We would always recommend having a breathable membrane in place between the tiles and the rafters too to allow the roof to breathe.
How to install insulation in roof rafters.
Air circulates from the lower part of the roof overhang through the rafters.
How to install rafter insulation.
Install ventilation baffles into the space between the rafters before you install the insulation usually by stapling into place.
Use your straightedge to compress the insulation before cutting and use a very sharp utility knife to assure a good straight cut across the fiberglass batt.
Once you have these in place measure the length of the spaces between your rafters.
When you use solid insulation board to create a warm loft the key thing to remember is ventilation you must keep a 50mm gap between the roof and the rigid insulation board.
The baffles will allow circulation to continue even though insulation has been installed directly in the path of the rafter air movement.
Cut each piece of insulation an inch or two longer than needed to assure a snug fit.
When installing rigid insulation in an attic to be finished as a living space you must provide a route by which hot air can escape.
If you have a solid floor covering the roof rafters use the claw end of a hammer to pull the floor up in several locations and blow the insulation under the boards.
Typically you will fill the insulation blower with the insulation and blow it between the attic floor joists.
Then blow another layer on top of the roof rafters.
They are directly below the roof sheathing and are were insulated with fiberglass batts stapled between the 2x4 rafters.
Garages built without attics are.