Use night flushing / cross-ventilation

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During heatwaves, nighttime is often the coolest part of the 24-hour cycle. Opening windows and vents overnight — or automating mechanical ventilation to do the same — lets cool outdoor air flush out the heat stored in walls, floors, and ceilings, a technique known as 'night flushing' or night purge cooling.

Walls, floors, and ceilings absorb heat during the day (thermal mass). Circulating cool night air through the building releases that stored heat before it builds up again the next day — so the building starts the morning from a cooler baseline.

• Free cooling with no extra equipment required
• Especially effective in buildings with high thermal mass (stone, concrete)
• Can be automated with motorized windows or a controlled ventilation system
• Reduces the number of daytime hours air conditioning needs to run

1. Identify which windows or vents can be safely opened overnight (security grilles, tilt-and-turn)
2. Set a nighttime ventilation window (typically midnight to 6am)
3. Close the building as soon as outdoor temperature exceeds indoor temperature in the morning
4. Consider motorized windows with temperature sensors to automate the process
5. Secure access and assets if the site is unoccupied overnight