The minimum thickness of a zinc phosphate cement base required to provide effective thermal insulation is 0.75 mm. This thickness is critical because it ensures adequate protection of the pulp from thermal stimuli caused by temperature changes during restorative procedures. Thinner layers (<0.75 mm) do not offer sufficient insulation, increasing the risk of pulpal irritation or sensitivity. Conversely, excessive thickness can compromise the mechanical properties of the restoration without added benefit. Therefore, maintaining a base thickness of at least 0.75 mm optimizes both thermal protection and material performance.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.