At the Réunion Island, trade winds are moderate on the north side of the island.
The climatology behaviours
The average velocity is close to 2 m/s at 10 metres above the ground and the buildings may be natural ventilated in this area. But due to the close neighbouring buildings, the wind should be computed and the mass flow rates entering the flat should be assessed. The pressure field on the building’s envelope generates flows through openings allowing the indoor space refreshment.
Experience of consulting services for natural ventilation design
For each set of pressure, the flow rate was evaluated following the macroscopic method with UrbaWind. P(ACH<15) is the probability for ACH to not exceed 15 vol/h. ACH(P<0.05) is the ACH corresponding to the exceed probability of 5% that means ACH is lower than this value 5% of the time (18 days a year). These statistics are usefull data to evaluate the efficiency of a cross ventilation configuration and to assess the thermal behavior of the buildings.
The mass flow rate defined as an input was considered as the air change rate calculated with UrbaWind. Nevertheless further computations were carried out by varying the ACH in order to evaluate sensibility of the system to the air exchange rate. Without additional insulation, the mean overheating is in the range [4-6] °C for ACH in the range [10-20] volume/hour exceeding the “2°C” limit.
Hence, the external treatment of the roof was necessary. The overheating became close to 2°C with ACH between 10 to 20 Vol/h. The overheating mainly due to the sun exposure of the northeast wall can be reduced with an additional outside insulation for instance with a thin polyurethane layer about 20 mm. Finally, the overheating is lower than 2°C for the range of ACH expected to be reach during summer. Note for the summer months, statically speaking, ACH is lower than 10 volume/hour, 5 hours per month and lower than 20 volume/hour 17 hours per month. With others words, the mean overheating of the renovated flat would exceed 1°C only one day a summer month and 2°C for few hours.