Building thermal-insulation effect on ambient and indoor thermal performance of green roofs

Ecological Engineering 69 (2014) 265–275

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Building thermal-insulation effect on ambient and indoor thermal performance of green roofs C.Y. Jim ∗ Department of Geography, The University of Hong Kong, Pokfulam Road, Hong Kong

a r t i c l e

i n f o

Article history: Received 8 November 2013 Received in revised form 26 March 2014 Accepted 19 April 2014 Keywords: Building thermal insulation Green-roof heat-sink effect Nocturnal thermal discharge Nocturnal thermal barrier Synergistic-dual thermal insulation Thermal insulation breaching

a b s t r a c t Building thermal insulation (BTI) may influence indoor cooling by green roofs. The hypothesis that buildings with poor BTI may benefit more from green-roof passive cooling is tested. An experiment was designed in humid-tropical Hong Kong to investigate two BTI states (good and poor) and three treatment plots (control-bare, succulent Mexican Sedum, and broadleaved Perennial Peanut). Temperature sensors were installed along a holistic vertical profile from outdoor air to green-roof layers and indoor ceiling and air. On summer-sunny day, poorly-insulated control roof allows notable daytime peak thermal intrusion, counterpoised by comparable nocturnal thermal discharge. It is the only roof with bidirectional heat fluxes and nearly-balanced diurnal heat-energy budget. Other roofs are predominantly unidirectional (downward) with considerable indoor heat gain. Well-insulated control roof experiences extended (morning to night) thermal intrusion, with depressed but delayed and prolonged heat influx, and thermal insulation breaching. At night time, it creates nocturnal thermal barrier to reduce heat escape from indoor space. Simple Sedum green roof on poorly-insulated roof establishes green-roof heat-sink effect to incur daytime and nocturnal aggravated thermal intrusion. On well-insulated roof, it generates a synergisticdual thermal barrier to suppress thermal insulation breaching. However, nocturnal thermal discharge has been constrained by persistent positive thermal gradient throughout the day. Thicker and more-elaborate Peanut green roof brings considerable evapotranspiration cooling and synergistic-dual thermal barrier to generate daytime subdued thermal intrusion and mask inherent BTI-differences. As omission of BTI for both Sedum and Peanut green roofs does not earn more indoor cooling, the hypothesis cannot be accepted. To optimize passive cooling, green roofs with thicker substrate and denser foliage should be installed on buildings with good BTI. © 2014 Elsevier B.V. All rights reserved.

1. Introduction A green roof can bring passive cooling of the ambient air above it and the indoor space below it (Köhler, 2004; Santamouris et al., 2007; Spolek, 2008; Teemusk and Mander, 2009, 2010). The cooled air at the building top may spread and descend to bring collateral or spillover effect to adjacent and street-level areas (Bruse and Skinner, 1999; Osmond, 2004; Peng and Jim, 2013). The upwardcum-downward cooling effect can be attributed to the multiple heat rejection, obstruction and dissipation capabilities of the vegetated roof system. Many studies have been conducted to evaluate the efficacy of these cooling functions under different site conditions and climatic zones. The study scale tends to polarize at either

∗ Tel.: +852 3917 7020; fax: +852 2559 8994. E-mail address: [email protected] http://dx.doi.org/10.1016/j.ecoleng.2014.04.016 0925-8574/© 2014 Elsevier B.V. All rights reserved.

the micro-scale individual sites or at macro-level district-city areas. The findings could inform design and management of green roofs, optimize thermal-energy benefits, and inform relevant policies in cities (Johnston and Newton, 2004; Banting et al., 2005; Carter and Fowler, 2008). The bulk of the heat absorbed by the roof creating an energetic liability is derived from solar radiation (Pearlmutter and Rosenfeld, 2008; Feng et al., 2010). Green-roof material layers shade the roof by blocking sunshine. They offer thermal insulation to retard downward passage of heat absorbed at the upper surface toward the roof slab (Lazzarin et al., 2005; Getter et al., 2011; Theodosiou et al., 2014). In particular, vegetation biomass in the upper part and plastic drainage sheet at the bottom have ample internal air spaces to foster thermal insulation effect. The foliage cover may have a higher reflectivity than some conventional bare roofs to reduce absorptivity for radiant energy and absorption of sensible heat brought by convection. The vegetation may also have higher emissivity than

