
To meet all three goals insulating your finished attic ventilating the roof and maximizing headroom use a combination of dense batt insulation rigid foam sheeting and air chutes.
The will hold open an air gap of approximately 1 inch to allow airflow past a layer of insulation.
If you re finishing your attic insulating it to the proper r value can cause a dramatic loss of headroom if you limit yourself to fiberglass batts.
Apply insulation between rafters when finishing an attic into a heated room or when building a cathedral ceiling in the home.
Consumers are often told by spray foam contractors to do the rafters.
If there is a floor you will need to remove it to properly insulate the roof rafters.