Gray Roof Research Articles

Understanding the hydrological performance of green and grey roofs during winter in cold climate regions

Green and grey roofs have emerged as promising and sustainable measures for effectively managing stormwater in urban catchments. However, there is a gap in the literature in understanding and modelling the hydrological performance of these roofs during winter and snow-covered periods in cold climate regions. The present study attempted to address this gap by validating the use of a snow module in simulating the dynamics of snow accumulation and melting of green and grey roofs. Then, the validated model was used to identify and separate the different events that occur in winter (melt only, rainfall only, rain-on-snow) to assess the hydrological performance of six different configurations of green and grey roofs in Trondheim, Norway. The snow module accurately simulated snow accumulation and melting of green and grey roofs. The results showed that rain-on-snow events in winter have longer duration compared to other events including rainfall events in summer. Consequently, rain-on-snow events yield a higher amount of inflow to the roofs compared to rainfall events in summer, despite summer events having higher intensities. The retention and detention performances of green and grey roofs were found to be lowest for rain-on-snow events compared to other types of events, but still yielding significantly lower peak runoffs when compared to standard black roofs. The decrease in retention and detention performances in winter were attributed to the long duration of events, accumulation effect of snow, freezing of roof surface layers, and reduction of evapotranspiration. The study highlights the importance of considering winter conditions in the design of green and grey roofs in cold climates to enhance stormwater management.

Green roof infrastructure outperforms grey technology in flood mitigation in São Paulo’s urbanized region

São Paulo City, Brazil, a densely urbanized area in a tropical climate region, is facing increasing challenges from flooding. However, a promising solution has emerged in the form of Nature-Based Solutions (NBS) infrastructure, surpassing traditional grey technology in flood mitigation. This article examines the effectiveness of green roofs in reducing flood risks and highlights their superiority over conventional approaches. We analyzed the direct impacts of surface permeabilization in public areas (sidewalks, streets, and squares) and private areas (urban lots, subdivisions) on runoff volume and peak flow in the Aricanduva River basin in São Paulo city. We used a thoroughly validated hydrological model to compare the interference in the land use of the watershed between a green infrastructure type called Green Roof (GR) and a gray technology called Permeable Pavement (PP), in terms of hydrological efficiency in mitigating floods. The flow and water level simulations for varying areas with GR or PP covered a wide range, allowing for situations above or below the canal overflow level, which provided a good distinction of the impact of boundary conditions. Overall, we observed a positive hydrological response, with a reduction in peak flow and runoff volume proportional to the increase in permeable area, and consequently, proportional to the increase in GR or PP surface within the contributing area. As the main contribution, the hydrological performance calculated with the variation of GR was remarkably superior to the variation of PP in terms of requiring a smaller coverage area with interference to prevent overflow or achieve equivalent efficiency. It is worth noting that the substitution of approximately 24% of the gray roof area with GR prevented canal overflow, which is notably lower than the required substitution of approximately 40% for PP interference. We reinforce the relevance of green roofs as an option to mitigate intense hydrological effects in urban areas and, more importantly, paves the way for other NBS with the same hydrological principles and environmental co-benefits to work individually or in combination.

Energy-saving, economic and environmental benefits analysis of cool roofs and green roofs for retail buildings in America

Green roofs and cool roofs are considered to improve energy efficiency, and provide economic benefits, energy-saving benefits, environmental benefits for the traditional grey roofs. Some national and local standards have analysed and recommended these technologies in existing building renovations, but their research subjects were mostly office or residential buildings, which didnt include the evaluation and analysis of specific roof renovations in different thermal zones in the United States. This paper conducted a 40-year life cycle cost analysis of retail buildings roofs in 16 cities across 8 different thermal zones in the United States. LCCA found that cool roofs showed a positive life cycle NPV in all cities, while green roofs showed a negative NPV. This article also analysed and compared the environmental benefits of retail buildings roofs in 16 cities, and calculated the emission reductions that they can bring to power plants. This paper also selected retail buildings roofs from 8 representative cities in 8 different thermal zones for energy savings and cost savings analysis. The results show that decision-makers can transform ordinary roofs into green roofs or cool roofs according to the characteristics of each thermal zone, combined with energy-saving benefits, economic benefits, environmental benefits and other factors, which is conducive to the implementation of new specifications.

