The Effect of a green roof on thermal comfort and learning
performance in a naturally ventilated classroom in a hot and
humid climate
Speakers:
Huang, Wen-Pin1; Ruey-Lung, Hwang2; Kuo-Tsang Huang3
1
Feng Chia University, Taichung, Taiwan
National United University, Miaoli, Taiwan
3
National Taiwan University, Taipei, Taiwan
2
Abstract: Green roofs have been considered as an ecological strategy for improving indoor
thermal comfort levels. By analyzing the results of an experiment, this paper discusses
overheating and learning performance in two naturally ventilated classrooms, one below a
green roof and one below a bare roof. In this study, indoor thermal conditions were
measured in order to determine the occurence of overheating during the hot periods of the
year and, as well, the severity of thermal discomfort was examined in each classroom. The
overheating periods were determined over a period of time where indoor operative
temperatures reached or went above the upper limits of acceptability for 3 categories
according to EN 15251. Besides assessing the risk of overheating, the study also investigates
the effects of green roofs on students’ learning performances. The conclusions highlight the
importance of green roofs for improving thermal comfort and students’ learning performance
in naturally ventilated classrooms in hot and humid climates.
Keywords: green roof, thermal comfort, schoolwork performance, overheating risk
Introduction
In Taiwan, elementary and high schools traditionally have been naturally ventilated.
Numerous Taiwan schools in the cities have been experiencing overheating in the summer
month and demands for cooling the classroom have increased. Outdoor air temperatures
continue to rise as a result of global warming and urban heat island. By providing better
thermal conditions in school classrooms students will have fewer complaints and, as well,
they will be able to focus more on their studies. This has convinced school administrators and
designers to treat the risk of overheating in the summer months as a real and serious issue,
particularly for the classrooms directly below the building’s roof.
Due to the intensity of strong solar radiation and long hours of direct sunlight, it is difficult
for a classroom directly below a roof to maintain proper indoor thermal comfort levels during
the warm and hot months of year, especially those classroom without sufficient insulation. A
green roof is an ecological design method, which uses the foliage of plants to reduce heat
going into the building. The vegetation layer limits heat entering the building and the solar
radiation is absorbed and converted by the plant’s photosynthesis, evapo-transpiration, and
respiration. The equivalent albedo of a green roof is in the range of 0.7-0.85 which is much
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higher than that (0.1-0.2) of a bare roof. Moreover, the layer of soil gives an added insulation
to the building’s roof and, as well, the water content of the soil increases thermal inertia.
Green roofs are typically used in warm and hot climates to enhance the insulation
performance of roofs, making the buildings less prone to overheating in the summer because
of their thermal behaviour with the solar radiation.
Here we presented a study that investigated the risk of overheating and the learning
performance of the students in two naturally ventilated classrooms directly below a building’s
roof, one having a green roof and the other a bare roof. Through measurements of the
microclimatic variables, this study assessed the risk of overheating and student’s learning
performance in two classrooms directly below both a green roof and a bare roof. The study’s
ultimate goal was to analyze the effects of a green roof in the naturally ventilated school
buildings and how it reduces the risk of overheating while improving the learning performace
of students.
Experiment set up, instrumentation and location
The experiment was conducted in a naturally ventilated, two-story, southwest facing school
building in Taichung, Taiwan. The experiment’s measurements were taken in two classrooms
directly below two different kinds roofs; a green roof and a bare roof (see figure 1). The size
of each room is 9.0 m x 8.0 m. For this study, a period of climatic monitoring was conducted
in both classrooms from September 7th to October the 25th, 2014, during the first six weeks of
the school semester. This period was selected as it is the time of year in Taiwan when the
weather gradually transitions from hot to cool.
(a) green roof
(b) bare roof
Fig. 1 The photographs of (a) green roof and (b) bare roof investigated in this study
The air temperature, relative humidity(RH), globe temperature, and wind speed were
continuously monitored during this period. Only the measurements recorded at the time when
the classroom was in sesssion were used for the analysis. The air temperature and RH were
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recorded using ESCORT iLog Temperature/Humidity Datalogger, and globe temperature
using the CENTER 314. A hot-wired omni-directional DeltaOHM thermo-anemometer
HD2103.2 was used to record the wind speed. All the instrumentation used are in accordance
with the ISO 7726 [1] requirements on equipments for evaluating the status of thermal
environment. A central spots in the classrooms were selected as points of climatic monitoring.
