Thermal Performance Of Green Roof Research Articles

Energy Saving Effects of Green Roof in Exiting Buildings according to Different Insulation Levels

Energy performance of building envelope components, including external walls, floors, roofs, windows and doors, is crutial for determining how much energy is required for heating and cooling in a building. Among various building technologies, a green roof system can be a good option for reducing heat gain and loss in new buildings as well as existing buildings for green remodeling. This paper evaluates the performance of green roof systems according to soil depth and Leaf Area Index (LAI) for existing buildings. It also attempts to quantify the energy saving effects on new and existing buildings with different insulation levels. Thermal performance of green roofs is mainly dependent on soil thickness and LAI. Installation of green roofs in deteriorated existing buildings can lead to improvements in roof insulation, due to the soil layer. An increase in soil depth leads to a decrease in heating load, regardless of conditions of vegetation on the green roof. Larger LAI values may reduce cooling loads in the cooling season. Installation of green roof in deteriorated existing buildings showed bigger energy saving effect in comparison to a case in new buildings. A simulation study showed that the installation of green roof systems in deteriorated existing buildings with low insulation levels, due to low thermal performance requirements when constructed, could improve the energy performance of the buildings similar or better to the peformance on new buildings with the most updated insulation standard. Thus, when remodeling a deteriorated building, green roofs could be a good option to meet the most recent energy requirements.

Snow depth and vegetation type affect green roof thermal performance in winter

Green roofs reduce building energy consumption in hot seasons, but cold season thermal performance has received little attention. The goals of this study were to quantify heat flux in an extensive green roof system, relate heat flux to solar radiation, substrate temperature and snow depth, and to determine the relationships between vegetation type, snow accumulation, and substrate temperature. Over the building heating season, we found lower net heat loss from green compared to conventional roofs. Doubling green roof substrate depth had no additional impact in lowering net heat loss. We also quantified substrate temperatures and snow depths from green roofs in different microclimates and vegetation types. Different roof microclimates (sheltered, exposed, over unheated building) resulted in differential snow accumulation; deeper snow resulted in lower variability in heat flux. The benefits of green relative to conventional roofs were lower in extreme winter conditions when the substrate was frozen and/or had snow cover, but also during sunny conditions. Plant species differentially affected depth and duration of snow coverage. Substrate temperatures also differed according to plant growth form during both snow-covered and bare conditions. Net thermal benefits of green roofs in winter will depend on climate, plant choice, roof construction and location.

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

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

Impact of plants transpiration, grey and clean water irrigation on the thermal resistance of green roofs

Evapotranspiration is one of the important processes in green roof heat and mass transfers. It is usually given in the literature by direct measurement or developed models to estimate both transpiration of plants and evaporation of soil at same time. In this paper, transpiration and evaporation were studied and evaluated separately. The thermal performance of green roof was evaluated in controlled weather conditions using a low-speed wind tunnel and the plants transpiration was measured in term of green roof thermal resistance. Green roof samples with two types of plants were tested. The results showed that plants evapotranspiration represented about 13% of the thermal resistance for ryegrass and about 37% of the thermal resistance for periwinkle. As an application, the impact of grey water on evapotranspiration and green roof thermal resistance was studied using the same laboratory facility. Green roof samples were tested in side-by-side studies using either grey water or clean water for long-term irrigation. Synthetic grey water of repeatable composition was used. Results showed visible effects of grey water on periwinkle but not on ryegrass. The thermal performance for green roofs irrigated with grey water was about 30% lower than those irrigated with clean water.

Effects of plant and substrate selection on thermal performance of green roofs during the summer

Green roof assemblies influence the total roof surface energy balance for a building. The energy balance for a green roof depends mostly on the selection of plants and substrates suitable for the building's location. This study measured thermal properties of common green roof materials and selected two types of plants and substrates to simulate transient thermal performance of different green roof assemblies. The selected plants and substrates have the highest and lowest reflectivity values to establish upper and lower bounds of thermal performance. The simulations use a previously developed green roof model including weather data for four cities representing different climate zones in the U.S. Based on the simulations, substrate heat fluxes and net radiation fluxes are compared for five days in July of the typical meteorological year. The results show that green roof assemblies receive net radiation fluxes that differ by 20%, and peak net radiation fluxes that differ by 16%, due to their different spectral reflectivity values. However, the substrate heat fluxes are similar for different green roof assemblies, as a roof insulation layer diminished this flux. Overall, the material selection of green roof assemblies is more important for buildings located in climate zone 4 or 5 than buildings located in climate zone 2 or 3, where limited water availability for evapotranspiration during hot, dry summers results in little thermal performance variability. Independent of the climate zones, simulation results show that the plant type has an important effect on the net radiation.

Validation of predictive heat and mass transfer green roof model with extensive green roof field data

Green roof technology has been adopted in the United States as a specialized roofing system and as a sustainable technology capable of saving energy. Most of the previous thermal performance models for green roofs have had the main goal of quantifying these energy savings. However, until recently, none of the models had been fully validated with laboratory and experimental data including both heat flux and surface temperature data. A recently developed green roof thermal performance model was validated with detailed experimental data from a new experimental apparatus called a Cold Plate, which is specifically designed and built for that purpose. In order to further examine the accuracy of the model, this paper describes the dynamic validation of the model using field data from a green roof installed on a commercial roof in Chicago. The dynamic validation consists of comparing substrate surface temperature, heat flux through the roof, and net radiation. The validated results show that the green roof thermal model predicts the heat and mass transfer appropriately as long as the long-wave radiation data from a weather station are used to reduce a possible bias resulting from the sky condition.

