A method for forming a vacuum insulated structure using a prepared core material includes preparing a powder insulation material defining a bulk density, pre-densifying the powder insulation material to form a pre-densified insulation base, crushing the pre-densified insulation base into granular core insulation to define a core density of the granular core insulation, disposing the granular core insulation having the core density into an insulating cavity defined within an insulating structure and expressing gas from the interior cavity of the insulating structure to further densify the granular core insulation to define a target density. The granular core insulation defines the target density disposed within the insulating structure defines the vacuum insulation structure, wherein the target density defines a density in the range of from approximately 80 grams per liter to approximately 350 grams per liter.