The present disclosure provides a method of producing an insulation preform having graded porosity for an exhaust treatment component of a vehicle. The method includes obtaining a first granular insulating material having a first diameter, a second granular insulating material having a second diameter less than the first diameter, an inorganic binder, and water. The method further includes producing a slurry comprising the first granular insulating material, the second granular insulating material, the inorganic binder, and water. The slurry is introduced into a mold having at least one surface adapted for vacuum extraction. A liquid phase of the slurry is evacuated from the mold using vacuum extraction to produce a moist preform. The moist preform has graded porosity such that a greater concentration of the second insulation material is adjacent to the at least one surface than the first insulating material. The moist preform is heated to produce the insulation preform.