Air leaks have a considerable impact on the energy load and durability of buildings, particularly in cold climates. In wood construction using cross-laminated timber (CLT), air leaks are most likely to be concentrated at the joints between panels and other elements. This study used simulations of heat, air, and moisture transfers through a gap between two CLT panels causing air leakage in winter conditions under a cold climate. A real leakage occurrence was sized to validate the simulations. The aim of this work was to assess the impact on the energy loads and the durability of an air leak, as either infiltration or exfiltration, for different gap widths and relative humidity levels. The results showed that infiltrations had a greater impact on the energy load than exfiltrations but did not pose a threat to the durability, as opposed to exfiltrations. Gap sizes in CLT may vary, but the effect on the energy load was sensitive to the leakage path in the rest of the wall. As expected, a combination of winter exfiltration and a high level of interior relative humidity was particularly detrimental.