Horizontal openings in dwellings, such as staircases, provide large paths for air and moisture transport between floors. They play an important role in the moisture distribution from a moisture-source room to the rest of the house. The phenomenon, however, has only been barely studied. The objective of this work is to study the moisture transport through a horizontal opening under different ventilation strategies, ventilation rates, temperature differences between the upper and lower room, and locations of the moisture source. The study was carried out in a full-scale test-hut composed of two rooms connected by a large horizontal opening of 1.19m x 0.91m x 0.22 m. The inner dimensions of each room are 3.62m x 2.44m x 2.43m. Moisture was generated for 10 hours at known rates (107±2 and 113±2 g/h in the first and second floor, respectively) followed by 14 hours without moisture generation; the RH and temperatures were measured across both rooms. Different ventilation strategies were studied: no ventilation, net flow up-to-down, and independent ventilation for each room. Also, temperature difference between the two rooms was set between -3.5ºK to 3ºK. The main results show that moisture exchange between the two rooms starts when the moisture generation begins and two-way airflow exists through the opening. A colder upper room promotes moisture and air exchange between the two rooms even if the net flow is downward. Additionally, it was found that air was not well mixed in each room, thus significant humidity ratio variations were found across each room and between the both rooms, especially when the upper room temperature was higher.