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A comparative analysis of a white-roof installation during a New Zealand autumn and winter
Abstract
It has been asserted that a white roof reflects the sun’s rays back into space, reducing the heat absorbed into the structure beneath it and therefore reducing the cooling requirements of the occupied space. This paper presents comparative field data on structurally identical buildings with an installation of a painted white (Titania) iron roof compared to the neighbouring existing control (red) iron roof for autumn and winter. It is shown that there were significant variances in roof surface and roof cavity temperatures between the building with the white roof and the building with the red roof, and that the occupied rooms in the two near-identical buildings showed very similar temperatures during the cooler parts of the day. It is suggested that the roof colour of a ceiling-insulated structure has very little impact on the temperature of the occupied and insulated space, but that during the hotter parts of the day there can be up to a 10°C temperature differential in the roof. From this preliminary data of two neighbouring near-identical buildings, it appears that white roofs do help to reduce the internal temperature of a structure during hotter periods, even when that structure is insulated with ceiling batts, and does not impinge on the internal temperature of structures during cooler periods.
... For example, the structure of parks, squares and streets governs factors such as sunlight and shade access, heat accumulation, wind nuisance, downwash, wind channelling effects and ventilation (Blocken and Carmeliet 2004;Lenzholzer and van der Wulp 2010;Nugroho, Triyadi, and Wonorahardjo 2022;Speak and Salbitano 2022). Further, urban surface materials exhibit a range of thermal characteristics, including reflectivity, absorptivity, conductivity, and emissivity (Oke et al. 2017;Taleghani et al. 2021;Taylor 2016). Finally, urban density and roughness, and presence or absence of green structures influence relative humidity, air (Uittenbroek, Janssen-Jansen, and Runhaar 2013). ...
... For example, the structure of parks, squares and streets governs factors such as sunlight and shade access, heat accumulation, wind nuisance, downwash, wind channelling effects and ventilation (Blocken and Carmeliet 2004;Lenzholzer and van der Wulp 2010;Nugroho, Triyadi, and Wonorahardjo 2022;Speak and Salbitano 2022). Further, urban surface materials exhibit a range of thermal characteristics, including reflectivity, absorptivity, conductivity, and emissivity (Oke et al. 2017;Taleghani et al. 2021;Taylor 2016). Finally, urban density and roughness, and presence or absence of green structures influence relative humidity, air (Uittenbroek, Janssen-Jansen, and Runhaar 2013). ...
In recent years there has been a growing interest in the development and thermal-energy analysis of passive solutions for reducing building cooling needs and thus improving indoor thermal comfort conditions. In this view, several studies were carried out about cool roofs and cool coatings, producing acknowledged mitigation effects on urban heat island phenomenon. The purpose of this work is to investigate the thermal-energy performance of cool louvers of shutters, usually installed in residential buildings, compared to dark color traditional shading systems. To this aim, two full-scale prototype buildings were continuously monitored under summer conditions and the role of the cool shutter in reducing the overheating of the shading system and the energy requirements for cooling was analyzed. After an in-lab optical analysis of the cool coating, showing a huge solar reflectance increase with respect to the traditional configuration, i.e., by about 75%, field monitoring results showed that the cool shutter is able to decrease the indoor air temperature up to 2 °C under free floating conditions. The corresponding energy saving was about 25%, with even much higher peaks during very hot summer conditions.
Passive solutions for building energy efficiency represent an interesting research focus nowadays. In particular, natural materials are widely investigated for their potential intrinsic high thermal energy and environmental performance. In this view, natural stones represent a promising solution as building envelope covering and urban pavement. This paper concerns the experimental characterization of several low-cost and local gravel coverings for roofs and urban paving, properly selected for their natural high albedo characteristics. To this aim, the in-field albedo of gravel samples is measured with varying grain size. These in-field measurements are compared to in-lab measurements of solar reflectance and thermal emissivity. The analysis shows a significant variation of the albedo with varying grain size. Both in-lab and in-field measurements agree that the stones with the finest grain size, i.e., fine sand, have the best optic-thermal performance in terms of solar reflectance (62%). This feature results in the reduction of the surface temperature when exposed to solar radiation. Moreover, a natural mixed stone is compared to the high reflectance stone, demonstrating that the chosen stone presents an intrinsic “cool” behavior. Therefore, this natural, low-cost, durable and sustainable material could be successfully considered as a natural cool roof or cool paving solution.
