The book explores the physical influences and boundary conditions surrounding interior insulation. It focuses on minimum thermal protection and moisture protection. The first part provides a simple introduction to the physical relationships of interior insulation. In more in-depth sections, commonly used terms such as condensate, dew point, and capillary activity are examined. Additionally, professional relationships concerning the hygrothermal material properties and potential failure criteria are explained. The second part, based on the established fundamentals, addresses the practical approach to planning and sizing interior insulation. Starting from the assessment and goal definition of the measure, the legal and normative requirements are presented. The associated calculation and verification methods are explained and illustrated with examples.