Economic comparison of cool and grey roofs of a small office building in the USA

EnergyPlus V9.0 simulations of a typical small office building prototype in five USA cities (Albuquerque, Atlanta, Buffalo, San Diego, Rochester) indicate that adding a cool roof (albedo 0.6) for an aged grey roof (albedo 0.2) would reduce the annual energy and annual energy cost in all hot dry or humid cities (Albuquerque, Atlanta, San Diego) and in severe cold or mixed humid cities (Buffalo, Rochester). The CO2, SO2, and NOX savings are positive in all selected cities. This work considers the economics of using cool roofs of an office building in the USA by performing a 20-year life-cycle cost analysis that compares each type of roof to a grey roof. The 20-year net present values (NPVs) of annual conditioning (heating + cooling) energy cost savings were calculated. The life cycle cost savings (NPV of annual streams – initial cost premium) of cool roofs have both positive and negative values. The NPV with the cool roofs is from -17.3 USD/m2 in Rochester to 6.5 USD/m2 in San Diego. Among the five cities in the USA, Albuquerque, Atlanta, and San Diego were able to apply the cool roofs since they all have positive NPV, while Buffalo and Rochester were unsuitable to apply the cool roofs methods because of the negative NPV. Owners concerned with urban heat island mitigation and slowing climate change may prefer cool roofs

Sub-ambient cooling effect and net energy efficiency of a super-amphiphobic self-cleaning passive sub-ambient daytime radiative cooling coating applied to various buildings

In this study, a super-amphiphobic passive sub-ambient daytime radiative cooling (PSDRC) coating was applied to various buildings in different climatic regions in China. Irrespective of sunny, cloudy, and rainy weather conditions, the roof and west-facing wall surfaces coated with the PSDRC coating exhibited distinct sub-ambient cooling effects. Compared with the cool-white and grey roof surface temperatures, the roof surface temperature due to PSDRC coating was lower by 10.1 °C and 33.1 °C, respectively. Consequently, the room interior temperatures of different buildings were lower than the ambient air temperature during the day because of heat conduction and convection. The room with a window coated by the PSDRC coating had 18.2 °C lower interior temperature than that with a west-facing glass window. Thus, the PSDRC coating decreased the cooling energy consumption of buildings in the summer but increased the heating energy consumption in the winter. However, the net energy savings were considerable because of low solar irradiance and high heating energy demand when the sub-ambient cooling effect was diminished, which minimised the difference between the roof surface and room interior temperatures of the PSDRC and control buildings. After five-cycle dirt resistance tests, the solar reflectance of the super-amphiphobic PSDRC coating was reduced only by 0.0051. After 2000 h of artificial accelerated weathering tests, the solar reflectance of the superamphiphobic PSDRC coating was attenuated by 0.021 on average. This indicates the significance of the super-amphiphobic self-cleaning capability, which endowed the coating with a long-term sub-ambient cooling effect.

Environmental Identities and the Sustainable City. The Green Roof Prospect for the Ecological Transition