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some conventional roofing materials to facilitate longwave radiant cooling (Gaffin et al., 2005, 2006). Solar radiation plays dual and diametrically opposite roles in human settlements. It can work consumptively to heat up cities and impose cooling load and heat stress (EPA, 2009). In conjunction with green spaces and greenery, solar energy can drive evapo´ 2001; transpiration to bring productive passive cooling (Pokorny, Hamada and Ohta, 2010; Declet-Barreto et al., 2013). In compact urban areas, the lack of urban green spaces (UGS) and shortage of solution spaces have curtailed increased use of this free and clean ecosystem service. Building envelopes, however, offer a new dimension to dress up the city with green roofs and green walls (Skinner, 2006; Tian and Jim, 2011, 2012). Such additional greening in greenery-deficit areas could ameliorate the urban heat island (UHI) effect which has been accentuated by urban intensification and superposition of global warming. Cities contain many old structures which may not have efficient modern building thermal insulation (BTI) at the roof. Without vegetation protection, such poorly-insulated roofs can absorb and transmit an appreciable quantity of heat to underlying indoor spaces. In addition, the heat retained by such roof slab can transfer to near-ground air by convection and advection to raise ambient air temperature. Retrofitting such bare building tops with green roofs could bring notable cooling effects. The inadequate ability to ward off heat ingress may permit more effective passive cooling of indoor air by green roof. Some recent studies proposed that green roofs could provide more cooling benefits to poorly-insulated buildings (Castleton et al., 2010; Jaffal et al., 2012). Other findings pointed out that they cannot compensate for the deletion of normal BTI (Eumorfopoulou and Aravantinos, 1998). Such observations thus far have been mainly inferred from on green-roof thermal studies. They deserve to be verified by a dedicated controlled experiment. This study has been developed to fulfill four objectives: (1) to design a controlled field experiment to compare the thermal performance of green roofs on buildings with and without BTI; (2) to evaluate the thermal-insulation effect of green roofs along a holistic vertical temperature profile extending from outdoor air to greenroof layers and to indoor ceiling and air; (3) to compare the thermal performance of two plant species with different growth forms and photosynthesis-transpiration physiology in relation to BTI status; and (4) to assess the heat flux patterns between experimental plots and underlying apartments.

spaces (Planning Department, 2013) is probably one of the lowest in the world for cities of a comparable size. Developed areas are excessively covered by impermeable buildings, roads and paved surfaces to increase specific-heat capacity and suppress heat dissipation by evapotranspiration. The grave shortage of UGS has limited cooling by vegetation and unsealed soil, and contributed to intensification of UHI effect (Hong Kong Observatory, 2013) with heat stress and health impacts (Cheng et al., 2012). The experimental site was located in a public housing estate in Tseung Kwan O new town of Hong Kong. The recently built estate allowed installation of custom-made roof slabs in two highrise blocks. One wing of each block’s roof top and underlying apartments were reserved for the study. Six domestic units were left vacant throughout the study period to provide standardized experimental conditions. The roofs were kept off-limits to exclude extraneous influences.

2.2. Green roof installation and experimental design At Block 1, roof BTI layers were omitted to emulate a poorlyinsulated building (Fig. 1). At Block 2, normal BTI layers were installed. Both blocks contain normal waterproof membrane resting on a screed layer placed on reinforced concrete slab. The thermal insulation and waterproof design and materials are commonly adopted in Hong Kong. All apartments have main windows facing southeast. To standardize their environmental conditions, small side windows of the terminal unit at each block were shielded by thermal-insulation gypsum boards. Windows and doors were kept closed in the study period to minimize external influence on temperature. Identical experimental designs were adopted at both blocks. Each block’s roof site of about 85 m2 is divided into three roughly equal plots, which correspond to the three apartments lying underneath (Fig. 2). Plot A (control plot) was left bare to serve as the baseline. Plots B and C were assigned to two green roof types. A proprietary green-roof system (Nophadrain, Kirkrade, The Netherlands) conforming to stringent German specifications (FLL, 2008) was installed. The sequence of its multiple materials layers is shown in Fig. 2.

2. Study area and methods 2.1. Study area The study was conducted in humid-subtropical Hong Kong in the southern coast of China (latitude 22◦ N and longitude 114◦ E). The weather is dominated by the regional Asian monsoon system, with occasional typhoons and thunderstorms (Hong Kong Observatory, 2013). The hot-rainy period lasts five months per year, with maximum summer temperature exceeding 33 ◦ C. The total rainfall of about 2300 mm per annum is dropped mainly in the wet season from May to September, which coincides with warm months. Winter is cool-dry with average temperature staying above 10 ◦ C. The population of 7.18 million is accommodated in about 24% of the small territory with merely 1108 km2 of land (Census and Statistics Department, 2013). The urban form is dominated by highdensity packing of buildings and roads, and pervasive occurrence of high-rise structures for both commercial and residential uses. The ultra-compact city lacks green and open spaces in built-up areas. The average provision of only 3.55 m2 per capita of public open

Fig. 1. The edge of Plot A at Block 1 showing the thermal insulation layers which were omitted at the experimental plots. The layers installed on the (A) waterproof membrane (the dark sheet on the surface in the foreground) include: (B) polystyrene foam 40 mm, (C) cement-sand bedding 25 mm, and (D) precast concrete tile 35 mm. These layers were installed on the roof of Block 2 to denote good building thermal insulation.