Cool roofs—roofs that stay cool in the sun by minimizing solar absorption and maximizing thermal emission—lessen the flow of heat from the roof into the building, reducing the need for space cooling energy in conditioned buildings. Cool roofs may also increase the need for heating energy in cold climates. For a commercial building, the decrease in annual cooling load is typically much greater than the increase in annual heating load. This study combines building energy simulations, local energy prices, local electricity emission factors, and local estimates of building density to characterize local, state average, and national average cooling energy savings, heating energy penalties, energy cost savings, and emission reductions per unit conditioned roof area. The annual heating and cooling energy uses of four commercial building prototypes—new office (1980+), old office (pre-1980), new retail (1980+), and old retail (pre-1980)—were simulated in 236 US cities. Substituting a weathered cool white roof (solar reflectance 0.55) for a weathered conventional gray roof (solar reflectance 0.20) yielded annually a cooling energy saving per unit conditioned roof area ranging from 3.30 kWh/m2 in Alaska to 7.69 kWh/m2 in Arizona (5.02 kWh/m2 nationwide); a heating energy penalty ranging from 0.003 therm/m2 in Hawaii to 0.14 therm/m2 in Wyoming (0.065 therm/m2 nationwide); and an energy cost saving ranging from
0.126/m2 in West Virginia to
1.14/m2 in Arizona (
0.356/m2 nationwide). It also offered annually a CO2 reduction ranging from 1.07 kg/m2 in Alaska to 4.97 kg/m2 in Hawaii (3.02 kg/m2 nationwide); an NOx reduction ranging from 1.70 g/m2 in New York to 11.7 g/m2 in Hawaii (4.81 g/m2 nationwide); an SO2 reduction ranging from 1.79 g/m2 in California to 26.1 g/m2 in Alabama (12.4 g/m2 nationwide); and an Hg reduction ranging from 1.08 μg/m2 in Alaska to 105 μg/m2 in Alabama (61.2 μg/m2 nationwide). Retrofitting 80% of the 2.58 billion square meters of commercial building conditioned roof area in the USA would yield an annual cooling energy saving of 10.4 TWh; an annual heating energy penalty of 133 million therms; and an annual energy cost saving of
735 million. It would also offer an annual CO2 reduction of 6.23 Mt, offsetting the annual CO2 emissions of 1.20 million typical cars or 25.4 typical peak power plants; an annual NOx reduction of 9.93 kt, offsetting the annual NOx emissions of 0.57 million cars or 65.7 peak power plants; an annual SO2 reduction of 25.6 kt, offsetting the annual SO2 emissions of 815 peak power plants; and an annual Hg reduction of 126 kg.
High-albedo white and cool roofing membranes are recognized as a fundamental strategy that dense urban areas can deploy on a large scale, at low cost, to mitigate the urban heat island effect. We are monitoring three generic white membranes within New York City that represent a cross section of the dominant white membrane options for US flat roofs: (1) an ethylene–propylene–diene monomer (EPDM) rubber membrane; (2) a thermoplastic polyolefin (TPO) membrane; and (3) an asphaltic multi-ply built-up membrane coated with white elastomeric acrylic paint. The paint product is being used by New York City's government for the first major urban albedo enhancement program in its history. We report on the temperature and related albedo performance of these three membranes at three different sites over a multi-year period. The results indicate that the professionally installed white membranes are maintaining their temperature control effectively and are meeting the Energy Star Cool Roofing performance standards requiring a three-year aged albedo above 0.50. The EPDM membrane shows evidence of low emissivity; however this had the interesting effect of avoiding any 'winter heat penalty' for this building. The painted asphaltic surface shows high emissivity but lost about half of its initial albedo within two years of installation. Given that the acrylic approach is such an important 'do-it-yourself', low-cost, retrofit technique, and, as such, offers the most rapid technique for increasing urban albedo, further product performance research is recommended to identify conditions that optimize its long-term albedo control. Even so, its current multi-year performance still represents a significant albedo enhancement for urban heat island mitigation.
