a study on roof design strategies for energy conservation

Revisiting the Role of Architecture for 'Surviving’ Development. 53rd International Conference of the Architectural Science Association 2019, Avlokita Agrawal and Rajat Gupta (eds), pp. 781–790. © 2019 and published by the Architectural Science Association (ANZAScA).

A study on Roof Design strategies for Energy Conservation in Indian Buildings

Kuladeep Kumar Sadevi1, Avlokita Agrawal2 1, 2 Indian Institute of Technology, Roorkee, India

[email protected], [email protected]

Abstract: Buildings contribute a major portion of total primary energy demand across the world. Energy efficiency in buildings is considered as high priority subject in order to mitigate the adverse effects of energy consumption in buildings. Heating and cooling loads have been observed to take a major share in buildings’ energy demand, which are further contributed by the performance of building envelope components. Designing a Roof has always been a significant factor of heat gain in buildings for passive cooling/ heating strategies. Unlike walls, the roof is exposed to solar Radiation throughout the day and hence heat transfer from the roof is considerably notable for its impact on the user comfort. Various research studies have focused on the properties of roof’s surface to reduce radiative and conductive heat gain through the roof in buildings. This study attempts to carry out a comprehensive technical review of the various Roof design strategies such as Shaded roofs though canopy/ pergola, ventilated double roofs, and roof shading through photovoltaic panels etc. Research papers which aimed at assessing the above-mentioned roof design strategies, either for the impact on the heat gain/ heat loss through the roof structure or for the overall energy conservation in buildings, are considered for this study. The study reveals that the roof design strategies have significant impact on the energy performance of buildings. Also, it is observed across the studies that optimising the parameters of roof design is very important to maximise the impact on energy performance of buildings.

Keywords: Building Energy, Roof design strategies, Energy conservation

1. Introduction With the background of nations aiming for net zero and nearly zero energy buildings across the

world (IPEEC 2014) (European Commission 2018) (Agne Toleikyte (TU Wien) et al. 2016), India has been taking serious steps to promote energy efficiency across building sector (UNDP- GEF and BEE 2017). ECBC is one such initiative to promote energy conservation in Buildings targeting to reduce the energy demands by at least 30% compared to the as usual case.

Building energy codes have specific role in shaping up the way we build our buildings. With more and more built up area envisaged in near future, it is more important for the codes be robust enough to shape a better built environment. Air conditioning usage has been identified as one of the prime reasons

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for the raise in electricity consumption in recent years and the trend is expected to be increasing multi-folded in coming years. (Thambi, Bhatacharya, and Fricko 2018). Heating and cooling loads have been observed to take a major share in buildings’ energy demand, which are further contributed by the performance of building envelope components. Hence, Architects and designers always try to design buildings in a way that improves the thermal performance of buildings.

ECBC code has prescribed various envelope features for enhanced building performance. In the 2017 revision of ECBC incorporated higher thresholds for various envelope parameters in addition to the base level code compliance criteria. ECBC 2017 also rewards such buildings with better performance through higher levels of certification such as ‘ECBC Plus’ and ‘Super ECBC’ Buildings. These higher performance levels can be achieved by meeting further stringent threshold levels of parameters of various building components. For example adding more insulation to the roof and walls can help in meeting the prescriptive criteria for the enhanced performance. However, there are lot of passive strategies in order to achieve the same performance of the envelope behaviour which are not factored in any means through prescriptive approach. One needs to rely upon the computer modelling and simulation tools in order to factor the performance of such passive cooling techniques for energy conservation. Some of such passive techniques which are not included in prescriptive measures include roof shading, double skin ventilated roofs, double skin facades, vegetated walls, opaque wall shading through double skin all shading etc. Further, studies indicate that the early design decisions impact the overall building construction costs and operative performance (Østergård, Jensen, and Maagaard 2016). While the building simulation is not being used by most of the Architects (74% of the surveyed respondents) in their day to day practice (Soebarto et al. 2015), it is important to have early design assessment tools in place to assess the passive cooling design strategies, especially included in the energy codes, to help architects explore such strategies for their better performance. Also, simplified early design assessment tools makes architects take individual decisions on such passive design features. (Nault et al. 2015).