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Fig. 2. Cross-sections of: (a) the three experimental plots (control, Sedum and Peanut) which were installed, respectively, at (b) Block 1 (without thermal insulation) and Block 2 (with thermal insulation) of the study area.

Plot B (Sedum plot) received a drought-tolerant succulent plant, Mexican Sedum (Sedum mexicanum Britton, Crassulaceae) (Fig. 3). It is an evergreen perennial herb native to Central American (Stephenson, 1994; Snodgrass and Snodgrass, 2006). With fairly fast establishment rate, it can form a full cover in one growing season and has been adopted for roof greening in tropical areas (Chen, 2013; Dvorak and Volder, 2013). Sedum species assumes the Crassulacean acid metabolism (CAM) photosynthetic mode by closing their stomata in daytime under water deficit condition (Sayed, 2001). At night time, stomata open to absorb carbon dioxide which is stored in leaf tissues for use in daytime photosynthesis when stomata are closed. Thus the transpiration rate of Sedum could be suppressed on hot days with dry substrate. Plots C (Peanut plot) took a ground-hugging trailing plant, Perennial Peanut (Arachis pintoi Krapov. & W.C. Greg., Fabaceae) (Fig. 4). Broadleaf herbs have been advocated as alternatives to Sedums for better passive cooling (Blanusa et al., 2013). The evergreen perennial groundcover herb, native to tropical South America, is characterized by vigorous-fast growth and tolerance of high temperature and strong insolation. It has been used successfully as a groundcover or living mulch crop for tropical orchards (Radovich et al., 2009), and as a tropical forage legume with nitrogen-fixing capability (Kerridge and Hardy, 1994). The species suits the harsh tropical rooftop microclimate (Chung and Chan, 2001). The common C3 photosynthesis mode is adopted. The Peanut plot had dual-strata substrate composed of an upper soil layer over a rockwool layer (Fig. 2). The soil was composed of

local completely decomposed granite obtained from the weathering crust below natural pedological profile. The mineral soil with a sandy loam texture (US Department of Agriculture scheme; Soil Survey Staff, 1999) was mixed thoroughly with 20% (v/v) of mature compost. The amendment enriched the organic matter and nutrient contents and improved soil structure, drainage and water storage capacity (Jim, 1996). The rockwool is composed of silica fibers made from molten volcanic rock and compressed into boards for horticultural use as a soil substitute. It has a high 80% (v/v) porosity to hold water, yet exceptionally light weight (dry weight 6 kg m−2 ). It can partly substitute the heavy mineral soil to reduce loading on the roof slab, yet offering considerable plant-available water capacity. The Sedum roof demands a substrate with less water-holding capacity as the species cannot tolerate high soil moisture content. Instead of using ordinary mineral soil, calcined clay in the form of spherical pottery pellets was used (Handreck and Black, 2002) (Fig. 2) It has large interstitial pores between pellets and limited moisture and exchangeable cation storage within the pellets. The rockwool layer was omitted as the drought-tolerant plant does not need the extra water supply. The vegetated plots were equipped with an automatic sprinkler irrigation system (Rain Bird, Tucson, AZ, USA) that delivered about 5 L m−2 per day of water in early morning. The system has an automatic timer control and a rainfall detector set to stop watering when antecedent rainfall reaches 10 mm conserve water.

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Fig. 4. Plot B is planted with Perennial Peanut (Arachis pintoi) with an instrument stand placed near the center of the experimental plot. It has achieved a full vegetation cover after one growing season.

3. Results and discussion 3.1. Data use and presentation Fig. 3. Plot B is planted with Mexican Sedum (Sedum mexicanum) with an instrument stand placed near the center of the experimental plot. It has achieved a full vegetation cover after one growing season.

Environmental sensors were installed along a holistic vertical temperature profile extending from outdoor to green-roof layers to indoor. The locations and types of temperature sensors are summarized in Table 1. The six plots at the two blocks received similar treatments. After establishing full vegetation cover on green-roof plots, the instruments (sensors and data loggers) were subject to one month of pilot testing and adjustment. All sensors were synchronized to collect data at 15-min interval in data loggers for periodic downloading. Live data collection ran through the summer of 2012. The weather parameters were monitored by a microclimatic station.

The study explores BTI effect on green-roof thermal performance. It could hint whether green-roof installation could serve as a sufficient substitute of BTI layers with cost-saving implications. The temperatures above (ambience), within (green-roof layers) and below (indoor) the roof are compared between two blocks and amongst three plots using control plot as baseline. In addition, differences in heat flux between roof and indoor space are compared. Instead of comparing graphs of the two blocks side-by-side, Block 2 (with BTI) data were subtracted from Block 1 (without BTI) to highlight inter-block differences which are depicted in graphs. Sunny days with high solar input and hence significant thermal impacts were chosen with reference to local meteorological data. Data collected on sunny days with comparable weather conditions were compared and found to be similar, which could be considered as replications. Instead of averaging the data of similar sunny days, a typical summer-sunny day was chosen for the present study. Main weather data on the sample days are given in Table 2. In the graphs,

