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Passive Ventilation

Ventilation is needed in all buildings to remove odours and provide us with oxygen. In hot climates, like East Africa,

adequate ventilation is essential to provide cooling.

 

‘Passive’ ventilation through windows and opening vents is the cheapest and simplest form of providing fresh air. Where there are problems with ambient noise or pollution, often in more urban locations, mechanical ventilation can be necessary, but passive ventilation should always be the default solution. Openings should be carefully designed to improve thermal comfort.

Prevailing Wind

Check wind speed and direction during the hottest season and design to capture prevailing winds.

Air movement can have a cooling effect when temperatures are uncomfortably high. Air movement of around three meters per second can provide an apparent cooling effect equivalent to around 2 ̊C. The prevailing wind tends to come from east through to west with a predominance of wind from the south east during the hottest season but there are inevitably significant regional and local variations caused by topography, trees or nearby buildings. Learning from locals, and if possible, installing a weather station during the early project stages can help to identify local wind conditions relevant for building orientation.

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Transsolar has installed a number of local weather stations to determine the prevailing winds on particular sites.

Openings

Ventilation openings can be created using hit and miss bricks, metal, timber, or concrete screens or louvres.

Openings for natural ventilation include glazed casements or shutters, grilles or metal louvres and 'hit and miss' brick openings that are most effectively positioned at the highest level of each space. The aerodynamic free open area (excluding mosquito netting, louvre blades and framing) should be equivalent to at least 10% of the floor area of the room. Louvres should be made to be 'storm proof' with upstands at the back and downstands at the front of a sloping section of metal.

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Nakapiripirit VI   Thousands of louvred blocks create a large surface area of permanent ventilation openings.

Cross Ventilation

Windows and vents on opposing sides of a building benefit from wind driven cross ventilation, particularly if they face the prevailing wind direction.

Wind creates pressure differences between one side of the building and the other. This results in air being drawn through openings on the high pressure side and drawn out on the low pressure side. Openings in adjacent walls are less effective at creating air movement than those on opposite walls and single sided ventilation provides even less. Uncomfortably high wind speeds are rare in East Africa so permanent ventilation openings such as hit and miss brickwork can be used to improve thermal comfort.

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COF Primaries  enjoy good cross ventilation due to shallow building depths and generous evenly spaced window openings.

Stack Ventilation

Bouyancy-driven stack ventilation uses temperature uses temperature differences to move air. This can be enhanced by solar chimneys driving faster ventilation.

Typically stack ventilation will not induce measurable air movement of any more than 0.5m/s. However, when there is no wind available this is a useful strategy to induce hygienic air exchange The speed of air movement is proportional to the height of the chimney, measuring from above the solar collector to the chimney exhaust. The exhaust must be horizontally oriented with a large free open area in order to maintain a negative pressure, irrespective of wind direction. The whole system must be airtight with a uniquely defined flow path.

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NTC Kaliro  Solar chimneys were designed incorrectly and consequently failed to provide a significant stack ventilation effect.

Night Cooling

Permanent ventilation openings allow cool night air to circulate the building, reducing heat build-up gathered by the structure during the day.

The impact of night time cooling is greater when there is substantial difference between day and nigh time temperatures. Measured data from earth or brick buildings with high levels of thermal mass shows that peak temperatures in the middle of the day and peal lows in the middle of the night can be reduced by allowing cool airflow through the building at night time. This creates cooler surfaces in the room during the day, which helps to counteract the impact of hotter air temperatures.

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ECD&F Centres  Permanent openings allow cool air to circulate at night-time helping to reduce heat build-up in the masonry walls.