This article develops a thermodynamic description of hysteretic behaviour of moisture in materials. The moisture sorption is described by a ‘condensation potential’ using a ‘pore interaction energy’ that is considered as an attribute of the condensate. Surface subsystems, (liquid–solid) and (liquid–gaseous), are not needed. In conformity with the classical domain theory, hysteretic effects can be explained as metastable states of the condensation potential (static, time-independent hysteresis) or as unstable states (dynamic, time-dependent hysteresis). The hysteretic extension of the standard heat, air and moisture transport model is derived in an easy and efficient way using a combined-phase thermodynamic approach. This article comprises three parts. First, the general thermodynamic description is given, and later, the condensation potential will be calculated for many different physical sorption mechanisms.