Table 1 The positions of temperature sensors installed along a holistic vertical temperature profile at the three experimental plots at each of the two blocks. Vertical profile

Position or height (cm)

Control plot

Sedum plot

Peanut plot

Sensora

Type

Accuracy

Outdoor

Air at 150 cm Air at 15 cm Surfaceb Soil Rockwool Drainage Tiled Ceiling-large Ceiling-small Air at 150 cm-large Air at 150 cm-small

Y Y Y

Y Y Y Y

Y Y Y Y Y Y Y Y Y Y HoboU15

Lufft8160.TFF Lufft8160.TFF Apogee S1-111 Lufft8160.TF Lufft8160.TF Lufft8160.TF Lufft8160.TF Apogee S1-111 Apogee SI-112 HoboU14 Thermister

Pt100 Pt100 Infrared radiometer Pt100 Pt100 Pt100 Pt100 Infrared radiometer Infrared radiometer Thermister ±0.21 ◦ C

±0.20 ◦ C ±0.20 ◦ C ±0.20 ◦ C ±0.40 ◦ C ±0.40 ◦ C ±0.40 ◦ C ±0.40 ◦ C ±0.20 ◦ C ±0.20 ◦ C ±0.21 ◦ C ±0.21 ◦ C

Roof Materialc

lndoore

a

Y Y Y Y

Y Y Y Y Y Y

The sensor manufactureres are Apogee (Logan, UT, USA), Hobo (Bourne, MA, USA), and Lufft (Fellbach, Germany). For the two green-roof plots, the surface temperature of the vegetation was measured. For the control plot, the temperature of the bare roof surface was measured. The infrared radiometer was a non-contact detector of material surface temperature. c The temperature sensors were placed in the middle of the soil, rockwoool and drainage layers d For the green-roof plots, the temperature sensor was placed at the bottom of the green roof below the root barrier sheet (refer to Fig. 2 for the layers). e Each apartment was partitioned into a bedroom (small room) and a living room (large room). b

C.Y. Jim / Ecological Engineering 69 (2014) 265–275

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Table 2 Key weather parameters on the three season–weather sample days. Season

Weather

Date (yyyymmdd)

Max temperature (◦ C)

Min temperature (◦ C)

Temperature range (◦ C)

Max relative humidity (%)

Min relative humidity (%)

Aggregate rainfall (mm)

Aggregate solar radiation (W m−2 )

Summer

Sunny

20120819

34.52

26.37

8.15

100.00

58.64

0.20

24 666.68

the positive (upper) side denotes that Block 1 is warmer than Block 2, and vice versa for the negative (lower) side. 3.2. Temperature difference at control plot The inter-block temperature differences are relatively small before sunrise, and widens to a limited range before noon (Fig. 5a). They then depart to reach maximum divergence in late afternoon to evening. Thereafter, the differences get narrower. In outdoor, air temperature at 150 cm displays the least inter-block difference, and at 15 cm more difference. In outdoor, roof surface temperature displays more prominent difference, with Block 1 warmer than Block 2 before noon, and switching to Block 2 warmer than Block 1 after midday. In indoor, ceiling temperatures at Block 1 is consistently and notably warmer than Block 2 throughout the day, reaching a maximum difference of 7.6 ◦ C at 1830 h. Indoor air temperatures at Block 1 are also warmer than Block 2 throughout the day, but less so than ceiling. The sunny day experiences high solar radiation input, pushing maximum temperature to 38.6 ◦ C and minimum 27.6 ◦ C (Table 2). Under the intense energy regime, pronounced differences in thermal effect at the two blocks are expressed. The dry bare roof with high Bowen ratio cools down main by convective dissipation of sensible heat (Gaffin et al., 2006). The diurnal records signify temperature divergence at the two blocks in heat gains and losses. Directly reached by incident insolation, roof surface echoes changes in the diurnal solar-energy regime (Fig. 5a). Heat is transferred upward from roof surface to ambient air, with comparable air warming at both 15 cm and 150 cm. Around midday, Block 2 is slightly warmer than Block 1 at 15 cm due to a warmer roof surface transferring more sensible heat by convection to near-ground air. This thermal effect is rather short-range, as it has less influence on temperature difference at 150 cm. Block 1 surface is covered by waterproof membrane without BTI layers (Figs. 1 and 2). Its roof slab with less conduction resistance has a smaller mass and a correspondingly lower specific-heat capacity. The roof surface heats up and cools down more readily than Block 2. Sensible heat acquired from solar radiation can conduct to indoor ceiling, and in turn to indoor air. The roof slab has limited heat-storage capacity but efficient conduction. At Block 1, the indoor warming effect reaches maximum difference in late afternoon, indicating time lag in thermal intrusion and reflecting gradual penetration of conduction heating front through the roof slab. This downward heat transmission is sustained at a relatively high level toward midnight, indicating heat-sink formation in the reinforced concrete layer. Without BTI, Block 1 has built a notable thermal gradient to push significantly more heat from roof surface to indoor space. The daytime heat gain sustains the night time heat ingress, which echoes a key contribution to UHI effect and associated hot night phenomenon. Block 2 roof surface is warmer than Block 1 from about noon to midnight. The solar heat absorbed by its bare concrete tile is retained to raise its material temperature. Heat accumulation in the course of the day forms a heat sink which is conspicuously expressed in the afternoon. The maximum difference occurs 2 h 15 m before the indoor ceiling maximum, suggesting more direct and faster warming of exposed concrete tile. It takes time for heat energy to build up and widen inter-block difference. The BTI layers

