Roof Surface Research Articles

Where Are White Roofs More Effective in Cooling the Surface?

AbstractBy reflecting more sunlight, white (cool) roofs experience lower surface temperatures and produce less heating of the surrounding air. The effectiveness of cool roofs, quantified by the surface temperature difference between cool and regular roofs (ΔTs), is known to vary spatially. A common perception is that ΔTs is controlled by solar radiation that reaches the roof surface. Here we use an Earth System Model and a surface energy balance model to show that the spatial variability of ΔTs, when normalized by the albedo difference between cool and regular roofs Δα, is also controlled by an energy distribution factor that encodes the efficiencies of surface energy balance components in dissipating heat. Our results suggest that painting the roof white is more effective when the roof has less water‐holding capacity and smaller thermal admittance and is located in places with more solar radiation, less precipitation, and lower wind speed.

Extraction of urban built-up surfaces and its subclasses using existing built-up indices with separability analysis of spectrally mixed classes in AVIRIS-NG imagery

Understanding the urban environments and their spatio-temporal behavior is necessary for local and regional planning along with environmental management. For monitoring and analyzing the urban environment, remote sensing imagery has been widely used due to its ability for repetitive coverage over large geographical areas. Compared with conventional per-pixel and sub-pixel analysis of remote sensing imagery, spectral indices have noticeable advantages because of their easy implementation and fast execution. However, most of the spectral indices are designed for multispectral imagery to extract only one land cover class, and confusion between other land cover classes still persists. This research explores the most significant spectral bands in AVIRIS-NG hyperspectral imagery for detection of built-up surfaces and its subclasses i.e. roads and roofs. Further, this study utilizes existing built-up indices for detection of urban built-up surfaces in the first level followed by its subcategories in the second level. Finally, a separability analysis between spectrally mixed urban land cover classes using various measures is also addressed. Results of the analysis indicate that BSI, NBI, and BAEI can prove to be effective for extraction of built-up surfaces with an overall accuracy (OA) of 93.89%, 90.11%, and 85.15%, respectively. Further, REI with OA of 94.40% appears to be suitable for extraction of road surfaces while NBAI with 95% OA can prove its efficacy for extraction of rooftops in AVIRIS-NG imagery. It also concludes that, for aforesaid indices, built-up surfaces (Level-1 and 2) can be effectively separated from the bare soil in hyperspectral imagery with slight confusion between road and roof surfaces.

On the Wind Energy Resource above High-Rise Buildings

One of the main challenges of urban wind energy harvesting is the understanding of the flow characteristics where urban wind turbines are to be installed. Among viable locations within the urban environment, high-rise buildings are particularly promising due to the elevated height and relatively undisturbed wind conditions. Most research studies on high-rise buildings deal with the calculation of the wind loads in terms of surface pressure. In the present paper, flow pattern characteristics are investigated for a typical high-rise building in a variety of configurations and wind directions in wind tunnel tests. The aim is to improve the understanding of the wind energy resource in the built environment and give designers meaningful data on the positioning strategy of wind turbines to improve performance. In addition, the study provides suitable and realistic turbulence characteristics to be reproduced in physical or numerical simulations of urban wind turbines for several locations above the roof region of the building. The study showed that at a height of 10 m from the roof surface, the flow resembles atmospheric turbulence with an enhanced turbulence intensity above 10% combined with large length scales of about 200 m. Results also showed that high-rise buildings in clusters might provide a very suitable configuration for the installation of urban wind turbines, although there is a strong difference between the performance of a wind turbine installed at the centre of the roof and one installed on the leeward and windward corners or edges, depending on the wind direction.

Comparative Analysis of Ground-Mounted vs. Rooftop Photovoltaic Systems Optimized for Interrow Distance between Parallel Arrays