Increasing urban albedo can reduce summertime temperatures, resulting in better air quality and savings from reduced air-conditioning
costs. In addition, increasing urban albedo can result in less absorption of incoming solar radiation by the surface-troposphere
system, countering to some extent the global scale effects of increasing greenhouse gas concentrations. Pavements and roofs
typically constitute over 60% of urban surfaces (roof 20–25%, pavements about 40%). Using reflective materials, both roof
and pavement albedos can be increased by about 0.25 and 0.15, respectively, resulting in a net albedo increase for urban areas
of about 0.1. On a global basis, we estimate that increasing the world-wide albedos of urban roofs and paved surfaces will
induce a negative radiative forcing on the earth equivalent to offsetting about 44Gt of CO2 emissions. At ∼25/tonne of CO2, a 44Gt CO2 emission offset from changing the albedo of roofs and paved surfaces is worth about 1,100 billion. Furthermore, many studies
have demonstrated reductions of more than 20% in cooling costs for buildings whose rooftop albedo has been increased from
10–20% to about 60% (in the US, potential savings exceed $1 billion per year). Our estimated CO2 offsets from albedo modifications are dependent on assumptions used in this study, but nevertheless demonstrate remarkable
global cooling potentials that may be obtained from cooler roofs and pavements.
Greenhouse horticulture has experienced in recent decades a dramatic spatial expansion in the semiarid province of Almeria, in southeastern (SE) Spain, reaching a continuous area of 26,000 ha in 2007, the widest greenhouse area in the world. A significant surface air temperature trend of -0.3°C decade-1 in this area during the period 1983-2006 is first time reported here. This local cooling trend shows no correlation with Spanish regional and global warming trends. Radiative forcing (RF) is widely used to assess and compare the climate change mechanisms. Surface shortwave RF (SWRF) caused through clearing of pasture land for greenhouse farming development in this area is estimated here. We present the first empirical evidences to support the working hypothesis of the development of a localized forcing created by surface albedo change to explain the differences in temperature trends among stations either inside or far from this agricultural land. SWRF was estimated from satellite-retrieved surface albedo data and calculated shortwave outgoing fluxes associated with either uses of land under typical incoming solar radiation. Outgoing fluxes were calculated from Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance data. A difference in mean annual surface albedo of +0.09 was measured comparing greenhouses surface to a typical pasture land. Strong negative forcing associated with land use change was estimated all, year round, ranging ftorn -5.0 W m-2 to -34.8 W m-2, with a mean annual value of -19.8 W m-2. According to our data of SWRF and local temperatures trends, recent development of greenhouse horticulture in this area may have masked local warming signals associated to greenhouse gases increase.
One of the primary reasons for the application of cool materials is their energy and associated environmental impact on the built environment. Cool materials are usually applied on the roof of buildings to reduce cooling energy demand. The relative benefits of this reduction depend on the construction of the building, external weather conditions and use of the building. Through experimental and computational studies, it has been demonstrated that energy reduction benefits are significant in cooling dominated climates but is also observed in moderate climates. This chapter reviews available literature on this and also presents available simplified toolkits that can be used in feasibility studies to determine whether or not energy benefits are likely to materialize. The toolkits also calculate environmental benefits related to energy use by buildings and related cost benefits to the user. This chapter also presents additional environmental benefits related to the improvement of thermal comfort inside buildings which are not air-conditioned, the improvement of external thermal comfort which results from the mitigation of the urban heat island because of the use of cool materials and the effect on general pollution in the cities and health.
