HAM-Tools is a software library specially constructed for hygrothermal system analysis in building physics. The software is developed as a modular structure of standard building elements and systems using a graphical programming environment. Due to its modular structure and transparency, each of the existing components can be easily changed and adjusted to specific user demands. In order to asses the software’s ability to predict the building hygrothermal response under given climatic and operating conditions, validation tests against measurements have been performed. As a case study, temperature and relative humidity of a cold ventilated attic space in real operating conditions was selected. Validation results are presented as a comparison between measurements and calculations. Parametric sensitivity analyses were performed and necessary adjustments for the missing input parameters have been fully documented. The code has shown a high degree of reliability, both in a qualitative and a quantitative way.