The aim of this research is to perform an in-depth performance comparison of ground-mounted and rooftop photovoltaic (PV) systems. The PV modules are tilted to receive maximum solar irradiance. The efficiency of the PV system decreases due to the mutual shading impact of parallel tilted PV modules. The mutual shading decreases with the increasing interrow distance of parallel PV modules, but a distance that is too large causes an increase in land cost in the case of ground-mounted configuration and a decrease in roof surface shading in the case of rooftop configuration, because larger sections of roof are exposed to sun radiation. Therefore, an optimized interrow distance for the two PV configurations is determined with the aim being to minimize the levelized cost of energy (LCoE) and maximize the energy yield. The model of the building is simulated in EnergyPlus software to determine the cooling load requirement and roof surface temperatures under different shading scenarios. The layout of the rooftop PV system is designed in Helioscope software. A detailed comparison of the two systems is carried out based on energy output, performance ratio, capacity utilization factor (CUF), energy yield, and LCoE. Compared to ground-mounted configuration, the rooftop PV configuration results in a 2.9% increase in CUF, and up to a 23.7% decrease in LCoE. The results of this research show that installing a PV system on a roof has many distinct advantages over ground-mounted PV systems such as the shading of the roof, which leads to the curtailment of the cooling energy requirements of the buildings in hot regions and land cost savings, especially for urban environments.

Looking for a needle in a haystack: Probability density based classification and reconstruction of dormers from 3D point clouds

AbstractAccurate reconstruction of roofs with dormers is challenging. Without careful separation of the dormer points from the points on the roof surface, the estimation of the roof areas is distorted. The characteristic distortion of the density distribution in comparison to the expected normal distribution is the starting point of our method. We propose a hierarchical method which improves roof reconstruction from LiDAR point clouds in a model‐based manner, separating dormer points from roof points using classification methods. The key idea is to exploit probability density functions to reveal roof properties and to skilfully design the features for a supervised learning method using support vector machines. The approach is tested based on real data as well as simulated point clouds.

Analysis of Rainwater Runoff Pollution Characteristics of Various Typical Underlying Surfaces in Ningbo

The pollution of rainwater runoff in urban areas can cause nutrient enrichment and eutrophication in receiving waters. To explore the pollution characteristics of rainwater runoff in Ningbo, eight sampling campaigns were carried out during rainfall events from 2009 to 2019. Samples of rainwater runoff were collected from underlying surfaces of roofs, squares, grassland, main roads, and the roads in commercial streets and residential districts. The concentrations of runoff pollutants, their sources and correlations, and first flush effects were studied using frequency statistical analysis and correlation analysis. The average event mean concentrations of the chemical oxygen demand (COD), ammonia nitrogen, total nitrogen, total phosphorus, and total suspended solids (TSS) in the rainwater runoff in Ningbo were in the ranges of 23.88-102.31, 0.40-1.69, 3.41-8.71, 0.09-0.50, and 37.6-323.4 mg·L-1, respectively. Apart from the square surfaces, the COD and total nitrogen pollution of the underlying surfaces was severe. The ammonia nitrogen concentrations from the roof, commercial street, and residential district surfaces were significantly higher (P<0.05) than those from the square, grassland, and main road surfaces. The concentrations of total phosphorus from the commercial street, main road, and grassland surfaces were significantly higher (P<0.05) than those of the other underlying surfaces. The correlations of TSS with COD, ammonia nitrogen, and total phosphorus showed that the pollutants and TSS have the same original sources in roof, square, main road, and commercial street runoff, while ammonia nitrogen and total nitrogen have the same original sources in runoff from grassland and residential areas. Under the meteorological conditions of light and moderate rain, the first flush effects of ammonia nitrogen in the runoff of roofs and grassland were observed clearly, whereas this was not the same for the pollutants of total nitrogen and total phosphorus in the residential area, main road, and square runoff.

Performance evaluation and comparative assessment of Sentinel-2B, Landsat-8, and AVIRIS-NG sensor imageries for extraction of road and roof surfaces using different algorithms

Urbanization in India has been rapid over the past few decades, which results in a substantial replacement of the natural surfaces into built-up lands. We present a performance evaluation of Sentinel-2B, Landsat-8 multispectral, and AVIRIS-NG hyperspectral imagery for extraction of road and roof surfaces using proposed spectral index-based and other conventional algorithms. The new road extraction index (NREI) and new building extraction index (NBEI) are developed for extraction of road and roof surfaces, respectively. Moreover, existing spectral angle mapper (SAM), spectral information divergence (SID), matched filter (MF), and support vector machine (SVM) are utilized as angle, information, filtering, and machine learning-based algorithms, respectively, for detection of both the surfaces. The results of our study suggest that the performance of AVIRIS-NG sensor is the best in comparison to aforesaid multispectral sensors, whereas Sentinel-2B performs better in comparison to Landsat-8 for extraction of road and roof surfaces. The comparison of various algorithms suggests that proposed indices, MF, and SVM produce the best results for extraction of road and roof surfaces, while SAM and SID are superior algorithms for extraction of both the surfaces in AVIRIS-NG imagery. Further, NREI and MF performed well for extraction of roads followed by NBEI and SAM for roofs in Landsat-8. Finally, NREI, SAM, and SID are found to be efficient for extraction of roads subsequently NBEI, SAM, and SID for roofs in Sentinel-2B imagery.

