This research focuses on comparing Accoya® with unmodified wood and investigates Accoya®’s structural performance in environments with varying relative humidity levels. The wood’s moisture content, and consequently its swelling and shrinking behavior, fluctuates on an annual basis due to these varying relative humidity levels, especially when exposed to outdoor conditions. FEM analyses were conducted to investigate physical properties and the performance of moment-resistant connections, with their implementation aimed at incorporating rotational stiffness into a structural portal frame. Accoya® demonstrates excellent moisture resistance and an increase in dimensional stability of approximately 80%, based on Dutch climate conditions. The reduction in swelling alleviates internal stresses within the connections, enhancing the strength and stiffness of moment-resisting connections. Specifically, a clamped connection and a circular dowel connection were analysed using a linear elastic static FEM model, revealing internal stress reductions of 81% and 52%, respectively. This reduction was observed during the simulated initial annual swelling cycle that the wood may undergo. With the use of Accoya®, significantly less plastic deformation is expected in connections due to swelling issues compared to unmodified wood and reduced deflection in structures is expected. An increase of 219% and 58% in rotational stiffness was observed for the respective cases. To evaluate the impact on overall stiffness, these observed values were implemented in a portal frame structure. A reduction in horizontal displacement was observed ranging from 31% to 66%. This opens up new possibilities in structural wood design, allowing for slimmer and lighter wood constructions. Due to Accoya®’s lower property degradation and more stable structural performance in high-humidity conditions, an adjustment of the kmod and kddef factors is suggested; however, this is not sufficiently substantiated in the current study. Future research could explore long-term performance factors with experiments such as creep and fatigue to validate Accoya®’s structural reliability further.