Abstract
Timber houses are a cornerstone of Nordic construction, evolving from traditional log structures to modern energy-efficient timber frame systems. Increased insulation requirements and airtightness have raised concerns about moisture accumulation and mould growth, particularly at the floor-to-wall connection in multi-story timber buildings. This study investigates moisture performance in rim joists, a critical component often exposed to high humidity and considered a common air leakage path. Three rim joist solutions were evaluated: the traditional airtight ceiling approach and two new alternatives featuring recessed blocking using structural timber or technical insulation integrated with the vapor barrier. Moisture and temperature sensors were installed in a detached dwelling in southeastern Norway, monitoring conditions during construction and 2.5 years of occupancy. Results show that all solutions maintained moisture levels well below the 20 weight-% threshold associated with mould growth risk. Seasonal variations were observed, with higher moisture in winter and lower in summer, consistent with previous studies. The new solutions demonstrated improved robustness, with the structural timber blocking performing best. Findings indicate that both new alternatives are promising for Norwegian dwellings, whereas the traditional solution remains reliable but less moisture resilient. These results provide practical guidance for reducing moisture-related risks in timber frame construction.