The Effect of a green roof on thermal comfort and learning performance in a naturally ventilated classroom in a hot and humid climate

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1 The Effect of green roof on therml comfort nd lerning performnce in nturlly ventilted clssroom in hot nd humid climte Spekers: Hung, Wen-Pin 1 ; Ruey-Lung, Hwng 2 ; Kuo-Tsng Hung 3 1 Feng Chi University, Tichung, Tiwn 2 Ntionl United University, Mioli, Tiwn 3 Ntionl Tiwn University, Tipei, Tiwn Abstrct: Green roofs hve been considered s n ecologicl strtegy for improving indoor therml comfort levels. By nlyzing the results of n experiment, this pper discusses overheting nd lerning performnce in two nturlly ventilted clssrooms, one below green roof nd one below bre roof. In this study, indoor therml conditions were mesured in order to determine the occurence of overheting during the hot periods of the yer nd, s well, the severity of therml discomfort ws exmined in ech clssroom. The overheting periods were determined over period of time where indoor opertive tempertures reched or went bove the upper limits of cceptbility for 3 ctegories ccording to EN Besides ssessing the risk of overheting, the study lso investigtes the effects of green roofs on students lerning performnces. The conclusions highlight the importnce of green roofs for improving therml comfort nd students lerning performnce in nturlly ventilted clssrooms in hot nd humid climtes. Keywords: green roof, therml comfort, schoolwork performnce, overheting risk Introduction In Tiwn, elementry nd high schools trditionlly hve been nturlly ventilted. Numerous Tiwn schools in the cities hve been experiencing overheting in the summer month nd demnds for cooling the clssroom hve incresed. Outdoor ir tempertures continue to rise s result of globl wrming nd urbn het islnd. By providing better therml conditions in school clssrooms students will hve fewer complints nd, s well, they will be ble to focus more on their studies. This hs convinced school dministrtors nd designers to tret the risk of overheting in the summer months s rel nd serious issue, prticulrly for the clssrooms directly below the building s roof. Due to the intensity of strong solr rdition nd long hours of direct sunlight, it is difficult for clssroom directly below roof to mintin proper indoor therml comfort levels during the wrm nd hot months of yer, especilly those clssroom without sufficient insultion. A green roof is n ecologicl design method, which uses the folige of plnts to reduce het going into the building. The vegettion lyer limits het entering the building nd the solr rdition is bsorbed nd converted by the plnt s photosynthesis, evpo-trnspirtion, nd respirtion. The equivlent lbedo of green roof is in the rnge of which is much 1

higher thn tht (0.1-0.2) of bre roof. Moreover, the lyer of soil gives n dded insultion to the building s roof nd, s well, the wter content of the soil increses therml inerti.

2 higher thn tht ( ) of bre roof. Moreover, the lyer of soil gives n dded insultion to the building s roof nd, s well, the wter content of the soil increses therml inerti. Green roofs re typiclly used in wrm nd hot climtes to enhnce the insultion performnce of roofs, mking the buildings less prone to overheting in the summer becuse of their therml behviour with the solr rdition. Here we presented study tht investigted the risk of overheting nd the lerning performnce of the students in two nturlly ventilted clssrooms directly below building s roof, one hving green roof nd the other bre roof. Through mesurements of the microclimtic vribles, this study ssessed the risk of overheting nd student s lerning performnce in two clssrooms directly below both green roof nd bre roof. The study s ultimte gol ws to nlyze the effects of green roof in the nturlly ventilted school buildings nd how it reduces the risk of overheting while improving the lerning performce of students. Experiment set up, instrumenttion nd loction The experiment ws conducted in nturlly ventilted, two-story, southwest fcing school building in Tichung, Tiwn. The experiment s mesurements were tken in two clssrooms directly below two different kinds roofs; green roof nd bre roof (see figure 1). The size of ech room is 9.0 m x 8.0 m. For this study, period of climtic monitoring ws conducted in both clssrooms from September 7 th to October the 25 th, 2014, during the first six weeks of the school semester. This period ws selected s it is the time of yer in Tiwn when the wether grdully trnsitions from hot to cool. () green roof (b) bre roof Fig. 1 The photogrphs of () green roof nd (b) bre roof investigted in this study The ir temperture, reltive humidity(rh), globe temperture, nd wind speed were continuously monitored during this period. Only the mesurements recorded t the time when the clssroom ws in sesssion were used for the nlysis. The ir temperture nd RH were 2

