Because most flat roofs don t have an attic crawl.
Insulate roof with no attic space.
It creates a breeding ground for mold and mildew and ruins the air trapping pockets that block heat flow.
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.
Materials commonly used for the insulation of the roof deck include.
In practical terms you can choose between mineral wool or insulation boards.
An attic you want vented but in your case you want the ceiling roof space sealed.
Insulating the attic floor.
Since your home s heating and cooling system usually does not extend into the garage insulating this area does little to change the cost of running the system.
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.
By laurie brenner homes with flat roofs require a different approach to insulation than the standard approach used for homes with pitched roofs.
With 2x6 rafters this will give about an r 19 roof higher if you use thicker solid foam insulation.
Effects of insulating the garage attic.
If you do not use the attic much it is pointless heating this area.
Works well costs more.
So insulating the floor of the attic is not only the best solution but it is also the simplest and cheapest.
Water is insulation s enemy.
For such constructions the principles and benefits of unvented attic construction are similar to that occurring in a single family residence.
The general answer to the question of whether insulating a garage attic will lower energy costs is no at least not appreciably.
You may also insulate your roof to help regulate the temperature in an attic that is a part of your living space.
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.