which include 40-mm of polystyrene foam (Figs. 1 and 2) restrict conduction to underlying roof slab. Thus ceiling and indoor air can remain cooler than Block 1 throughout the day and especially after midday. 3.3. Temperature difference at Sedum plot Inter-block differences happen mainly in daytime, displaying peaks and troughs mainly from morning to late afternoon (Fig. 5b). The differences are compressed to a narrow margin at night time. At most sensor positions, Block 1 is warmer than Block 2. The most pronounced differences occur in the top (Sedum surface) and bottom (tile) of the green roof. The peak temperature of Sedum surface at Block 1 exceeds Block 2 by 13.2 ◦ C, which occurs somewhat early at 1130 h. Tile temperature at Block 1 exceeds Block 2 by a maximum of 7.6 ◦ C, which occurs somewhat late at 1445 h. Air temperatures at 15 cm and 150 cm and in soil have less differences. From sunrise to 1800 h, soil temperature at Block 1 is warmer than Block 2, reaching maximum divergence of 2.9 ◦ C at 1045 h. The drainage layer and indoor (ceiling and air) sensors experience little temperature differences throughout the day. On the hot sunny day, a considerable amount of solar energy (Table 2) is absorbed by Sedum roof. The thin substrate offers limited insulation (Del Barrio, 1998). The CAM photosynthesis of Sedum has less efficient transpiration and cooling when moisture supply is insufficient (Ohno and Maenaka, 2006). At Block 1, outdoor temperatures are notably higher than Block 2 at most sensor positions. The widest difference occurs at Sedum surface, followed by tile. Higher Sedum surface temperature at Block 1 indicates low moisture content especially around midday (Yamamoto et al., 2006). The green-roof materials sandwiched between them also record higher temperatures at Block 1 but to a lesser degree. At Block 1, outdoor air temperatures are warmed more than Block 2, with somewhat delayed peak differences at both 15 cm and 150 cm. They indicate that warmed green-roof materials can transmit heat mainly by convection to ambient air. Without BTI, heat absorbed by green-roof materials can conduct downward with little restriction to roof slab. The tile temperature probes the thermal state of roof slab. Block 1 is notably warmer than Block 2 in daytime but somewhat cooler at night time. The pattern indicates that heat is transmitted downward from Sedum surface through green-roof layers to roof slab, which behaves as a heat sink to store the received energy. At night time, Block 1 is cooled down relatively faster than Block 2 due to the small roofslab mass. Thus Block 1 materials tend to warm up and cool down more readily than Block 2. Indoor temperatures of the two blocks are similar. A small amount of heat absorbed at the roof has moved into indoor space. BTI omission does not offer more cooling benefits to Sedum roof. Rather, it permits marginally more heat to transmit into indoor space. 3.4. Temperature difference at Peanut plot Inter-block differences are confined to about ±4 ◦ C (Fig. 5c). Daily variations occur mainly in outdoor, especially at Peanut surface, 15 cm and 150 cm. They are more subdued in subsurface soil, rockwool, drainage and tile, and similarly limited in indoor.

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Fig. 5. Differences (Block 1 minus Block 2) in diurnal temperatures on summer sunny day along the holistic vertical temperature profile of: (a) control plot, (b) Sedum plot; and (c) Peanut plot.