Comprehensive evaluation of thermal and energy performance of radiative roof cooling in buildings

This paper newly established a radiative roof cooling performance model to conduct thorough parametric studies on pivotal factors governing achievable equilibrium temperature and net cooling power in building roofs. A detailed building model integrated with radiative roof cooling considering typical climate regions in China was correspondingly developed to evaluate building thermal response and energy efficiency. The results indicate that compared with broadband and transmittance based adaptive emitters as roof surfaces, atmospheric window based selective emitter can achieve the minimum sub-ambient equilibrium temperature. Net cooling powers are reduced by 13–33.6 W/m2 and sub-ambient equilibrium temperatures are weakened by 7.1–13.5 °C with atmospheric precipitable water vapor increasing from 2.3 to 10 mm. The cooler degree of roof top temperature can reach over 30 °C under radiative roof surface which is conducive to avoiding thermal shock. The roof bottom temperature reduction noticeably reaches up to 13.8 °C for non-air-conditioned buildings, guaranteeing a more thermally comfortable built environment. The annual energy savings turn out to be positive for five typical cities, where cooling load dominant regions exhibit the most saving potentials with the saving rates of 7.1% for Hong Kong and 9.3% for Kunming respectively. With the application of a radiative roof cooling surface, the heat resistance layer can be removed from the thermal perspective for cost savings. The study sheds light on the promising benefits of radiative roof cooling for achieving energy efficient buildings in China.

The impact of cool and green roofs on summertime temperatures in the cities of Jerusalem and Tel Aviv

Experiments were conducted to assess the potential impact of cool and/or green roofs to mitigate summertime conditions in two cities of Israel, Jerusalem and Tel Aviv, which differ in elevation, humidity, and housing density. Tel Aviv is located along the coastal plain and characterized by low- and medium-density housing in a humid climate. Jerusalem is in the central mountains (elevation >750 m), and characterized by medium- and high-density housing in a dry climate. The fraction of potential roofs available for reflective cooling and the fraction of green versus impervious surface areas were estimated from “Google Earth.” Both were utilized as input into the Urban Canopy Model (UCM) within the Weather Research and Forecasting model, along with the residential building density. Increasing the albedo (cool roofs) had a larger impact on roof surface radiometric temperatures than covering the roof with irrigated soil and vegetation. Cool roof surface temperature differences were about 20 °C, compared to between 10 and 15 °C for wet soils with vegetation. The impact of varying albedo on 2-m. surface temperatures was about 0.4 °C, and the impact of varying soil moisture 0.1 °C. Increasing the leaf area index from 1.5 to 4.5 had a comparatively small impact on 2 m temperatures. Imposed anthropomorphic heating added about 0.2 °C to 2-m surface temperatures, which was less than values found in other studies conducted with more sophisticated building energy models. The surface temperature and heat index within Tel Aviv were more sensitive to mitigation efforts than those in Jerusalem, but both cities could benefit from mitigation efforts.

Experimental Study of Bond Strength between Existing Roof Slab and Concrete Support of Flat Roof Mounted Solar Arrays

Rooftop mounted solar array systems are connected to the existing concrete roof using casting concrete blocks over the existing concrete roof surface or by using precast concrete blocks. Since fixing the mounting system directly on the roof surface using a bolted connection is not advisable since it can affect roof strength, concrete blocks, either precast or cast in situ, are desirable to support the mounting system. To better understand the bonding mechanism at the interface between old and new concrete surfaces, overlaid specimens were fabricated to measure the various bond strengths. Three different bonding methods, which include cement slurry, casting over existing concrete, and epoxy, were used. The tests performed found that epoxy bonding provides better bond strength for new and existing concrete. To withstand wind loads, the foundation of the solar system should resist horizontal drag and vertical lift forces safely. Self-weight of supporting concrete blocks significantly reduces drag and lift forces coming over the solar system due to wind loads. The bonding strength between the existing concrete roof surface and array supporting concrete blocks also plays a vital role in resisting drag and lift forces. Since the weight of concrete blocks cannot exceed the design load for the roof, drag and lift forces can be resisted by increasing bond strength either by increasing the surface area of blocks (which would increase weight thus may defy purpose) or by using a superior bonding agent.