3 recorded using ESCORT ilog Temperture/Humidity Dtlogger, nd globe temperture using the CENTER 314. A hot-wired omni-directionl DeltOHM thermo-nemometer HD ws used to record the wind speed. All the instrumenttion used re in ccordnce with the ISO 7726 [1] requirements on equipments for evluting the sttus of therml environment. A centrl spots in the clssrooms were selected s points of climtic monitoring. Methodology for ssessing risk of overheting Current interntionl stndrds for indoor therml environment in nturlly ventilted buildings re the ASHRAE Stndrd 55:2010-Therml Environmentl Conditions for Humn Occupncy [2] nd Europen Stndrd EN 15251:2007-Indoor environmentl input prmeters for design nd ssessment of energy performnce of buildings ddressing indoor ir qulity, therml environment, lighting nd coustics [3]. Bsed on the dtbse developed in the 2000s from field surveys in nturlly ventilted school buildings, when the upper limit of the comfort zones for the wrm seson of Tiwn projected by these two stndrds ws compred, Hwng nd Shih[4] found tht the predictbility of the dptive model in ASHRAE 55 ppered to be less thn tht of the model in EN Thus, EN ws dopted s the therml comfort stndrd in this study. In EN 15251, the optiml indoor opertive temperture (T c ) in nturlly ventilted buildings is predicted s: T 0.33 T c = rm Trm = ( T T T T T T T 7 ) / 3.8 (1) (2) where T rm is exponentilly weighted running men of the outdoor temperture; T -1 is dily men outdoor temperture for the previous dy; T -2, is dily men outdoor temperture for the dy before nd so forth. The ceptble bnds for the 3 ctegories of buildings used in EN re: T c ±2, T c ±3 nd T c ±4 C for ctegories I, II nd III respectively. Furthermore, the risk nd mgnitude of overheting cn be clculted ccording to the mount by which the opertive temperture for ny given hour exceeds the predicted comfort temperture clculted using eqution (1). The Europen Stndrd EN defines the likelihood of overheting s eqution (3) exp[ T 2.607] PD H = 1+ exp[ T 2.607] (3) where PD H is the proportion in het discomfort nd Τ is the difference between the ctul opertive temperture nd the comfort temperture. Opertive temperture is the therml index used throughout this study s it is the one used for the ssessment of the indoor environment in EN The opertive temperture is the 3

4 weighted verge of the ir temperture nd the men rdint temperture nd expresses their combined effect. The weights depend on the convective nd the rdint het-trnsfer coefficients of the clothed body of the occupnt. The opertive temperture is clculted using Eqution (4) [5]. T 10v + Tr T op = 1+ 10v (4) T = [( T r g + 273) ( d 0.4 T ) 273 where T g, T dnd v re the mesured vlues of globe temperture, ir temperture nd ir velocity, respectively. The globe s dimeter d used in this study is 150mm. Results of Mesurements Figures 2 show the mximum, minimum nd dily men outside temperture for the monitoring periods. Also shown re the dptive therml comfort (T c ) nd upper limits for Ctegory I nd II. ) v 0.6 ( T g 0.25 (5) 35 Temperture, C dily rng men temp optiml temp Upper limit for Ct. I Upper limit for Ct. II 9/7 9/15 9/23 10/1 10/9 10/17 10/25 Fig. 2 Distribution of mbient temperture, optiml comfort temperture nd upper limits for Ctegory I nd II in Tichung, Tiwn over the whole monitoring period (from Sept. 7 th to Oct. 25 th, 2013) The mesured indoor ir tempertures during school hours (08:00-12:00 for Wednesdy nd 08:00-16:00 for other weekdys) of the clssroom directly below both the green roof nd bre roofsre presented in Figure 3. The mesured ir tempertures fluctutes betwee 23.5 to 32.2 C inside both clssrooms, indicting tht it is sometimes too hot during sunny fternoons by referred to by the upper limits of Cttgory II nd depicted by the dshed line in Figure 3. It is lso observed tht the ppliction of green roof contributes to the reduction of the indoor temperture during most of the monitored period. Dte 4