C.Y. Jim / Ecological Engineering 69 (2014) 265–275

Differences at 15 cm and 150 cm shift from positive in the morning to negative in the afternoon. Except for a short period in the morning, Peanut surface at Block 1 is cooler than Block 2 in the 1.4 ◦ C to 4.6 ◦ C range. Indoor air temperatures at both blocks are similar in the whole day. The intensive solar energy regime has created inter-block differences mainly in outdoor and secondarily green-roof material temperatures. However, the differences are smaller than control and Sedum plots. Strong solar radiation input at Peanut roof has incurred convergence of thermal responses at outdoor. The irrigated Peanut roof has a low Bowen ratio, with cooling mainly contributed by evapotranspiration (Gaffin et al., 2006). Solar radiation is the main determinant of this cooling process (Jim and Peng, 2012). The Perennial Peanut plant with C3 photosynthetic mode promotes a high transpiration rate when soil moisture supply is adequate. The more efficient biologically-driven cooling can achieve more notable cooling than Sedum. The rather strong cooling mechanism has masked inherent differential BTI treatments of the two blocks. The thicker dual-layered substrate, composed of soil plus rockwool (Fig. 2), provides more mass for thermal insulation (Liu and Bass, 2005; Jim and Tsang, 2011a) to roof slab. For Block 1, this critical supplementation can partly compensate for the absence of BTI. For Block 2 with BTI, this effect is not so evident. With moisture and solar-heat absorption, however, the same substrate can become an effective heat sink to raise green-roof temperature (Jim and Tsang, 2011a). The sunny day with high evapotranspiration rate has extracted sufficient substrate moisture by around midday to dampen the heat-sink effect. The resulting indoor air temperatures at the two blocks are similar. Overall, the thicker and more elaborate Peanut green roof in conjunction with productive solarradiation use have largely nullified the ill impacts of BTI omission at Block 1. 3.5. Heat flux pattern at control plot Diurnal heat flux results at the three plots are summarized in Fig. 6. Their heat-flux budgets are depicted in Fig. 7, and their notable thermal processes are summarized in Table 3. The heat flux pattern at control plot resembles the bell-shaped diurnal solar radiation regime (Figs. 6a and 7; Table 3). Substantial amounts of heat energy, reaching nearly 100 W m−2 around midday, move into indoor space at Block 1 in daytime to generate conspicuous daytime peak thermal intrusion. The curve rises sharply in the morning and drops precipitously in late afternoon, indicating close adherence to solar input with little time lag. Comparable level and pattern of heat flux for bare roofs are recorded in tropical Singapore (Wong et al., 2007). The pattern indicates that exclusion of BTI, hence less resistance to conduction, has permitted heat gain in roof to transmit promptly into indoor space in daytime. At Block 2, heat flux maximum approaches 90 W m−2 at 1400 h despite BTI. The curve rises more gently in the morning and falls even more gently from afternoon through evening to night time to generate extended thermal intrusion. Its peak demonstrates a time lag of 1.25 h behind Block 1. BTI at Block 2 has enhanced the roofslab heat-sink effect and thermal inertia, inducing depressed but delayed heat-flux peak, but sustaining thermal intrusion into the night. In other words, BTI has moderated heat flux and spread it over a longer duration, but it has increased its total quantity especially in daytime. The long-intense tropical insolation has created high temperature, large heat storage and steep thermal gradient in the bare roof slab, overcoming the conductance-suppression function of BTI to cause thermal insulation breaching. Under the pressure of a steep thermal gradient, BTI may retard the rate but it is unable to reduce the absolute quantity of heat ingress into indoor space.

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At night time, fast radiative cooling of bare-roof surface creates a negative thermal gradient to draw heat upward. A similar night time heat loss is found in tropical Singapore (Tan et al., 2003). BTI deletion at Block 1 facilitates heat release from indoor space to generate nocturnal thermal discharge to counterpoise daytime heat gain. Thus the poorly-insulated roof slab is the only one that demonstrates bidirectional heat flux; the remaining five roofs are predominantly unidirectional (downward). All six roofs experience whole day net positive (downward) heat gain, but this roof registers the lowest amount (Fig. 7). However, BTI at Block 2 sustains at night time a positive thermal gradient to resist heat loss from indoor space through the roof to ambience to generate nocturnal thermal barrier. The thicker roof with a larger mass maintains the roof-slab heat-sink effect even at night. Night time heat loss from indoor space can reduce cooling load and air-conditioning energy consumption. If air conditioning is not used, heat dissipation is important to human comfort and health. Thus BTI’s role is somewhat ambivalent if not disappointing: it fails to reduce daytime heat entry into indoor space but suppresses night time heat escape from indoor space. 3.6. Heat flux pattern at Sedum plot Comparing with control plot, Sedum heat-flux pattern is exclusively positive (Figs. 6b and 7; Table 3). At Block 1, it is strongly positive in daytime to follow closely the bell-shaped solar input regime to create daytime peak thermal intrusion, attaining a high value of 117.4 W m−2 at 1500 h. A similar result was obtained by Yamamoto et al. using the same Sedum species (2006). The intense daytime peak and total influx, exceeding control plot, is attributed to moisture held by vegetation and substrate to enhance the thermal capacity of Sedum roof. Serves as frontline recipient of solar radiation, Sedum roof passes heat to the underlying roof slab. The simple Sedum roof creates a green-roof heat-sink effect (GHE) to store heat (Jim, 2014) for gradual and prolonged downward transmission from early morning to midnight as extended thermal intrusion to cause daytime aggravated thermal intrusion. BTI omission facilitates and accentuates heat passage into indoor space. For poorly-insulated buildings, Sedum is not a climate-appropriate species for tropical roof greening (Farrell et al., 2012). Unlike control plot, at night time at Block 1, heat flux remains positive albeit lower than daytime. The green roof does not bring nocturnal cooling effect as good as control plot (Jim, 2012). Rather, its effect is tantamount to nocturnal thermal barrier, analogous to BTI at control plot at Block 2 (Section 3.5). The simple Sedum roof has maintained a positive thermal gradient throughout the day to restrict nocturnal thermal discharge and hence retains heat in indoor space. Moisture in its green-roof material layers retains heat to create GHE, which sustains a positive thermal gradient at night to drive nocturnal thermal intrusion. It contrasts with control plot at Block 1 where a negative thermal gradient is created at night to facilitate nocturnal thermal discharge (Section 3.5). Installing Sedum green roof on poorly-insulated Block 1 has induced both daytime and nocturnal aggravated thermal intrusion into indoor space. This finding contradicts the belief that poorly-insulated roofs can benefit more from passive cooling of green roofs. At Block 2, green roof layers plus BTI have jointly formed a highly effective heat barrier in both daytime and night time to minimize thermal intrusion. No sharp peak is displayed and the maximum is merely 45.9 W m−2 at 1815 h. The simple Sedum roof plays the role of supplementary thermal insulation to work synergistically with BTI to form synergistic-dual thermal barrier to circumscribe thermal intrusion. Thermal insulation breaching (Section 3.5) has been suppressed. The twin layers have notably reduced the roof’s aggregate thermal conductance (Niachou et al., 2001). A simple Sedum green