Methodology for assessing risk of overheating
Current international standards for indoor thermal environment in naturally ventilated
buildings are the “ASHRAE Standard 55:2010-Thermal Environmental Conditions for
Human Occupancy” [2] and “European Standard EN 15251:2007-Indoor environmental input
parameters for design and assessment of energy performance of buildings addressing indoor
air quality, thermal environment, lighting and acoustics” [3]. Based on the database developed
in the 2000s from field surveys in naturally ventilated school buildings, when the upper limit
of the comfort zones for the warm season of Taiwan projected by these two standards was
compared, Hwang and Shih[4] found that the predictability of the adaptive model in
ASHRAE 55 appeared to be less than that of the model in EN 15251. Thus, EN 15251 was
adopted as the thermal comfort standard in this study.
In EN 15251, the optimal indoor operative temperature (Tc) in naturally ventilated buildings is
predicted as:
Tc = 0.33 × Trm + 18 .8
(1)
Trm = (T−1 + 0.8 × T− 2 + 0.6 × T−3 + 0.5 × T− 4 + 0.4 × T−5 + 0.3 × T− 6 + 0.2 × T−7 ) / 3.8
(2)
where Trm is exponentially weighted running mean of the outdoor temperature; T-1 is daily
mean outdoor temperature for the previous day; T-2,…is daily mean outdoor temperature for
the day before and so forth.
The aceptable bands for the 3 categories of buildings used in EN 15251 are: Tc ±2, Tc ±3 and
Tc ±4°C for categories I, II and III respectively.
Furthermore, the risk and magnitude of overheating can be calculated according to the amount
by which the operative temperature for any given hour exceeds the predicted comfort
temperature calculated using equation (1). The European Standard EN 15251 defines the
likelihood of overheating as equation (3)
PDH =
exp[0.4734 × ∆T − 2.607]
1 + exp[0.4734 × ∆T − 2.607]
(3)
where PDH is the proportion in heat discomfort and ∆Τ is the difference between the actual
operative temperature and the comfort temperature.
Operative temperature is the thermal index used throughout this study as it is the one used for
the assessment of the indoor environment in EN 15251. The operative temperature is the
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weighted average of the air temperature and the mean radiant temperature and expresses their
combined effect. The weights depend on the convective and the radiant heat-transfer
coefficients of the clothed body of the occupant. The operative temperature is calculated using
Equation (4) [5].
Top =
(4)
Ta ⋅ 10v + Tr
1 + 10v
(5)
Tr = [(Tg + 273)4 + (1.2 ×105 ⋅ d −0.4 )v0.6 (Tg − Ta )0.25 − 273
where Tg , Ta adnd v are the measured values of globe temperature, air temperature and air
velocity, respectively. The globe’s diameter d used in this study is 150mm.
Results of Measurements
Figures 2 show the maximum, minimum and daily mean outside temperature for the
monitoring periods. Also shown are the adaptive thermal comfort (Tc) and upper limits for
Category I and II.
35
Temperature, °C
30
25
20
daily rang
mean temp
optimal temp
Upper limit for Cat. I
Upper limit for Cat. II
15
9/7
9/15
9/23
10/1
10/9
10/17
10/25
Date
Fig. 2 Distribution of ambient temperature, optimal comfort temperature and upper limits for
Category I and II in Taichung, Taiwan over the whole monitoring period (from Sept. 7th to Oct.
25th, 2013)
The measured indoor air temperatures during school hours (08:00-12:00 for Wednesday and
08:00-16:00 for other weekdays) of the classroom directly below both the green roof and
bare roofsare presented in Figure 3. The measured air temperatures fluctuates betwee 23.5 to
32.2 °C inside both classrooms, indicating that it is sometimes too hot during sunny
afternoons by referred to by the upper limits of Catatgory II and depicted by the dashed line in
Figure 3. It is also observed that the application of a green roof contributes to the reduction of
the indoor temperature during most of the monitored period.
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Table 1 summarises the cumulative hours where temperatures reached or went above the
upper limits of category I, II and III in EN 15251. The number of hours exceeding Category I
limits is reduced from 159 to 130 when a green roof is in place, while the hours exceeding
Category II limit is reduced by two-third when a green roof is applied. The number of hours
of temperature higher than Catgoery III limits is 34 for bare roof compared to 10 for green
roof.
