If you put cellulose up there it will help insulate but you need a vapor barrier above the ceiling because you ll have warm ceiling air hiiting colder roof surface and condensing which will turn the cellulose into a moldy mess.
Insulate roof with no attic space.
For such constructions the principles and benefits of unvented attic construction are similar to that occurring in a single family residence.
Materials commonly used for the insulation of the roof deck include.
With 2x6 rafters this will give about an r 19 roof higher if you use thicker solid foam insulation.
Fill the remaining space with fiberglass or cellulose.
But spray foam insulation used at the underside of the roof deck would need to be covered by a thermal barrier and intumescent coatings are commonly used.
Insulating the attic floor.
The general answer to the question of whether insulating a garage attic will lower energy costs is no at least not appreciably.
Insulating the roof is a priority if the ceilings of your home happen to be part of the roof deck as is the case with cathedral ceilings.
By laurie brenner homes with flat roofs require a different approach to insulation than the standard approach used for homes with pitched roofs.
This gives the best air path eaves to ridge but costs more for the labor to nail furring and cut insulation into pieces to fit between rafters.
In practical terms you can choose between mineral wool or insulation boards.
Water is insulation s enemy.
If you do not use the attic much it is pointless heating this area.
Works well costs more.
Look for water stains on the roof sheathing or damp or moldy spots on attic joists and existing insulation as a clue to where leaks might be.
An attic you want vented but in your case you want the ceiling roof space sealed.
Box out light fixtures.
Effects of insulating the garage attic.
So insulating the floor of the attic is not only the best solution but it is also the simplest and cheapest.
You may also insulate your roof to help regulate the temperature in an attic that is a part of your living space.