This research deals with the issue of the recovery of the historic urban fabric with a view towards ecological transition, nowadays considered the preferable direction of sustainability for the reform of the house–city–landscape system. The massive incentives provided by the Italian government for sustainable building, in view of the post-pandemic economic recovery, risk being reduced to mere support for the real estate sector, which turns the financial transfer from the public into an increase in asset value for the private sector. Such an incentive system could contradict the original function of the city, which is to be the privileged place for social communication and the creation of the identity of settled communities. A process of property development that disregards the distribution of income favors the most valuable property, thus increasing the socioeconomic distance between centrality and marginality. The latter is a condition that often characterizes the parts of the historic city affected by extensive phenomena of physical and functional obsolescence of the built heritage, and it is less capable of attracting public funding. The increase of building decay and social filtering-down accelerates the loss and involution of neighborhood identities; the latter constitutes the psycho-social energy that helps preserve the physical, functional and anthropological integrity of the city, due to the differences that make its parts recognizable. This study, with reference to a neighborhood in the historic city of Syracuse (Italy), proposes a model of analysis, evaluation and planning of interventions on the buildings’ roofs, aimed at defining the best strategy for ecological–environmental regeneration. The model presented allows one to generate a multiplicity of alternative strategies that combine different uses of roofs: from the most sustainable green roofs, but that are less cost-effective from the identity and landscape point of view; to the most efficient photovoltaic roofs from the energy–environmental point of view; and up to the most cost-effective ones, the vertical extensions with an increase in building volume. The proposed tool is an inter-scalar multidimensional valuation model that connects the multiple eco-socio-systemic attitudes of individual buildings to the landscape, identity, energy–environmental and economic overall dimensions of the urban fabric and allows one to define and compare multiple alternative recovery hypotheses, evaluating their potential impacts on the built environment. The model allows the formation of 100 different strategies, which are internally coherent and differently satisfy the above four perspectives, and it provides the preferable ones for each of the five approaches practiced. The best strategy characterizes most green roofs, 427 out of 1075 building units, 277 blue roofs, 121 green–blue roofs and 46 grey roofs.

Increasing water retention capacity via Grey roof to green roof transformation

AbstractThis study evaluated the potential of transforming nonvegetated (grey) roofs into vegetated (green) roofs to increase their water retention capacity and the retention of surface runoff in Taipei City. Of the total area of Taipei City (26 855 ha), we found that 304.5 ha of grey roof could be converted to green roof. Residential areas with higher living standards had a greater potential for grey roof to green roof transformation. The sampled substrate had an average porosity of 59.2% and a bulk density of 1.00 g/cm3. Surface runoff is likely to occur when the level of precipitation exceeds the field capacity of the green roof (13.3 mm/day). The highest percentage of runoff reduction (59.0%) occurred in the cold season. During the rainy months, the runoff reduction ranged from 14% to 28%. The water retention capacity of the green roof was higher during low‐intensity rainfall events than during high‐intensity rainfall events.

Influence of Traditional and Solar Reflective Coatings on the Heat Transfer of Building Roofs in Mexico

Building roofs are sources of unwanted heat for buildings situated in zones with a warm climate. Thus, reflective coatings have emerged as an alternative to reject a fraction of the solar energy received by roofs. In this research, the thermal behavior of concrete slab roofs with traditional and solar reflective coatings was simulated using a computational tool. The studied slab configurations belong to two groups, non-insulated and insulated roofs. In the second group, the thermal insulation thickness complies with the value recommended by a national building energy standard. Weather data from four cities in Mexico with a warm climate were used as boundary conditions for the exterior surface of the roofs. The computational tool consisted of a numerical model based on the finite volume method, which was validated with experimental data. A series of comparative simulations was developed, taking a gray roof as the control case. The results demonstrated that white roofs without insulation had an exterior surface temperature between 11 and 16 °C lower than the gray roof without insulation. Thus, the daily heat gain of these white roofs was reduced by a factor ranging between 41 and 54%. On the other hand, white roofs with insulation reduced the exterior surface temperature between 17 and 21 °C compared to the gray roof with insulation. This temperature reduction caused insulated white roofs to have a daily heat gain between 37 and 56% smaller than the control case. Another contribution of this research is the assessment of two retrofitting techniques when they are applied at once. In other words, a comparison between a non-insulated gray roof and an insulated white roof revealed that the latter roof had a daily heat gain up to 6.4-times smaller than the first.

