Natural Ventilation
Builders who reconnect with those ancestral techniques take advantage of possibilities of the climate and the immediate environment to meet the needs of renewal of air and comfort in a building. There are 2 good reasons for ventilate (health and thermal comfort), 3 ways to cool buildings (direct overventilation, delayed overventilation and air movement) and 4 technical and architectural types (through ventilation, mono-oriented, by draft thermal or assisted and controlled).
AIR RENEWAL RATE
Hygienic ventilation (biocontaminants and particles emitted by the coatings of the room): 0.5 to 1 volume/hour and up to 4 volumes/hour for a classroom.
Thermal comfort: from 10 to 30 volumes/hour
3 WAYS TO COOL BUILDINGS
Free-cooling or direct overventilation
It is used during occupancy of the premises, when the temperature is cooler outside than inside. In residential, this concerns the ventilation of the rooms thanks to freshness of nights.
Delayed or nocturnal overventilation
This overventilation with phase shift is adapted to inter occupancy premises mixed, like the offices or school equipment. The coolness of the night is then stored in available inertia of the structure (slab of floor without false ceiling, wall interior) so that users benefit by retransmission the following day.
Ventilation by air movement
Here, we seek less the evacuation of overheating than the ability of a current of air to accelerate evapotranspiration cutaneous with production of adiabatic coolness (by evaporation) and disposal wetting on the skin. An air speed of 0.5 to 1 meter/second provides a pleasant sensation of freshness.
AIR SPEED
An increase in air speed of 1m/s improves the feeling temperature comfort from 4 to 5°C. The ceiling fans (which consume relatively little energy) are good solutions to support a heat episode.
EVAPO-PERSPIRATION
The presence of plantations around buildings causes increased humidity ambient air and promotes evaporation sweat.
THE 4 FAMILIES OF NATURAL VENTILATION
Natural through ventilation
The pressure difference between two facades, from wind or differentiated sunlight, is the main driver of the interior air flow. Of all natural ventilation engines, the effect of the wind on a through room is by far the most effective, several tens of volumes per hour for wind speeds of the order of 1 m/s.
Mono-oriented natural ventilation
If there is only one opening in the room, a air movement is created under the double effect of the external pressure of the wind and the difference temperature between indoors and outdoors. This creates an overpressure in the lower part of the window and a depression in height. For a same wind speed, this motor is five to six times lower than that of through ventilation.
Natural ventilation by thermal draft
The main driver of air movement can also be thermal draft, i.e. the pressure difference created by a gradient of altitude between the air inlet and outlet, and a delta temperature between indoors and outdoors. We also speaks of stack effect, because it is often achieved by making the air “rise” in a duct. The greater the height, the higher the temperature indoor air temperature is high compared to that of outside air plus engine operation
thermal draft is optimized.
Assisted and controlled natural ventilation
The randomness of the climate (outside temperatures and winds) is the main drawback of natural ventilation. It is therefore a matter of overcoming this weakness by reinforcing the system with natural devices to increase or to control flows. As input, it may be the degree of opening of windows or vents suitable self-adjusting. On output, we can imagine: registers slaved to the speed of the air in the chimney, the wind by venturi effect or by static fan, the sun artificially overheating the air in the duct, etc.
POROSITY
It is defined by the ratio between the free surface of all the openings of a part and the floor space of this room. For the natural comfort ventilation, it must be at least 6%. But a doubling of the porosity leads to an increase in the order of the effectiveness of the natural ventilation.
Source: Ecole des Ingénieurs de la Ville de Paris, translated and summed up by Greengreat.org – October 2022
