Box out light fixtures.
Insulate roof with no attic space.
Water is insulation s enemy.
You may also insulate your roof to help regulate the temperature in an attic that is a part of your living space.
Effects of insulating the garage attic.
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 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.
An attic you want vented but in your case you want the ceiling roof space sealed.
Because most flat roofs don t have an attic crawl.
Materials commonly used for the insulation of the roof deck include.
Fill the remaining space with fiberglass or cellulose.
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.
If you do not use the attic much it is pointless heating this area.
With 2x6 rafters this will give about an r 19 roof higher if you use thicker solid foam insulation.
Works well costs more.
The general answer to the question of whether insulating a garage attic will lower energy costs is no at least not appreciably.
It creates a breeding ground for mold and mildew and ruins the air trapping pockets that block heat flow.
Insulating the attic floor.
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.
For such constructions the principles and benefits of unvented attic construction are similar to that occurring in a single family residence.
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.
So insulating the floor of the attic is not only the best solution but it is also the simplest and cheapest.