33
Temperature, C
31
°
29
27
green roof
bare roof
upper limits for Cat. II
25
23
9/9
9/16
9/24
10/1
10/9
10/17
10/24
Date
Fig. 3 Indoor operative temperature profile during school hours over the whole monitoring period
Table 1 Cumulative hours exceeding three reference temperatures
Green roof
Bare roof
Category I
130 (52%)
159 (63%)
Category II
66 (26%)
100 (40%)
Category III
10 (4%)
34 (13%)
For each hour of occupation over the whole monitoring period, the difference between the
actual operative temperature and the comfort temperature was calculated, and the proportion
of human occupants not comfortable was evaluated using equation(3). Figure 4 shows a
histogram of the PDH profiles recorded for both the green roof and bare roof. The maximum
PDH was 28% in the case of the green room, compare to 34% in the case of the bare roof.
Similary, the averaged PDH over the whole monitoring period increased from 11.6% in the
case of green roof and by 14.3% in the case of bare roof.
Impact on student’s learning performance
To calculate the effects of the thermal climate on student’s learning performance, the hourly
measured temperatures for each classroom were substituted into an empirical equation,
suggested by Seppanen et al [6], to translate indoor temperature to schoolwork performance.
The formula is shown in equation (6) and applicable for an indoor air temperature range of
15-35°C. According to equation (6), maximum performance is reached at an indoor
temperature of 21.8℃. The productivity is reduced by 11.7% when the indoor temperature
reaches the maximum of Tmax (31.7°C) over the whole monitoring period.
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RP = 0.1647524 × Ta - 0.0058274 × Ta2 + 0.0000623 × Ta3 - 0.4685328
(6)
where RP is relative performance, and Ta is room temperature (°C).
80
72
70
green roof
59 58
count of hours
60
50
56
47
bare roof
45 43
39
40
30
29
21
20
20
8
10
7
0
0
<5%
5-10% 10-15% 15-20% 20-25% 25-30% 30-35%
0 0
>35%
PDH intervals
Fig. 4 PDH histogram for the whole monitoring period
learning performance, %
100
95
99.4
max
95.4
99.2
Q3
94.5
Q1
90.1
91.1
90
88.4
min
87.3
85
80
green roof
bare roof
Fig. 5 Box plots for the predicted learning performance (Q1=25% percentile, Q3=75% percentile)
With the measured temperature profiles as input, equation(6) was used to determine hourly
performance indices in both indoor and outdoor environments. Figure 5 showsthe predicted
students performance in the classrooms below a green roof and a bare roof. The resulting
average performances were 93.3% and 92.5% for the green roof and the bare roof,
respectively. The students’ learning performance in the classroom with a green roof is slightly
increased compared to that of the classroom with a bare roof.
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Conclusion
This study investigated the contribution of a green roof and it affects on of the thermal
comfort levels in the school building where it has been place on. The conclusions are
sumarized as follows:
The green roof reduced the occurance of overheating. Concluded from the measured indoor
operative temperatures in each classroom, lower occurence of overheating was measured in
the classroom covered by green roof and likewise there was a higher occurence of overheating
in the classroom covered by a bare roof. When a green roof was applied, the percentage of
school hours where temperatures exceeding the upper limit of category II in EN 15251 was
reduced from 63% to 52%, from 40% to 26% for Category II and from 13% to 4% for
Category III.
As well, the green roof reduced the severity of discomfort during hot periods. The maximum
PDH in the case of green roof was 28%, compare to 34% in the case of the bare roof. Similary,
the average PDH over the whole monitoring period increased by 11.6% in the case of green
roof and by 14.3% in the case of bare roof.
Finally, the impact of a green roof on students’ learning performance is slightly less than 1%.
Acknowledgement
We offer our sincere appreciation for assistance in grant to the Ministry of Science and
Technology of Taiwan under the project number NSC-102-2221-E-239-028.
Reference
[1] ISO(1988), ISO Standard 7726:1998, Ergonomics of the Thermal Environment—
Instruments for Measuring Physical quantities. Geneva: International Standard
Organization.
[2] ASHRAE (2010), ASHRAE Standard 55:2010, Thermal Environmental Conditions for
Human Occupancy. Atlanda: American Society of Heating, Refrigerating and AirConditioning Engineers.
[3] CEN(2010), EN 15251:2007, Indoor environmental input parameters for design and
assessment of energy performance of buildings addressing indoor air quality, thermal
environment, lighting and acoustics, Brussels: CEN.
[4] Hwang R.L., Shih W.M. (2014), Appicability of different adaptive thermal comfort
standard in Taiwan, HV&AC, 44(1):23-27.
[5] CIBSE (2006), Guide A-Environmental design, London, Chartered Institution of Building
Services Engineers.
[6] Seppanen O., Fisk W., Lei Q. (2006), Effect of temperature on task performance in office
environment. Lawrence Berkeley National Laboratory, LNBL report 60946.
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