Thermal behavior of a phase change material in a building roof with and without reflective coating in a warm humid zone

In this paper, the heat transfer in a concrete roof with an intermediate layer of PCM was studied numerically. Three positions and three thicknesses of the PCM layer and two values of solar absorptance (gray and white) of the roof were evaluated. The period studied was a typical summer week of a city in Mexico with warm humid weather. The interior boundary was in contact with the indoor air at a constant temperature. The unsteady two-dimensional model was solved by the Finite Volume Method using an implicit scheme. For the gray roof, the greatest reduction in the temperature of the interior surface (6.4°C) and in the heat gains (22.2%) was obtained when a PCM layer with a thickness of 2 cm is close to the interior surface. For the white roof, the results of the maximum temperatures of the interior surface and the heat gains between the different thicknesses and positions of the PCM layer showed negligible differences. However, the melting cycles of the PCM layer were affected as it was closer to the interior surface of the roof, and its thickness increased. The incorporation of a white reflective coating reduced the maximum interior surface temperature and the cooling load by 14.7–15.4⋄C, and 58.1–62.7%, respectively.

Limitations in using runoff coefficients for green and gray roof design

AbstractClimate change combined with urbanization increases the performance demand on urban drainage systems. Green roofs are one of the most used green infrastructure measures to alleviate the pressure on the urban drainage system through the detention and retention of runoff. The rational method with the runoff coefficient (C) is one of the most commonly used design tools for stormwater design in Norway. This method relies on a runoff coefficient being available for green roofs, which is typically not the case. This paper compares laboratory and experimental field studies to investigate runoff coefficients from different types of detention-based roofs. The methodology described in the German ‘FLL Guideline’, one of the world's most commonly used green roof standards, was used to measure the runoff coefficients for the different components making up a typical green roof. The contribution from each layer is reflected in the runoff coefficients. The runoff coefficients from the field experiments were calculated using observed precipitation and runoff from existing green roofs in Oslo, Trondheim, Sandnes, and Bergen, Norway. Events that had a cumulative precipitation comparable to the laboratory events, but longer durations, were selected. These events gave significantly lower and varying runoff coefficients, clearly demonstrating the limitation of choosing a suitable runoff coefficient for a given roof. However, laboratory experiments are important in understanding the underlying flow processes in the different layers in a detention-based roof.

Hydrological modelling of green and grey roofs in cold climate with the SWMM model

Rooftop retrofitting targets the largest land-use type available for reduction in impervious surfaces area in urban areas. Extensive green and grey roofs offer solution for retention and detention of stormwater in densely developed urban areas. Among the available green roof types, the extensive green roof has become a popular selection and commonly adopted choice. These solutions provide multiple benefits for stormwater and environmental management due to stormwater retention and detention capacities. The Storm Water Management Model (SWMM) 5.1.012 with Low Impact Development (LID) Controls was used to model the hydrological performance of a green and a grey (non-vegetated detention roof based on extruded lightweight aggregates) roof (located in the coastal area of Trondheim, Norway) by defining the physical parameters of individual layers in LID Control editor. High-resolution 1-min data from a previously monitored green and grey roof were used for calibration. Six parameters within the individual LID layers: soil (four parameters) and drainage mat (two parameters) were selected for calibration. After calibration, the SWMM model simulated runoff with a Nash-Sutcliffe model efficiency (NSME) of 0.94 (green roof) and 0.78 (grey roof) and a volume error of 3% for the green roof, and 10% for the grey roof. Validation of the calibrated model indicates good fit between observed and simulated runoff with a NSME of 0.88 (green roof) and 0.81 (grey roof) and with volume errors of 29% (green roof) and 11% (grey roof). Concerning the snowmelt modelling, the calibrated model showed a NSME of 0.56 (green roof) and 0.37 (grey roof) through the winter period. However, regarding volume errors, additional model development for winter conditions is needed; 30% (green roof) and 11% (grey roof). Optimal parameter sets were proposed within both the green and grey configurations. The results from calibration and especially validation indicated that SWMM could be used to simulate the performance of different rooftop solutions. The study provides insight for urban planners of how to target and focus the implementation of rooftop solutions as stormwater measures.