Impacts of Roof-top Solar Photovoltaic Modules on Building Energy Performance: Case Study of a Residence in HCM City, Vietnam

Vietnam is considered a country with high solar potential and solar irradiation is comparable to most countries in the region. However, high density solar project in areas causes overload state of transmission grid system and some plants wouldn’t operate at full design capacity. Therefore, roof-top solar PV projects in provinces and cities, which have large potential of solar irradiation, are promoted to reduce pressure on power supply. Besides, rooftop solar would be also affordable for home and business owners and communities by helping households halve electricity bills and gain energy efficiency. In this paper, we investigate case study of a residential building in HCM city, Vietnam for analysing impacts of roof-top solar photovoltaic modules on energy performance. The results showed not only the potential photovoltaic power, but also the reduction of roof heat transfer. In consequence, the building cooling load and total energy consumption are significantly reduced thanks to Roof-top PV installation. On the other hand, the reduction of solar incident on the building roof keeps cooling roof surface (outer and inner) and improves thermal comfort of attic space. Our research objective is to quantify indirect benefit of Roof-top PV on energy efficiency and to provide ideas for further researches.

Fusion of multispectral imagery and LiDAR data for roofing materials and roofing surface conditions assessment

ABSTRACT Assessment of rooftop rainwater harvesting (RRWH) quality and suitability requires detail and reliable information on roofs. Characterization of roof surface conditions affects the quality of harvested rainwater. Nevertheless, the implementation of the system requires improvement in terms of the roof detection techniques to ensure the roof of the building is selected appropriately. Thus, the classification techniques need to be optimized to detect roof materials and roof surface conditions (new or old) with high accuracy. This study aimed to produce high precision detailed roof materials and roof surface conditions map with using high-resolution remote sensing imagery, WorldView-3 (WV3) and light detection and ranging (LiDAR) data. Three different fusion methods; layer stacking (LS), Gram-Schmidt (GS) and principal components spectral sharpening (PCSS) were explored and their performances were compared to improve the spatial and spectral richness of the image. Subsequently, the roof materials and roof surface conditions classes which include old concrete, new concrete, old metal, new metal, old asbestos and new asbestos had been discriminated by employing support vector machine (SVM) and the rule-based technique known as a decision tree (DT). Generally, generated rule-sets present a higher overall accuracy with 87%, 72% and 66% for LS, GS and PCSS, respectively. For SVM classifier, the maximum accuracy recorded for LS, PCSS and GS were 70%, 63% and 43% respectively. Therefore, rule-based classification via LS fusion technique was utilized to identify suitable rooftops for the development of harvested rainwater system in the urban area. Findings indicate that the degradation status of a roof in heterogenous urban environments could be determined from satellite observation and the quality of roof-based harvested rainwater affected by roofing materials and roofing surface conditions can be analysed effectively.

Architectural Sustainability and Efficiency of Enhanced Waterproof Coating from Utilization of Waterborne Poly (Siloxane-Imide-Urethane) Copolymers on Roof Surfaces

According to Taiwan’s Ministry of the Interior, from 2017 to 2019, more than 12% of house-purchase disputes were due to water leakage caused by frequent tropical rains, which have long troubled engineers. The thermal stability resistance, water resistance, and ultraviolet resistance of existing polyurethane formulations have been limited by environmental aging. Thus, the lifespan of commercial PU-coated resins (typical PU) for the waterproofing of roof surfaces is merely two to three years. Accordingly, this study proposed the introduction of siloxane and imide groups to produce waterborne poly(urethane-siloxane-imide) (Si-imide-WPU) copolymers to improve the resistance of environmental aging in typical PU. The waterproof coating resin made of Si-imide-WPU copolymers was environmentally friendly, safe to use, and free of organic solvents. The results showed that the optimal Si-imide-WPU-2 sample in the study made improvements on the defects of polyurethane (PU) including its thermal properties, mechanical properties, environmental resistance, and lifespan which could be extended up to 5.4 years. Consequently, the studied Si-imide-WPU copolymers could reduce material waste while enhancing the sustainability and efficiency of the architecture.

