If the garage is open to the rafters the boards that support the roof use r 38 insulation between the rafters at the roof if they are 10 inches deep.
How to insulate garage ceiling rafters.
The fiberglass insulation comes in rolls that are 16 to 24 inches wide.
To get more insulation in the garage install a ceiling and provide access to the space between the ceiling and the roof.
The paper will fold over the rafter framing thereby allowing for attachment with staples.
In the case of a cold roof structure the insulation is placed between or between and under the rafters or at ceiling joist level and the roof is ventilated by leaving a 50mm gap above the insulation which vents air to the outside.
Be sure to count the spaces above doors and windows.
To prepare determine the area of the space you plan to insulate and add 10 to 20 percent to account for mistakes and odd spaces you need to fill.
So before insulating go around the garage with a can of low expanding spray foam and seal all gaps and cracks that let in the daylight.
This will allow you to add the needed insulation.
The insulation should be installed with the paper facing out and attached using staples.
Push the insulation in place between the rafters but be careful not to compress the material.
Measure the space between joists and studs to determine the width of the insulation you need.
You can insulate the walls ceiling and door of the garage to the highest r value possible but if you fail to fill those air gaps you ll still be wasting a lot of heat.