2.1. Object Study
As mentioned in the introduction, to study the effect of the greenery layer on building thermal characteristics, three kinds of green façades were installed with different leaves covered areas in Semarang City, around Architecture Department at Diponegoro University. They included a non planted wall (experiment I), green façade with 50% leaves covered area (experiment II), and green façade with 90% leaves covered area (experiment III) [
15]. At that moment, the building model placed behind other buildings and surrounded by large trees can be seen in
Figure 1. Later in the data measurement, it would affect wind velocity, causing insignificant data. Comparisons were done of the three data measurements by analyzing the temperatures characteristics, decrement factors, and heat flows.
The field measurements were done at rainy season on December 10th, 2013 for experiment I; December 13th, 2013 for experiment II, and December 16th for experiment III. This is different with previous studies that carried out during summer time [
3,
14].
Due to the limited number of models, the data measurements for each experiment were done on different days. However, during data measurement, local weather conditions were relatively similar, as can be seen in
Table 1. Data of environmental conditions were adopted from Maritime Meteorology Station of Semarang City. The different in each data measurement are 2% for humidity, 0.52 °C–1.4 °C for air temperature and 0.2 m/s–0.5 m/s for air velocity.
2.2. Model of Green Façades
As previously already mentioned, in this study, three field measurements with different leaves covered area were done on the building model, which was like a house miniature with a scale of 1:4 compared to an original building [
15,
19,
20]. Details of the dimensions of the building model can be seen in
Figure 2. Walls as measurement objects had dimensions of 1 m × 1 m. The east façade was chosen as the object for measurement, because in tropical areas it gains more direct solar radiation, especially during morning until evening [
21].
The thickness of the wall constructions were 12 cm bricks, without layer materials (plasters).
Table 2 shows details of the building material.
Table 3 shows data of the geometrical and thermo–physical properties of the walls structure.
Figure 3 illustrates the configuration of the measured wall with or without a green façade.
In this study, exterior and interior temperatures, as well as wall surface temperatures, were measured as dependent variable that were affected by the green façade, while data measurement related to environmental factors, such as the averages of wind velocity, air humidity, and air temperature, were obtained from a nearby observation station, the Maritime Meteorology Station of Semarang City.
In order to collect data measurements, a number of measurement tools were employed. A surface thermometer was used for surface temperature measurement. An infrared thermometer was used as a measurement tool that was directed on a specific measurement point of the façade surface. The temperature range was −50 °C to 280 °C and measuring accuracy was ±2%.
Since one specific measurement point could not cover all surface data measurements, the authors divided façade surfaces into fifteen measurement points [
15]. There were three rows, where each row contained five measurement points. The range of differences in each measurement point was 0–0.1 °C. The average temperature on the fifteen measurement points was used as data for surface temperature and also for data analysis.
While space thermometers were used for space temperature measurement. Data measurement were carried out using a hygro thermometer placed at a distance 30 cm from the façade for the exterior space and in the middle of the building model for the interior space. The temperature range of this measurement tool was −10 °C to 60 °C and the measuring accuracy was ±1 °C.
Wind velocity measurement was also conducted using a 4 in 1 environment tester LM-8000. The specific range of this measurement tool was 0.4 m/s to 30.0 m/s. As for the measuring accuracy, it was ±0.1 m/s. However, the results were too weak around 0.4 m/s to 0.6 m/s due to surrounding buildings acting as wind barriers that reduced air flow around measurement objects. This condition was similar with the study conducted by Eumorfopoulou and Kontoleon [
3], where winds often did not blow since the built environment around measurement object acted as wind barrier and generated the insignificant data of air velocities around object measurement. Due to these factors, the wind velocity through leaves foliage was negligible.
All data measurements were collected in one hour interval. Selected data for analysis were chosen from data measurement on clear days.
Figure 4 shows documentation of data measurement.
Based on previous studies, decrement factor,
f, can be defined as the ratio of the amplitude of the temperature on the inner surface of the multi-layer material to the outer surface temperature [
22,
23,
24]. The decrement factor on each experiment was calculated by the following formula [
24], which was also used in the study conducted by Eumorfopoulou and Kontoleon [
3].
where
f is achieved by the difference between maximum and minimum interior surface temperature (T
si,max − T
si,min) divided by the difference between maximum and minimum exterior surface temperature (T
se,max − T
se,min).
Decrement factor calculation was also followed by time lag,
(in hours), which refers to the time delay due to the thermal mass [
22,
23,
25,
26], where the difference is obtained from the time occurrence of maximum temperature on the exterior surface (T
se,max) and interior surface (T
si,max); P is the period of the wave, which can be seen in
Figure 5.
The analysis of dynamic thermal characteristics was calculated based on the value of heat flow, Q (W/m
2) [
3] that passed the entire wall section of the measured façade during the day period, as follows:
where,
is calculated from the difference between the average of surface temperature, T
se,mean − T
si,mean, °C. R
wall (K m
2/W) is the sum of thermal resistance based on the geometrical and thermophysical properties of the material layers that comprise the configuration, and finally n denotes the measured time intervals (n = 24 hourly intervals during measurement time).
In this regard, a simple way to characteristic the leaves covered area was adopted based on Odum theory [
27]. Traditionally, profile of leaves covered area on the plants was already employed in farming area to measure the growth of crops, including their irrigation schedule [
28]. As opposed to previous studies that mostly used homogeneous species [
3,
12,
14,
18,
29,
30,
31,
32,
33,
34], in this study
Passiflora flavicarva and
Pseudocalymma alliaceum were used as greenery elements [
15,
19]. They are evergreen species and based on the observations, the proportion of each vegetation had balanced distribution of leaves covered area.
To determine the proportion of leaves covered area, in every week, documentation of the green façade was taken as an initial simulation to determine the leaves densities. From the simulation, there were 10 variations of leaves densities, from 0–100%. Since previous studies [
14,
29] already used 37%, 64%, 87.5% and 100% as their quantity of leaves covered area, the chosen leaves covered area in this study are 0%, 50% and 90%.