Does the spatial location of green roofs affects runoff mitigation in small urbanized catchments?

Green roofs have been treated as practical low impact development (LID) strategies to retain stormwater runoff and alleviate the rainfall-induced flooding risks in urban regions. The purpose of this study was to analyze the hydrological effects of the spatial location of green roofs in urbanized catchments. In the built-up region of Beijing, 12 urbanized catchments with various architectural patterns were chosen as the study areas. To distinguish the spatial characteristics of roof surfaces, we defined the effective roof surfaces to distinguish from other types of roofs, which have more convenient or direct hydrological connections to drainage systems. A hydrological model was then used to simulate the stormwater mitigation performance of green roofs for the study catchments, which were assigned to different rainfall conditions. The simulation results confirmed the benefits of implementing green roofs for urban stormwater regulation. However, the spatial variability of green roofs showed inherent influences on the runoff mitigation capacity in urbanized catchments. Greening on effective roof surfaces would provide more effective stormwater regulation benefits, for reductions in both runoff volume and peak flow. In addition, the spatial arrangement characteristics of roof surfaces also influenced the hydrological efficiency of green roofs. The effect of the spatial location of green roofs on runoff mitigation was rainfall-dependent. These findings provide insights into the hydrological role of green roofs, and suggest that proper siting of LID facilities should be a consideration for urban stormwater management in order to fulfill the hydrological efficiency and cost-effectiveness planning target.

Above-roof air temperature effects on HVAC and cool roof performance: Experiments and development of a predictive model

In building performance simulations it is typically assumed that outdoor HVAC equipment is exposed to air at the outdoor ‘ambient’ temperature, obtained from a weather file. However, in reality, significant spatial variations in the outdoor air temperature can exist close to buildings. This simplification may lead to significant errors, especially when comparing simulations of buildings with and without cool roofs, since the colder surface of the cool roof would reduce above-roof air temperatures, thereby influencing the energy consumption of rooftop HVAC equipment.Above-roof temperature variations were measured in detail at three shopping centre buildings, and the experimental data was used to develop a model that can estimate air temperatures above hot or cold roof surfaces. The model requires only four input variables, all of which are available in typical building simulation tools.Implementation of the model in a set of case-study simulations revealed that above-roof air temperatures can have a large effect on the predicted performance of cool roofs. In cases where cooling equipment and ventilation inlet ducts were both located on the roof, the electricity savings and gas ‘penalties’ attributable to cool roofs would have been underestimated by 44–85% (61% on average) if above-roof air temperature variations had not been modelled accurately.

Progressive failures of batten to rafter connections under fluctuating wind loads

Batten to rafter connections in light framed timber housing can be vulnerable to wind loading, and failures of these connections are one of the more common failure modes seen in post windstorm damage surveys. Such failures often occur in a progressive or cascading manner resulting in the loss of a large section of the roof envelope. These progressive failures of batten to rafter connections are a complex process influenced by the pressure fluctuations on the roof surface, the response of individual connections and the behaviour of the structural system as a whole. This study presents a method for examining load redistribution and progressive failure behaviour of batten to rafter connections in light framed structures. Nonlinear time history analyses were performed using a finite element model using fluctuating pressures determined from a wind tunnel study and connection properties determined from laboratory testing of connections under dynamic loads.

A Comparison of Energy and Thermal Performance of Rooftop Greenhouses and Green Roofs in Mediterranean Climate: A Hygrothermal Assessment in WUFI

