Natural ventilation is air change driven purely by physics: wind pressure across the facade and the temperature difference between inside and out. No fan is involved. Air enters through trickle vents, vent windows or facade openings and leaves through openings on the opposite side or through a vertical shaft.
The two driving forces
- Wind pressure — the windward face of a building sees positive pressure, the leeward face negative. That difference pushes air through the building. Cross ventilation, with openings on two opposite facades, exploits it best
- Stack effect — warm indoor air is lighter and rises. Where there is a height difference between inlet and outlet, a flow develops. The bigger the height and temperature difference, the stronger the draw
Both forces are free, but capricious: on a still autumn day, when inside and outside are almost the same temperature, the flow rate falls to nearly nothing. That is the system’s weakness in one sentence.
Advantages and drawbacks
- Advantage: no electricity, no fan, no noise. There is nothing to break down
- Advantage: minimal maintenance — cleaning the vents is the whole job
- Drawback: no control. Too much in a gale, too little in still air
- Drawback: draughts and heat loss. Incoming air is at outdoor temperature, and heat recovery is impossible
- Drawback: no filtration of pollen, soot or particulates
Where it is used
In housing built before roughly 1990, natural ventilation is the norm: vents in the facade and a vertical shaft serving kitchen, bathroom and WC. In new build it rarely suffices on its own, because airtightness and energy standards are demanding and the accidental draught ventilation of older construction has gone. Natural supply combined with mechanical extract, however, remains very common.
Points to watch
- Vents must stay open. A taped-up trickle vent is the fastest route to condensation and mould on the coldest surface in the room
- Clean vents annually; dust, insects and paint halve the free area
- Never connect a cooker hood or fan to a natural shaft that also serves an open-flued appliance — that can pull combustion gases indoors
- Allow transfer air inside the dwelling: roughly a 10 to 20 mm gap under internal doors, or transfer grilles
- Combine with purge ventilation: opening windows wide for a short spell changes the air far faster than leaving one ajar all day, and cools the structure less
Related terms
- Mechanical ventilation
- Balanced ventilation
- Night ventilation
- Condensation
- Airtight construction
- Airtightness
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