Analyses of hygrothermal conditions in low-energy houses is important because of their likely sensitivity for excessive moisture. The presented work deals with real-time measurement of temperature and relative humidity at multiple locations inside a low-energy house envelope. The measured data allows diagnosing approaches towards building design and understanding and evaluating the house performance. Suitability and accuracy of numerical computation was analysed. The Finnish mould growth model was used to monitor risk and extent of mould growth under measured and computed conditions. The measured conditions represent more favourable environment to avoid mould growth than the design values recommended by national and international guidelines. There was no mould growth indicated at any monitored points of the envelope. Monitoring the hygrothermal conditions provides valuable information about the performance of structural elements, building material and the house envelope and it helps to predict moisture related risks during the building’s service life.