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Fig. 6. Heat flux through the roof slab, respectively, on summer-sunny day and at Blocks 1 and 2 at: (a) control plot; (b) Sedum plot; and (c) Peanut plot.

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Fig. 7. Heat flux budgets of the three experimental plots at two blocks on summer sunny day, showing partition into daytime, night time and whole day net components, and positive (downward) and negative (upward) heat flux directions.

Table 3 The notable thermal processes that occur at the roof slabs of the three experimental plots on summer sunny day. Plot

Time of daya

Notable green-roof thermal processb Block 1 (without building thermal insulation)

Block 2 (with building thermal insulation) [+] Mainly downward thermal conductance [+] Extended thermal intrusion [+] Daytime thermal insulation breaching [+] Nocturnal thermal barrier [+] Green-roof heat-sink effect [+] Downward thermal conductance [−] Supplementary thermal insulation [–] Synergistic-dual thermal barrier [−] Daytime subdued thermal intrusion

Control

Wholeday

[+][−] Bidirectional thermal conductance

Sedum

Daytime Night time Whole day

[+] Daytime peak thermal intrusion [−] Nocturnal thermal discharge [+] Green-roof heat-sink effect [+] Downward thermal conductance [+] Extended thermal intrusion

Daytime

[+] Daytime peak thermal intrusion [+] Daytime aggravated thermal intrusion [+] Nocturnal aggravated thermal intrusion [+] Green-roof heat-sink effect [+] Downward thermal conductance [−] Supplementary thermal insulation [−] Synergistic-dual thermal barrier [−] Daytime subdued thermal intrusion [+] Nocturnal aggravated thermal intrusion

Peanut

Night time Whole day

Daytime Night time a b

Daytime in summer is reckoned from 0700 to 1900 h, and night time from 1900 to 0700 h. [+] Incurs more heat retention in indoor space, and [−] less heat retention in indoor space.

[+] Nocturnal aggravated thermal intrusion [+] Green-roof heat-sink effect [+] Downward thermal conductance [−] Supplementary thermal insulation [−] Synergistic-dual thermal barrier [−] Daytime subdued thermal intrusion [+] Nocturnal aggravated thermal intrusion

