cost to insulate attic Insulating an attic is the best way to reduce energy costs. This is true for newly built homes and older homes. Blown-in attic insulation is a convenient way to seal all drafts in attics. Blown in insulation is far less expensive than the roll type insulation. However, determining the cost of blown in attic insulation is based on the same premise: the size of the attic. Attics vary in size. As such, the square footage of the attic comprises the size and is the main factor in the cost of insulation. For example, it costs less to insulate a smaller 1800 square foot attic than a 5,000 square foot attic. Insulating an attic is a tedious, labor intensive job best left to professionals.