Natural ventilation (NV) is one of the passive strategies, which can help to cool buildings and keep occupants thermally comfortable while reducing the need for active cooling systems. Natural ventilation along with elevated air speed, often provided by ceiling fans, can achieve desired thermal comfort levels. The following sections provide an overview of tools and rules of thumbs, which can be applied in the early design phases.
WIND SPEED & DIRECTIONS
The building should be orientated to maximize surface exposure and openings in the prevailing wind directions. The prevailing wind directions depend on the project location and can be found in the weather file for the location. Local green building certification schemes or codes can also provide information on the prevailing wind directions and wind speeds as shown in the table below for Kuala Lumpur and Singapore.
NATURAL VENTILATION METHODS
Natural ventilation can be implemented in designs by two methods:
Wind-driven ventilation, such as single-sided or cross ventilation
Buoyancy driven ventilation in the form of stack ventilation
1. Wind-driven ventilation
Both Malaysian Standard 1525 and Singaporean Green Mark give guidelines for room design dimensions. The below table summarises the recommendations.
2. Buoyancy driven ventilation
The buoyancy-driven or stack ventilation requires a sufficient differential height between the inlet and the outlet. The airflow can be maximised by providing an equally sized inlet and outlet.
The presence of air movement can further increase occupant thermal comfort by enhancing the evaporative and convective cooling from the skin. Thermal comfort standards such as ASAHRAE 55 sets the criteria and acceptability limits for the operative temperature in natural ventilated spaces. The online calculator below can help you to estimate the operative temperature acceptability limits for Kuala Lumpur and Singapore based on current weather file data for outdoor dry bulb temperature (°C). For further please refer Center for the Build Environment (CBE) Thermal Comfort Tool.
& ADAPTIVE THERMAL COMFORT
Certification schemes such as the Singaporean Green Mark require natural ventilated projects to conduct Computational Fluid Dynamics (CFD) simulations to verify that required average air speeds can be achieved. Green Mark further requires the project to verify that Thermal Comfort can be achieved, using the Predicted Mean Vote (PMV) method. The online calculator shown below can be used to estimate the required air speed for various building types for these to meet the thermally comfortable PMV levels of -0.5 to 0.5, Neutral.
Below table form Malaysian Standard 1525 gives an overview of the mechanical effect and occupant sensation of different air speeds.
COMPUTATIONAL FLUID DYNAMICS (CFD)
While the above guidelines and calculators can lead the project in the right direction more detailed and advanced CFD simulations will be required for verifying the design and for compliance documentation as required by Singaporean Green Mark.
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