Dynamic U-value estimation and energy simulation for green roofs

Green roofs cooling and thermal insulating features have been demonstrated in many research projects. However, all efforts mostly have assessed green roof thermal properties under steady state conditions or by computational modeling. The present study evaluated green roof thermal performance in terms of the thermal transmittance coefficient, in real scale and under dynamic conditions. For the study's purposes, five semi-intensive green roof systems were constructed on the roof of an outdoor test cell. It was found that the green roof with 8 cm thick rock wool substrate with 2 cm sod on top had a very low U-value. For the same level of substrate moisture content, the other two green roof systems made of 8 cm deep coarse aggregate substrates with 2 cm sod on top provided higher U-values. In contrast, deeper amounts of same substrates (20 cm) reduced the U-value. The relation between the estimated thermal transmittance and the substrate moisture content was investigated and found to be linear. The green roof systems were also simulated for a single-storey residential building in order to quantify their possible energy savings. The results from the simulation showed that shallow substrates conserve building energy mainly during the summer period of the year. Rock wool and deeper substrates showed significant cooling and thermal insulating features.

Investigation of green roof thermal performance in temperate climate: A case study of an experimental building in Florianópolis city, Southern Brazil

Green roofs have been investigated as a bioclimatic strategy to improve the energy efficiency of buildings. Quantitative data on this subject are still needed for many specific climatic conditions. This paper deals with the investigation of the green roof thermal performance of an experimental single-family residence in Florianópolis (SC, Brazil), a southern city with a temperate climate. Field measurements during a warm period (01-March-2008–07-March-2008) and during a cold period (25-May-2008–31-May-2008) included internal air temperature of rooms, internal and external surface temperature of three types of roofs (green, ceramic and metallic), heat fluxes through these roofs, green roof's temperature profile, water volumetric content in substrate layer and meteorological data. During the warm period, the green roof reduced heat gain by 92–97% in comparison to ceramic and metallic roofs, respectively, and enhanced the heat loss to 49 and 20%. During the cold period, the green roof reduced heat gain by 70 and 84%, and reduced the heat loss by 44 and 52% in comparison to ceramic and metallic roofs, respectively. From the derived data it has been confirmed that green roof contributes to the thermal benefits and energy efficiency of the building in temperate climate conditions.

Modeling the heat diffusion process in the abiotic layers of green roofs

Green roofs have been increasingly installed to alleviate some common environmental problems. The thermal benefit of living vegetation on rooftop has been extensively studied. The individual and joint contribution of the non-living green roof layers, namely soil, rockwool (water storage) and plastic drainage layers, to thermal performance of green roof has seldom been assessed. This study evaluates the insulating and cooling effects of these abiotic materials. A one-dimensional theoretical model was developed to assess the heat diffusion process in the layers. The model was validated with empirical results from three experimental plots. A calibration procedure was successfully applied to determine key model parameters. The model can capture the most critical features of temperature variations and thermal performance of common abiotic green roof materials. The appreciable water-retention capacity of rockwool plays the dual role of supplying water to the soil to enhance evaporative cooling, and increasing the specific heat capacity of the green roof. The plastic drainage sheet with ample air spaces serves as an excellent thermal insulator. The model remains robust despite seasonal and weather variabilities. Our research findings contradict with some researches in the temperate region that the thermal dissipation in green roofs with dense vegetation is lower than thermally insulated bare roofs. The theoretical model could be used to simulate the micro-environmental conditions and predict the thermal performance of different materials to improve green roof design.

Thermal Performance of Green Roof Panels in Sub-Zero Temperatures

Abstract To date, much of the research on green roof technology has focused on the capacity for these systems to contribute to the cooling of buildings during summer months. The thermal performance of green roofs in cold climate conditions is critical to understanding the potential of these roofs to decrease energy use in buildings during winter. This paper compares the behavior of two green roof systems with that of a conventional built-up roof by making use of a novel hot box testing apparatus. The green roofs tested are classified as extensive systems. Each system included: a 3 mm thick styrene butadiene rubber waterproofing membrane, 0.2 mm thick polyethylene slip sheet, a 76 mm thick extruded polystyrene insulation layer, 2 mm thick filter fabric, a 51 mm drainage layer followed by a 2 mm thick filter cloth, either 100 mm or 150 mm growing medium, and a 25 mm thick wild flower vegetated mat. The conventional roof consisted of a 2 mm thick layer of Kraft™ vapour retarder bonded with insulation adhesiv...

Performance evaluation of green roof and shading for thermal protection of buildings

The present paper describes a mathematical model for evaluating cooling potential of green roof and solar thermal shading in buildings. A control volume approach based on finite difference methods is used to analyze the components of green roof, viz. green canopy, soil and support layer. Further, these individual decoupled models are integrated using Newton's iterative algorithm until the convergence for continuity of interface state variables is achieved. The green roof model is incorporated in the building simulation code using fast Fourier transform (FFT) techniques in Matlab. The model is validated against the experimental data from a similar green roof-top garden in Yamuna Nagar (India), and is then used to predict variations in canopy air temperature, entering heat flux through roof and indoor air temperature. The model is found to be very accurate in predicting green canopy-air temperature and indoor-air temperature variations (error range ±3.3%, ±6.1%, respectively). These results are further used to study thermal performance of green roof combined with solar shading. Cooling potential of green roof is found adequate (3.02 kWh per day for LAI of 4.5) to maintain an average room air temperature of 25.7 °C. The present model can be easily coupled to different greenhouse and building simulation codes.