In this series of two papers, the humidity sensing of a carbon nanotube’s (CNTs) network-based material is studied through quartz crystal microbalance (QCM) sensors. To this aim, quartzes functionalized with different amounts of sensing material were realized, exposed to different humidity levels, and characterized. In this first paper, the theoretical framework is presented, whereas the second one presents the experimental study. This paper discusses at first the water adsorption and desorption on single-wall carbon nanotube (SWCNT) networks, and subsequently deeply investigates the behavior of QCM-based measurements. Numerical simulations based on the equivalent electrical model of the quartz were used for predicting the vibrational behavior of functionalized QCMs when exposed to different humidity levels, accounting for the effect of the different water adsorption mechanisms: chemisorption, physisorption, and capillary condensation.