This chapter is a state of the art report focusing on the aspects that concern cool materials for buildings that are white or light colored. It is structured in four sections; in the first section cool white or light colored materials i.e. materials characterized by high solar reflectance and high infrared emittance are defined. Typical values of the two properties are given according to independent studies, the U.S. Cool Roof Rating Council's database and the IEE Cool Roofs project database. The impact of solar reflectance and infrared emittance on the surface temperature is explained and performance examples according to experimental results are compiled. The second section focuses on the types of cool white colored materials that are commercially available per roof type. These include build up roofs, single ply membranes, modified bitumen, tiles, coatings, metal roofs etc. A short description of each technology is given and their main characteristics are reported. The benefits of using cool white colored materials on buildings and the urban environment are analyzed in the third section. The benefits include increased thermal comfort conditions in buildings, reduced cooling energy consumption and peak electricity loads, increased life span of the roof system, reduction of the air pollution and greenhouse gas emissions and mitigation of the heat island effect. Each one of these benefits will be supported by experimental and simulation data available in the bibliography. Finally, the problem of ageing is analyzed. Exposure of cool materials to outdoor conditions has the effect of changing their main properties: solar reflectance and infrared emittance, compromising their performance. The main mechanisms (photo-degradation, thermal stress, deposition of pollutants etc.) that are responsible for causing the ageing effect of the materials are reported. Experimental results that estimate the aging effect of various types of materials and for several outdoor conditions are presented. The case of artificial and natural ageing is discussed as well as any effort to model the ageing effect of cool materials.
More than one-third of energy expenditure is attributable to buildings. Urbanization is leading to tighter spatial interrelationships among buildings, which is escalating building energy consumption due to the impact of buildings on one another. This, in turn, is exacerbating Urban Heat Island (UHI) effects. We sought a bio-inspired solution to this significant engineering issue and discovered a similar heat island effect in flowers, namely the “micro-greenhouse effect.” However, a special cooling effect has been observed in a temperate flower—Galanthus nivalis—which generates cooler intrafloral temperatures. In this paper, we studied the special retro-reflecting property of the flower petals, which has been suggested as a possible contributor to this cooling effect, and modeled a bio-inspired retro-reflective building envelope. We conducted cross-regional energy simulation of building networks in order to examine its thermal-energy impact. We found that building surface temperatures dropped considerably when neighboring buildings were retrofitted with a bio-inspired retro-reflective façade. We concluded that a bio-inspired retro-reflective building envelope can; (1) lessen the reflected heat of solar radiation in spatially-proximal buildings leading to reduced UHI, and (2) reduce the energy required for cooling. These findings have broad implications for building design, urban planning, development of retro-reflective technology, and environmental conservation.
Urban heat island is the more documented phenomenon of climate change. Information on the magnitude and the characteristics of the canopy layer urban heat island measured in 101 cities and regions of Asia and Australia and collected through 88 scientific articles, are compiled, evaluated and presented. Data are classified in several clusters according to the experimental protocol used and the type of statistical information reported regarding the magnitude of the urban heat island. Results and detailed analysis are given for each defined cluster. Very significant differences on the UHI intensity are found between the clusters and analyzed in detail. The detailed impact of the main weather parameters and conditions on the magnitude of the UHI is also investigated. The specific influence of anthropogenic thermal fluxes as well as of the urban morphological and construction characteristics to UHI is thoroughly examined. The relation between the UHI intensity and the city size is assessed and global relationships of UHI as a function of the urban population are proposed. The seasonal and diurnal variability of the UHI is analyzed and discussed while specific features and conditions like the urban heat island characteristics in coastal cities and the existence of daytime cool islands are explored. Finally, the impact of the selected reference station and its characteristics is considered.
