The Night Flush

The physics are straightforward. During hot periods, outdoor air temperature typically drops 10-15°C between afternoon peak and pre-dawn low — even in continental climates like Edmonton's, where July afternoons reach 25°C but nights fall to 12°C. This diurnal swing is the resource. Thermal mass — concrete, masonry, stone — absorbs heat during the day, moderating indoor peaks. But that absorbed heat must go somewhere. If the building remains closed, the mass slowly releases its stored energy into the room, fighting against any mechanical cooling and warming the space precisely when occupants expect relief.

Night flush ventilation — sometimes called night purge or nocturnal precooling — uses the cool hours to draw heat out of the mass before dawn. The California Energy Commission's Title 24 standards recognize this strategy for commercial buildings, allowing reduced cooling equipment sizing when night flush is incorporated with exposed thermal mass. Givoni's landmark work on passive cooling (1994) documented that buildings with night ventilation and thermal mass maintained indoor temperatures 3-6°C below outdoor peaks in hot-dry climates — a margin that can mean the difference between comfort and heat stress. The strategy works best where the diurnal range exceeds 10°C and nighttime lows drop below 20°C — conditions common across the Canadian prairies, the American mountain west, and much of continental Europe during summer.

The practical challenge is moving enough air. Infiltration through cracks will not do it. The mass must feel a breeze. Santamouris et al., studying Greek office buildings in the 1990s, found that effective night cooling required air change rates of 8-12 per hour — far above the 0.5-1.0 typical of sealed buildings. This demands deliberate openings: large operable windows, or better, dedicated ventilation paths that can remain open while maintaining security. In hot-arid climates, wind scoops and thermal chimneys have served this purpose for centuries — the badgirs of Yazd, the malqafs of Cairo. In temperate climates, the solution is often simpler: secure louvered openings at low and high points, allowing stack-effect ventilation to run all night without leaving windows vulnerable.

Alexander did not address night flush directly, though his pattern Wings of Light (Alexander 107) — which calls for narrow buildings that permit natural ventilation — lays the groundwork. What he missed — writing from Berkeley, where summers are mild — was the critical importance of timing. Cross ventilation during a hot afternoon can make things worse, admitting 35°C air into a 28°C room. The same openings, used at night when outdoor air drops to 18°C, become the building's exhaust system. The distinction matters.