Test box experiment to assess the impact of waterproofing materials on the energy gain of building roofs in Mexico

In Mexico, the owners of homes use roof coating materials for waterproofing purposes and usually select them just for aesthetic reasons. In this work, four concrete roofs with different waterproofing materials (terracotta, white, gray, and black) were tested under weather conditions of Cuernavaca, Mexico. A set-up with two outdoor test boxes in which the roofs can be exchanged was monitored for six weeks of spring 2015. Three comparative tests (terracotta-black, white-black, and gray-black) were carried out during the period with the highest solar radiation of the year. The results showed that the white roof (WR) had the best thermal performance, under peak solar irradiance it was 29 °C cooler than the black roof (BR) and just 1.5 °C hotter than the outdoor air. Moreover, the average daily energy gain of the WR was 73% smaller than the BR. The terracotta roof (TR) and the gray roof (GR) had peak temperatures of 10 and 14 °C lower than the BR. Moreover, the TR and the GR had daily energy gains 33 and 38% smaller than the BR, respectively, but they are significant compared to one of the WR. Therefore, white waterproofing materials for roofs should be used in Mexico as a passive cooling strategy to prevent solar energy gains of buildings.

Life cycle assessment of white roof and sedum-tray garden roof for office buildings in China

White roof (WR) and Sedum lineare tray garden roof (STGR) have been convinced to improve the energy-efficiency and provide various benefits for conventional impervious grey roofs. Some national and local standards have standardized and recommended these technologies in existing building retrofits, however, they do not include assessment and choice of a particular roof retrofit in different climates. This paper presents a 40-year life-cycle cost analysis (LCCA) of an office building roof retrofitted by adding either WR or STGR over an existing grey roof in five cities, located in four Chinese climate zones. The LCCA find that the WR retrofits exhibit positive life-cycle net savings (NS) in warm winter zones, ranging 5.7–35.1 CNY/m2, and STGR retrofits have negative NS of -81.3– -16.7 CNY/m2 in all climate zones. The NS of both WR and STGR generally tend to improve as one moves from the coldest cities to the warmest cities.LCCA results suggest that adding new building codes concerning crediting or prescribing WR and STGR retrofits into office buildings with grey roofs in hot summer climate zones and warm winter zone in China, respectively. And featured by more specific requirements, the localized Technical Norms help promote the implementation of new building codes.

Experimental thermal evaluation of building roofs with conventional and reflective coatings

We evaluated the thermal performance of one conventional red and two white reflective roofs using two outdoor test cells situated in Cuernavaca, Mexico. The roofs of the cells are removable so that one can analyze different roofs with a different coating each. We monitored the test cells for two weeks and measured roofs surface temperature, indoor air temperature and determined the heat flux of the roofs. The red roof (RR) had a surface temperature 3°C higher than the gray roof (GR) and this difference caused the RR to have a peak heat flux 20% higher. On the other hand, the white reflective roofs (WR) reduced the surface temperature between 10 and 14.6°C compared with the GR. We obtained the daily heat gain of the roofs by calculating the area under the curve of the heat flux of each day. The WR had a daily heat gain between 59 and 80% lower than the GR, whereas the RR had a net heat gain 22% greater. Finally, using data from the experiments we validated roof model to determine the year-round performance of the roofs. From the results provided by the roof model, we estimated cost effectiveness of the roofs coatings. We found that WR are a cost-effective strategy to reduce building energy consumption in Mexico because its payback period is lower than 2 years.

Assessment of the impact of cool roofs in rural buildings in India

Increasing roof reflectance reduces absorption of solar radiation, roof surface temperature and heat ingress into the building. This leads to an increase in indoor comfort and reduction in room air temperature that in turn results in energy saving by reducing AC demand. To assess this impact of cool roofs on buildings in rural and peri-urban areas of India, a study was performed on two un-conditioned school buildings in Hyderabad (ZPH School) and Nagpur (VNHM School). Two classrooms of same size, function, and occupancy from both the school buildings were monitored for a period of ten weeks starting from March 2014 till May 2014. The data from the experiments and their analyses showed promising results. The average reduction and peak reduction in indoor air, roof underdeck and roof overdeck surface temperatures are 2.1°C, 5.0°C and 12.3°C and 4.3°C, 10.0°C and 26.3°C respectively for the room with white roof as opposed to grey roof in ZPH School and 1.5°C, 4.0°C and 9.5°C, and 3.3°C, 4.2°C, and 25.2°C respectively in VNHM School. The results from the experiment prove that white roof has a significant effect in reducing the indoor air temperatures in buildings due to reduction in outdoor and indoor roof surface temperature.