Copyright © 2015 Elsevier B.V. All rights reserved.
White roofs are alleged to reflect the sun's rays back into space, reducing absorbed heat in the structure, and therefore reducing the cooling requirements of the inhabited space. This paper presents comparative field data on structurally identical buildings with an installation of a newly painted white (Taitania) roof compared to the neighbouring existing control (red) roof. This paper is the first in a longitudinal series covering all seasons. During two weeks of autumn/fall, it was found that as expected, there were significant variances in roof and roof cavity temperatures between the white roofed building and the control (red) roofed building. Further, it was found that the inhabited rooms in the two near-identical structures showed very similar temperatures during the cooler parts of the day, initially suggesting roof colour of a ceiling-insulated structure has very little impact on the temperature of the inhabited space, but that during the hotter parts of the of day there could be up to a 7°C temperature differential. From this preliminary data of two neighbouring near-identical buildings, it appears that white roofs do help reduce the internal temperature of a structure during hotter periods, even when that structure is insulated with ceiling batts, and do not impinge on the internal temperature of a structure during cooler periods.
Cool roofs represent an acknowledged solution for cooling energy saving. However, the efficacy of such physical intervention can be affected by occupants’ attitudes. Human behavior, in fact, is often neglected or underestimated at the design stage, e.g. while assessing the effect of physical retrofits. Accordingly, the purpose of this study is the assessment of cool roof effect with varying those occupants’ attitudes having some effect on energy need for cooling and indoor Thermal Deviation Index (TDI). The analysis has been performed through calibrated dynamic simulation of a continuously monitored historic building. Innovative cool roof clay tiles, suitable for application in historic buildings, have been selected as physical retrofit. Main findings show that occupants’ role can dominate the thermal-energy effect of the selected physical retrofit. For instance, cool roof tiles contribute to save 50% of primary energy for attic cooling in the hottest month, from 782 kWh to 398 kWh. If occupants’ adaptation availability in adjusting temperature setups is taken into account in combination of the cool roof effect, the energy need for cooling becomes less than 100 kWh. Also, the same cool tile is able to reduce the TDI from 0.87 to 0.54. If occupants are able to implement effective natural ventilation programs, TDI decreases to 0.29. These results show the necessity to consider neither only the effect of physical retrofits, nor only the effect of human attitudes, but the combination of both of them, in order to perform reliable energy need estimation, in both ante and post occupancy assessment.
Rejection of solar gains is the aim of passive cooling strategies in any type of building and any climatic region. The extent of cool materials’ applicability depends on the external climatic conditions and internal heat gains. The aim of the present paper is to analyse the contribution of an innovative cool fluorocarbon coating in the reduction of energy demand for cooling when applied in an industrial building with increased heat gains under temperate climatic conditions. The material is tested using accelerated weathering procedures and its optical properties, i.e. solar reflectance and infrared emmitance are measured. There is an increase of 120% of the roof's albedo by the application of cool material. Regarding the heating and cooling loads there was a decrease of 73% for cooling while there was a minor heating penalty of 5%.
Air temperatures in urban areas continue to increase because of the heat island phenomenon (UHI) and the undeniable warming of the lower atmosphere during the past few decades. The observed high ambient air temperatures intensify the energy demand in cities, deteriorate urban comfort conditions, endanger the vulnerable population and amplify pollution problems especially in regions with hot climatic conditions. The present paper analyses 40 years of hourly data series from nine meteorological stations in Greece in order to understand the impact of air temperature and relative humidity trends on the energy consumption of buildings. Using a typical office building, the analysis showed that for the period in question the heating load in the Greek building sector has decreased by about 1 kWh/m2 per decade, while the cooling load increased by about 5 kWh/m2 per decade. This phenomenon has major environmental, economic and social consequences, which will be amplified in the upcoming decades in view of the expected man-made climatic changes in this geographic area.