In urban areas, a considerable proportion of energy demand is allocated to buildings. Since rooftops constitute one-fourth of all urban surfaces, an increasing amount of attention is paid to achieving the most efficient shapes and component designs compatible with every climate and urban context, for rooftops of varying sizes. In this study, three types of rooftop technologies, namely insulated, green roof, and rooftop greenhouse, are evaluated for energy and thermal performance using computer simulations. Water surface exposure, absorption, and intrusion are the three important factors in the calculation of hygrothermal models that impact energy consumption and building envelope performance; however, a few studies are specifically focused on providing realistic results in multi-dimensional hygrothermal models and the assessment of the impact of moisture in roofing solutions. This paper aims at evaluating the performance of three different roofing technologies through a two-dimensional hygrothermal simulation in software WUFI. To accomplish this, a precise localized microclimate model of a complex urban context on the scale of a neighborhood was employed to evaluate the cooling and heating loads of the buildings, the impact of the water content in the green roof on the thermal behavior of the roof surface, and the feasibility of designing a building with nearly zero cooling needs. A two-story building in the city center of Bologna, Italy is modelled. Simulation results have shown that during the cooling period, the performance of the designed rooftop greenhouse is the most effective by 50% reduction in cooling loads. Besides, the impact of moisture in green roofs has been detected as a negative factor for thermal and energy performance of the building in the Mediterranean climate. The results ultimately highlighted the capability of passively-designed rooftop greenhouses to create a building with nearly zero cooling needs.

Kneeling trunk kinematics during simulated sloped roof shingle installation

Trunk musculoskeletal disorders are common among residential roofers. Addressing this problem requires a better understanding of the movements required to complete working tasks, such as affixing shingles on a sloped residential roof. We analyzed the extent to which the trunk kinematics during a shingling process are altered due to different angles of roof slope. Eight male subjects completed a kneeling shingle installation process on three differently sloped roof surfaces. The magnitude of the trunk kinematics was significantly influenced by both slope and task phase of the shingling process, depending on the metric. The results unequivocally point to roof slope and task phase as significant factors altering trunk kinematics. However, extension of the results to roofing workers should be done carefully, depending on the degree to which the study protocol represents the natural setting. Future studies on shingle installation in residential roofing should absolutely consider capturing a wider array of shingling procedures in order to encapsulate all the possible methods that are used due to the lack of a standardized procedure.

RAINWATER HARVESTING (RWH) INSTALLATION FOR BUILDINGS: A SYSTEMATIC REVIEW AND META-ANALYSIS APPROACH

Rainwater that falls upon a roof surface which are collected and transferred to the storage facility for later use is a method known as Rainwater Harvesting (RWH). This method is a step towards conserving the public drinking water where the demand continues to grow worldwide. In Malaysia, there is a huge potential of rainwater harvesting based on the available rainfall amount. Even so, the existing RWH design guideline in Malaysia lacks the technical specification regarding congruousness of RWH installation method with the physical characteristic of the building. Therefore, systematic literature review and meta-analysis were done to identify and understand the suitability between the types of RWH installation system with the building’s physical characteristic. The main objective of this study is to review the existing articles from year 2000 to the present on the installation systems used in RWH for non-potable purposes. The RWH installation system, types of building and building characteristics used in the previous studies will be evaluated in order to understand the suitability or regularity of the combinations. This review uses the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) as reviewing method. The systematic review process involved four stages which are identification, screening, eligibility and included. The sources of this review are Science Direct and Google Scholar. Results show there are three types of RWH installation system that were identified namely Gravity Fed System (GFS), Indirect Pumping System (IPS) and Direct Pumping System (DPS). DPS is commonly applied for non-potable purposes in residential, commercial and educational buildings. Small yard area, sloping site condition and types of soft landscape will affect the type of RWH installation system to be used. The result can be used to determine the suitability of the RWH installation system for different types and characteristics of a building.

Predicting Event-Based Sediment and Heavy Metal Loads in Untreated Urban Runoff from Impermeable Surfaces

Understanding the amount of pollutants contributed by impermeable urban surfaces during rain events is necessary for developing effective stormwater management. A process-based pollutant load model, named Modelled Estimates of Discharges for Urban Stormwater Assessments (MEDUSA), was further developed (MEDUSA2.0; Christchurch, New Zealand) to include simulations of dissolved metal loadings and improve total suspended solids (TSS) loading estimations. The model uses antecedent dry days, rainfall pH, average event intensity and duration to predict sediment and heavy metal loads generated by individual surfaces. The MEDUSA2.0 improvements provided a moderate to strong degree of fit to observed sediment, copper, and zinc loads for each modelled road and roof surface type. The individual surface-scale modelling performed by MEDUSA2.0 allows for identification of specific source areas of high pollution for targeted surface management within urban catchments.