Cool roofs in China: Policy review, building simulations, and proof-of-concept experiments

While the concept of reflective roofing is not new to China, most Chinese cool roof research has taken place within the past decade. Some national and local Chinese building energy efficiency standards credit or recommend, but do not require, cool roofs or walls. EnergyPlus simulations of standard-compliant Chinese office and residential building prototypes in seven Chinese cities (Harbin, Changchun, Beijing, Chongqing, Shanghai, Wuhan, and Guangzhou) showed that substituting an aged white roof (albedo 0.6) for an aged gray roof (albedo 0.2) yields positive annual load, energy, energy cost, CO2, NOx, and SO2 savings in all hot-summer cities (Chongqing, Shanghai, Wuhan, and Guangzhou).Measurements in an office building in Chongqing in August 2012 found that a white coating lowered roof surface temperature by about 20°C, and reduced daily air conditioning energy use by about 9%. Measurements in a naturally ventilated factory in Guangdong Province in August 2011 showed that a white coating decreased roof surface temperature by about 17°C, lowered room air temperature by 1–3°C, and reduced daily roof heat flux by 66%.Simulation and experimental results suggest that cool roofs should be credited or prescribed in building energy efficiency standards for both hot summer/warm winter and hot summer/cold winter climates in China.

Rule-based classification of high-resolution imagery over urban areas in New York City

This paper describes the integration of results from different feature extraction algorithms using spectral and spatial attributes to detect specific urban features. Methodology includes segmentation of IKONOS data, computing attributes for creating image objects and classifying the objects with fuzzy logic and rule-based algorithms. Previous research reported low class accuracies for two specific classes – dark and grey roofs. A modified per-field approach was employed to extract urban features. New rule-sets were used on image objects having similar or near-similar spectral and spatial characteristics. Different algorithms using spectral and spatial attributes were developed to extract specific urban features from a time-series of Multi-Spectral Scanner (MSS) (4 m × 4 m) IKONOS data. The modified approach resulted in a remarkable improvement in the accuracy of classes that registered low spectral seperability and therefore low accuracy. The spectral and spatial based classification model may be useful in mapping heterogeneous and spectrally similar urban features.

Análise termográfica da temperatura superficial de telhas

Neste trabalho objetivou-se realizar imagens termográficas da superfície de telhas visando avaliar sua temperatura superficial. Os tratamentos mensurados se constituíram de: T1 -Telha Coppo Venneto Ondulada Cinza; T2 - Telha Coppo Venneto Ondulada Marfim; T3 - Telha Plana Marfim; T4 - Telha Plana Cinza; T5 - Telha de Barro Colonial esmaltada; T6 - Telha Ondulada de Cimento Amianto e T7 - Telha de Barro Francesa. Cada unidade experimental foi composta de três telhas sobrepostas uma sobre a outra, exceto no tratamento de telha ondulada de cimento amianto. O delineamento experimental foi em blocos casualizados com três repetições. Os dados coletados em cada horário foram analisados por meio do modelo de medidas repetidas, utilizando-se o procedimento MIXED do SAS. As temperaturas da superfície inferior das telhas foram muito parecidas ao longo do dia. As telhas com coloração marfim obtiveram melhores valores de temperatura superior e inferior que as telhas com coloração cinza, nas respectivas categorias. A telha de barro colonial obteve maior temperatura da superfície inferior que as demais telhas.

