<p>Natural ventilation is key for reducing energy demand and ensuring indoor air quality. We study the deterministic and stochastic dynamics of a naturally ventilated system where a room with a steady buoyancy source connects to an unforced atrium (an arrangement representative of many real buildings). The buoyant fluid accumulates at the ceiling of both spaces. A stratification arises, inducing the stack effect which drives the ventilation of the system. We consider the occurrence of an opposing wind. Firstly, a constant wind is imposed, and the steady states are analysed for different atrium geometries. It is known that when no external wind occurs the atrium may either enhance or worsen the ventilation of the room depending on its geometry. We find that for any geometry the atrium enhances the ventilation for a wind velocity large enough. Secondly, stochastic fluctuations are introduced in the wind velocity. These induce a ‘noise-induced transition’: the mean position of the interface between warm and cold layers lies below that observed under constant wind. This phenomenon was already known for single-room systems; however, we find that the room-atrium system is less sensitive to this effect and to variations in the coefficient of variation of wind velocity.</p>