5 Tble 1 summrises the cumultive hours where tempertures reched or went bove the upper limits of ctegory I, II nd III in EN The number of hours exceeding Ctegory I limits is reduced from 159 to 130 when green roof is in plce, while the hours exceeding Ctegory II limit is reduced by two-third when green roof is pplied. The number of hours of temperture higher thn Ctgoery III limits is 34 for bre roof compred to 10 for green roof Temperture, C green roof bre roof upper limits for Ct. II 23 9/9 9/16 9/24 10/1 10/9 10/17 10/24 Dte Fig. 3 Indoor opertive temperture profile during school hours over the whole monitoring period Tble 1 Cumultive hours exceeding three reference tempertures Ctegory I Ctegory II Ctegory III Green roof 130 (52%) 66 (26%) 10 (4%) Bre roof 159 (63%) 100 (40%) 34 (13%) For ech hour of occuption over the whole monitoring period, the difference between the ctul opertive temperture nd the comfort temperture ws clculted, nd the proportion of humn occupnts not comfortble ws evluted using eqution(3). Figure 4 shows histogrm of the PD H profiles recorded for both the green roof nd bre roof. The mximum PD H ws 28% in the cse of the green room, compre to 34% in the cse of the bre roof. Similry, the verged PD H over the whole monitoring period incresed from 11.6% in the cse of green roof nd by 14.3% in the cse of bre roof. Impct on student s lerning performnce To clculte the effects of the therml climte on student s lerning performnce, the hourly mesured tempertures for ech clssroom were substituted into n empiricl eqution, suggested by Seppnen et l [6], to trnslte indoor temperture to schoolwork performnce. The formul is shown in eqution (6) nd pplicble for n indoor ir temperture rnge of C. According to eqution (6), mximum performnce is reched t n indoor temperture of The productivity is reduced by 11.7% when the indoor temperture reches the mximum of Tmx (31.7 C) over the whole monitoring period. 5

6 RP = T T where RP is reltive performnce, nd T is room temperture ( C) T (6) green roof bre roof count of hours <5% 5-10% 10-15% 15-20% 20-25% 25-30% 30-35% >35% PD H intervls Fig. 4 PD H histogrm for the whole monitoring period mx 99.2 lerning performnce, % Q3 Q1 min green roof bre roof Fig. 5 Box plots for the predicted lerning performnce (Q1=25% percentile, Q3=75% percentile) With the mesured temperture profiles s input, eqution(6) ws used to determine hourly performnce indices in both indoor nd outdoor environments. Figure 5 showsthe predicted students performnce in the clssrooms below green roof nd bre roof. The resulting verge performnces were 93.3% nd 92.5% for the green roof nd the bre roof, respectively. The students lerning performnce in the clssroom with green roof is slightly incresed compred to tht of the clssroom with bre roof. 6

7 Conclusion This study investigted the contribution of green roof nd it ffects on of the therml comfort levels in the school building where it hs been plce on. The conclusions re sumrized s follows: The green roof reduced the occurnce of overheting. Concluded from the mesured indoor opertive tempertures in ech clssroom, lower occurence of overheting ws mesured in the clssroom covered by green roof nd likewise there ws higher occurence of overheting in the clssroom covered by bre roof. When green roof ws pplied, the percentge of school hours where tempertures exceeding the upper limit of ctegory II in EN ws reduced from 63% to 52%, from 40% to 26% for Ctegory II nd from 13% to 4% for Ctegory III. As well, the green roof reduced the severity of discomfort during hot periods. The mximum PD H in the cse of green roof ws 28%, compre to 34% in the cse of the bre roof. Similry, the verge PD H over the whole monitoring period incresed by 11.6% in the cse of green roof nd by 14.3% in the cse of bre roof. Finlly, the impct of green roof on students lerning performnce is slightly less thn 1%. Acknowledgement We offer our sincere pprecition for ssistnce in grnt to the Ministry of Science nd Technology of Tiwn under the project number NSC E Reference [1] ISO(1988), ISO Stndrd 7726:1998, Ergonomics of the Therml Environment Instruments for Mesuring Physicl quntities. Genev: Interntionl Stndrd Orgniztion. [2] ASHRAE (2010), ASHRAE Stndrd 55:2010, Therml Environmentl Conditions for Humn Occupncy. Atlnd: Americn Society of Heting, Refrigerting nd Air- Conditioning Engineers. [3] CEN(2010), EN 15251:2007, Indoor environmentl input prmeters for design nd ssessment of energy performnce of buildings ddressing indoor ir qulity, therml environment, lighting nd coustics, Brussels: CEN. [4] Hwng R.L., Shih W.M. (2014), Appicbility of different dptive therml comfort stndrd in Tiwn, HV&AC, 44(1): [5] CIBSE (2006), Guide A-Environmentl design, London, Chrtered Institution of Building Services Engineers. [6] Seppnen O., Fisk W., Lei Q. (2006), Effect of temperture on tsk performnce in office environment. Lwrence Berkeley Ntionl Lbortory, LNBL report

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