Potential benefits of cool roofs on commercial buildings: conserving energy, saving money, and reducing emission of greenhouse gases and air pollutants

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 Open image in new window0.126/m2 in West Virginia to Open image in new window1.14/m2 in Arizona (Open image in new window0.356/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 Open image in new window735 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.

PLANT MICROFOSSILS FROM A FORT UNION COAL OF MONTANA

FOSSIL SPORES and pollen are abundant in coals occurring in the Fort Union series near Red Lodge, Carbon County, Montana. Previous to this discovery only a single species of fossil spore (Miner, 1935) and no pollen grains were known from the Paleocene rocks. In North America, very few of the extensive Tertiary coal deposits have been explored for plant microfossils, although such study has great possibilities for the advancement of knowledge concerning the composition and stratigraphy of coal. The coal field in the vicinity of Red Lodge and Bear Creek, Montana, is extensive. According to Dr. Erling Dorf (personal written communication), coals are very high in the Fort Union series, are definitely post-Lebo, and probably postTongue River as now restricted (see Wood et al., Bull. Geol. Soc. Amer., vol. 52, plate 1, 1941). Woodruff (1909) has shown that there is a total of more than eighty-one feet of coal in the numerous seams. These occur through an 825-foot section of the Fort Union series. The coals range from smuts to seams twelve feet in thickness. There is a general thinning of the seams toward the southwest and thickening toward the northwest. The areal distribution of the seams indicates that they are more extensive than local lenticular deposits. The structure of the coal field is an eroded monocline which dips southwestward from the Pryor Mountains. At the foot of the Beartooth Range the monocline is terminated by a fault whose throw is several thousand feet. The dip of the beds in the vicinity of Red Lodge is 180, but south of Bear Creek the dip is much less. Minor undulations occur in the beds, and a small dome is present near the latter town. In 1940 and 1941 twenty-seven channel samples of coal were collected from seams one to eight inclusive. The seams are numbered from the top down. Maceration of the samples has revealed the presence of spores and pollen in varying degrees of abundance. Those coals composed mostly of modified woody tissues contain very few spores or pollen and have proved unsatisfactory for statistical correlation. This paper describes the fossil spores and pollen from one deposit, the Kolarich Mine. This mine is located in Section 25, Township 7 S., Range 20 E. It is approximately two miles east of Red Lodge. Some uncertainty exists concerning the position of this coal seam. Opinions of local residents are di1 Received for publication August 22, 1945. The authors wish to express their appreciation to Dr. Erling Dorf for information concerning the age of the Kolarich coal, to Dr. J. C. Gilman for examining the fungus spores, and to Dr. Theodor Just for reading the manuscript and checking the construction of the new names and combinations. For a grant-in-aid, the authors are indebted to the American Association for the Advancement of Science. vided as to whether the Kolarich Mine is exploiting seam No. 1 or No. 3. No conclusive evidence has yet been secured from microfossil studies that will solve the problem. The coal sample was secured from the wall several yards from the entrance of the drift mine. The rock section in the Kolarich Mine consists of (1) a gray undershale, (2) coal, 6 feet, (3) shale parting, 6 inches, (4) coal, 6 inches, (5) gray roof shale. METHODS.-Tertiary coals require various maceration treatments because they range widely in composition and structure. Since the Kolarich coal is both woody and close to sub-bituminous rank, it was necessary to use Schultz's solution for maceration. Pieces of coal were selected that did not contain visible woody structure. These were broken into pieces approximately one-half centimeter in diameter and covered with Schultz's solution for 24 hours. In the course of the maceration the mass was frequently stirred. This treatment was followed by washing with water, and decantation, until the odor of Schultz's solution was no longer apparent. The coal was then covered with ammonium hydroxide for another 24 hours. Again the material was washed and decanted until the odor of the ammonium hydroxide disappeared. The remaining sediment was examined for megaspores, but none were found. The finer sediment was stained with safranin, washed, centrifuged and transferred to glycerine jelly. Permanent slides were made from this preparation. All specimens are in the collection of the senior author.