It is not necessary to insulate the area between the rafters in an attic. Leaving the space between the rafters open promotes airflow and reduces moisture.
According to the Sustainable Energy Authority of Ireland, fiberglass insulation traps and holds heat, which can cause the roof and rafters to begin rotting within a short time. To turn an unfinished attic into a usable living space, homeowners should use rock wool or another non-fiberglass insulation. They should also install ventilation fans to reduce the risk of moisture that causes damage to the structure of the home.
Generally, it is best to install insulation in the floor and walls of the attic, rather than between the rafters, to reduce heating and cooling costs inside the home. Insulating the attic floor and walls maintains the temperature inside the home without the risk of damaging the rafters.