**3. Experimental Results**

The experimental campaigns check the green roof effects on the building energy efficiency during 20 months (July 2017 to March 2019). During the first period, from July 2017 to February 2018, measurements correspond to the "conventional" roof. In February 2018, the external gravel layer of the conventional roof was replaced by the green roof. Thus, from March 2018 to March 2019, registered data corresponded to the green roof effects. Figure 8 displays typical traces for the temperatures and solar radiation obtained for the conventional and green roof campaigns, respectively, along an entire week during the summer period.

**Figure 8.** Typical traces for different temperatures and solar radiation in summer period.

The obtained results in both situations are summarized in Table 2, where the consumption from the air conditioning system (peak demand, energy consumption range during the entire campaign and its average daily range) and the external roof temperature are detailed for both types of roofs. Because the operation time of the offices located in the building was restricted to the morning hours, the operating time of the Heating, Ventilating and Air Conditioning (HVAC) system was only from 8:00 to 15:00, thus, the time span for comparison purposes was fixed for the period 9:00–13:00.


**Table 2.** Energy demand for conventional and green roof.

We can deduct from these measurements that, during summer time, energy consumption was higher in the conventional roof in comparison to the winter one. However, energy consumption during winter is lower in the conventional roof. This behavior could be partly explained due to the presence of gravel in external layer of the roof, which acts as a heat storage system with high temperature. This fact introduces an additional load to be compensated by the air conditioning system during the summer. Given that the summer period is much longer than the winter one in the Mediterranean area, any e ffort to increase energy e fficiency of the building should be concentrated onthe summer months.

Once the green roof is installed, the di fferences in energy consumption between the summer and winter periods reduced significantly to less than 15%, higher for the winter period in this case, whichcould be explained by the absence of the gravel layer as aheat source. This interpretation is supported by the data presented in Figures 9 and 10, where the evolution of the temperature along a similar day during the summer and winter periods, respectively, is presented for the conventional and green roof situations. Similar days were selected in terms of similar ambient outdoor temperature, humidity and solar radiation. During the summer, ambient outdoor temperature was 35 ◦C and the gravel reaches 50 ◦C, while with the presence of the green roof, this e ffect is smoothed and the temperature in the roof does not exceed the ambient outdoor temperature; in fact, it is below that value, namely, 32 ◦C.

**Figure 9.** Temperature evolution for a typical summer day with and w/o green roof.

**Figure 10.** Temperatureevolution for a typical winter day with and w/o green roof.

Similar behaviorwasobtained for the winter period, as data plotted in Figure 10 shows. In this case, the smoothing of the temperature fluctuation due to the green roof reduces the heat input to the building and forces to higher energy consumption for the same level of indoor comfort.

Analysis of the data presented at Table 2 enables to evaluate the impact of the green roof on the energy requirements of the building. It can be deduced that in the winter period there is an increase for the total daily energy consumption and the requirement in electric power to be used by an order of 15%. This fact can be explained by the increase in the roof external layer temperature produced by the solar radiation, which is 50% higher in the case of the conventional roof than with the green roof due to the isolation produced by the former one. This heat source helps to heat up the building, reducing the energy requirements in winter, while this is not available when the green roof is installed. On the contrary, during the summer period, the energy saving increases up to 30%, with a reduction of the required peak power of about 15%. Given the higher percentage of savings and the longer duration of the summer period, it can be concluded that the global energy savings for the entire year is going to be highlysignificant. Table 3 summarizes the percentages in energy requirement variation due to installing the green roof for the winter and summer periods.


**Table 3.** Energy and power demand variation due to the green roof presence.

Data comparingtwo similar days during summer time with both types of roofs areshown in Figure 11. An initial peak power was observed to start building conditioning, as is the case for the conventional roof; once a stable situation is reached, however, the power demand with the green roof is 20% less than with the conventional one.

A similar comparison is presented in Figure 12 for two similar days in the winter period. Power demand is very similar for both types of roof, but slightly lower in the case of the conventional one.

Obtained experimental results are comparable with other experimental studies conducted in Mediterranean climate conditions [22], in which a 15% to 17% lower energy consumption was observed during warm periods, whilehigher energy consumption (10% to 12%) was observed during cold times.

**Figure 11.** Comparison of similar days with and without a green roof in the summer period.

**Figure 12.** Comparison of similar days with and without a green roof in the winter period.