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roof installed on a building with BTI can provide ample comparative thermal benefits. Similar to control plot at Block 2, nocturnal thermal discharge has been inhibited. With or without BTI, Sedum roof has nullified the night time upward heat-loss pathway. The findings verify that to optimize the critical function of trimming thermal intrusion, green roofs should be installed on buildings with effective BTI. Conversely, buildings with no or poor BTI will suffer from both daytime and nocturnal aggravated thermal intrusion after green-roof installation. 3.7. Heat flux pattern at Peanut plot All values are positive and the two curves are clustered together and confined largely to a narrow 20–40 W m−2 range throughout the day (Figs. 6c and 7; Table 3). The rather monotonous pattern has no notable peaks or troughs to denote suppression of thermal intrusion as recorded in other tropical studies (Wong et al., 2007; Lin et al., 2013). Peanut roof with thicker substrate and denser foliage cover offers more effective thermal insulation (Del Barrio, 1998; Takakura et al., 2000) to generate synergistic-dual thermal insulation. However, thermal intrusion continues in the whole day due to GHE formation. Comparing with Sedum roof, daytime subdued thermal intrusion has reduced more heat flux, but nocturnal aggravated thermal intrusion remains comparable. The green roof with C3 plant generates efficient passive cooling due to evapotranspiration. It brings overriding cooling at all three weather scenarios, playing a significant equalizing role, to mask inherent BTI-difference of the two blocks. More heat enters indoor space at Block 1 than Block 2. BTI omission at Block 1 does not bring thermal advantages over Block 2 with BTI. The results cannot support the belief that poor BTI can enhance green-roof cooling effect. The findings verify that for poorly-insulated buildings, a somewhat elaborate broadleaved herbaceous green roof (rather than simple Sedum type) should be installed to realize passive-cooling benefits. Adding BTI can avoid some heat ingress into indoor space. The study offers corroborating evidence that a green roof with 10-cm substrate thickness, which is not particularly thick or heavy, in conjunction with a luxuriant foliage cover can effectively reduce heat ingress into indoor space (Jim and Tsang, 2011b). 4. Implications and conclusion Passive cooling is generally considered as the key benefit of green roofs. Many studies from multidisciplinary perspectives have been conducted to enhance its understanding. Most investigations focus on upward cooling of ambient air, with inadequate assessment of downward indoor cooling. The knowledge gap on the intricate interactions between green roof and indoor thermal regime could be filled by research. The findings could inform green-roof design, management and policies through knowledge exchange endeavors. This study explores an outstanding research question: Will poorly-insulated buildings benefit more from green-roof passive cooling. The issue has far-reaching implications as cities have many old buildings with outdated and inadequate roof thermal insulation. With increasing adoption of the green-roof innovation, it is necessary to find satisfactory answers to this conundrum. This study adopts a field experimental approach by comparing three types of roof plots (control, Sedum and Peanut) on two building thermal-insulation (BTI) states (with and without). Temperatures along a holistic vertical profile (outdoor, green-roof layers, indoor) at each plot, and heat fluxes between each plot and the apartment underneath, were analyzed in detail to decipher underlying patterns and processes. From the findings, the following

main conclusion and implication statements can be distilled with reference mainly to the hot tropical summer sunny day scenario:

(1) Poorly-insulated bare roof permits daytime peak thermal intrusion into indoor space. However, it facilitates effective nocturnal thermal discharge to release indoor heat through the roof at night time. It is the only roof type that demonstrates bidirectional heat exchange, reflecting reversal of the thermal gradient in a diurnal cycle, and resulting in minimal net heat gain by indoor space. The remaining five roof types all incur significant unidirectional (downward) heat flux and notable heat gain by indoor space. (2) Well-insulated bare roof triggers extended thermal intrusion that stretches from morning to midnight to convey heat into indoor space. The thicker roof slab offers a larger mass to store heat to create a roof-slab heat-sink effect. It sustains a positive thermal gradient in the whole day, to form a nocturnal thermal barrier to suppress night time heat dissipation from indoor space upward through roof slab. Overall, it pushes more heat into indoor space than poorly-insulated bare roof to cause thermal insulation breaching. (3) Simple Sedum green roof establishes notable green-roof heatsink effect to warm ambient air and indoor space. On poorly-insulated roof, it transmits considerable amount of heat to indoor space by extended thermal intrusion, inducing both daytime and nocturnal aggravated thermal intrusion. On poorlyinsulated roof, it maintains a steep positive thermal gradient in the whole day to resist nocturnal thermal discharge from indoor space. On well-insulated roof, it generates supplementary thermal insulation to reinforce building thermal insulation, forming a synergistic-dual thermal barrier, to eliminate daytime peak thermal intrusion, resist nocturnal thermal discharge, and ameliorate thermal insulation breaching. (4) More complex Peanut green roof with thicker substrate and denser foliage also creates green-roof heat-sink effect to work in unison with the roof-slab heat-sink effect to warm ambient air and indoor space. The vegetated roof with relatively thick substrate and vegetation layers generates supplementary thermal insulation to reinforce building thermal insulation, forming a synergistic-dual thermal barrier. Effective passive cooling is also provided by relatively high evapotranspiration rate. Daytime subdued thermal intrusion is attributed mainly to evapotranspiration cooling, but at night time nocturnal aggravated thermal intrusion continues. The two efficient cooling processes work jointly to mask the inherent differences between poorly- and well-insulated roofs to produce similar thermal responses. Well-insulated roof has a small advantage over poor one in trimming heat ingress into indoor space. (5) The hypothesis that poorly-insulated building can derive more benefit from green-roof passive cooling cannot be accepted for both Sedum and Peanut green roofs. This observation refers to simple extensive green roofs, and is derived from a study under hot tropical summer-sunny weather conditions. Green roofs with more complex biomass structure and thicker substrate may bring more passive cooling benefits to building with poor thermal insulation. (6) To optimize ecosystem service of passive cooling, green roofs should be installed on buildings with good thermal insulation, and should have a thicker substrate and denser foliage cover. A 10 cm substrate is verified to be sufficient to form an effective thermal barrier to reduce heat ingress into indoor space. (7) Formation of green-roof heat-sink effect can push more heat into indoor space than bare roof with reference to both poorly- and well-insulated roofs.

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