Night ventilation, also called night cooling or night purging, is the flushing of a building with cool outside air during the night to remove the heat absorbed during the day. It is one of the cheapest defences against summer overheating: no refrigeration plant is needed, only a temperature difference and an air path.
How it works
During the day, the sun heats the indoor air and, more importantly, the building fabric: floors, walls and ceilings store heat. At night the outside air in a temperate climate is usually several degrees — sometimes more than ten — cooler than inside. Ventilating heavily at that point lets the fabric release its stored heat into the passing air. The next morning the building starts from a lower temperature, so it takes longer before comfort limits are exceeded.
Two conditions govern how well it works:
- Thermal mass — a house with concrete floors, solid internal walls and hard finishes benefits far more than a lightweight timber frame. If the mass sits behind a suspended ceiling or thick carpet, it does not take part
- Air flow rate — the normal ventilation setting is far too low. Effective night cooling needs several times that, in practice through open windows or a dedicated purge setting on the ventilation system
Applications
- Houses — opening windows on two facades or two storeys so cross ventilation or a stack effect develops
- Offices and schools — automatically controlled opening vents or roof lights, or a mechanical system running at full output overnight
- Combined with shading — night cooling is the final measure; external shading and a sensible glazed area on south and west elevations come first
Points to watch
- Security. Open windows are a risk. Use secured night-vent positions, restrictors, grilles or a ventilated parapet
- Rain and insects. Combine with insect screens and make sure an open vent does not let rain in
- Noise. On a busy road an open window at night is not an option; a mechanical solution with attenuated ducts is then needed
- Overcooling. After a cool night following a hot day the building can feel chilly by morning; a control that reads both internal and external temperature prevents this
- Humidity. On humid summer nights, cool moist air can condense on cold floors — relevant in cellars and on exposed concrete
Related terms
- Mechanical ventilation
- Natural ventilation
- Overheating
- Thermal mass
- Solar shading
- Indoor climate
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