In tropical climatic regions such as Indian sub-continent, extensive use of air-conditioning systems especially during the hot seasons increase the electricity usage, adversely affecting the temperature regulation of human body and also leading to urban heat islands. Hence, passive cooling strategies need to be given priority, which can provide both thermal comfort and lesser energy consumption. (IEA-ECBCS-Annex37, 2004).

Roof has always been a significant factor of heat gain in buildings and thus passive cooling/ heating strategies in roof design. Unlike walls, the roof is exposed to solar Radiation throughout the day and hence heat transfer from the roof is considerably notable for its impact on the user comfort. The effect is considerably high in small scale buildings such as individual homes, apartments and low-rise buildings, contributing to about 70% of the total heat gain in hot climates.

Various research studies have focused on the properties of roof’s surface to reduce radiative and conductive heat gain through the roof component. Building energy codes are addressing roof reflectivity and roof insulation to achieve energy efficiency in buildings. However, various other roof design strategies which impacts the energy consumption in buildings are being practised by Architects, without

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actually quantifying the impact. This study is to explore such roof design strategies which contribute to the better energy performance in buildings.

1.1 Aim and Approach:

This study attempts to carry out a comprehensive technical review of the various Roof design strategies and their respective parameters which affect energy performance of buildings. Various roof design strategies such as Shaded roofs though through canopy/ pergola, ventilated double roofs, roofs shaded through photovoltaic panels and evaporative roof cooling systems are discussed.

Research papers which aimed at assessing the above mentioned roof design strategies, either for the impact on the heat gain/ heat loss through the roof structure or for the overall energy conservation in buildings, are considered for this study. Various design parameters of roof design evaluated in these papers are studied to understand the significance of such parameters in energy conservation in buildings.

1.2 Scientific Innovation and relevance:

In tropical regions such as India, it is predominantly hot days with intense direct solar radiation for most of the year. The temperature difference between the outer and inner surfaces of roofs is great. Hence, this study would help Architects to consider such strategies in their projects for better energy performance of buildings.

Adapting such roof design strategies in buildings would further improve the energy performance of buildings, laying further support to achieve Net Zero Energy Buildings. The output of this study would pave a base for the policy makers to include such design strategies into the mandatory national energy codes and standards

2. Relevance of Roof design features for energy efficiency in buildings:

Building envelope design has direct impact on the energy consumption in the buildings, especially the HVAC component of the buildings’ energy consumption. Building envelope is considered to be the major component in the building impacting the energy consumption (Okba 2005; Sadineni, Madala, and Boehm 2011). While the traditional architecture used to emphasise on the building envelope design parameters to address the user comfort, today’s buildings are more and more dependent on the mechanical systems to meet the user comfort. Also, keeping the speedy pace of building design and construction in these days, very less time being given in exploring the design opportunities to address the user comfort. Thus, the building envelope design features have been given least importance in all the stages of design process. Reducing the energy demand in buildings before addressing the efficiency of the mechanical systems to meet the demand, is considered to be highly effective strategy. While, the whole world is aiming for net zero or nearly zero energy buildings, shortly called as NZEBs, it is highly important to reduce the energy demand in buildings through building design and passive strategies for


building heating/ cooling. A report on NZEB targets for India (Kapoor, Deshmukh, and Lal 2011) also stresses upon the urgent need to reduce the energy demand, while increasing the renewable energy supply, to meet NZEB targets in India by 2030.

Roof is the surface of the building which is highly exposed to the solar radiation for most of the day, compared to walls of any orientation. Though the diurnal variation of solar intensity over the roof surface is varying drastically, the overall radiation intensity received on the roof is the highest because of continuous exposure to the sun during the day time. Sometimes, if the roof area needs to be used for any functional purpose, the roof is often protected from sun and rain by providing a covering over the roof in the form of canopy or a pergola. Though the intent of the provision of pergola or canopy over the roof is largely to cover the roof space for functional use, the shading helps in reducing the insolation and thereby the heat gain into the building through the roof membrane. Roof shading can be seen as an alternative to the provision of insulation to the roof membrane, to meet the reduced heat gain in the floors below. This would typically show impact on the energy consumption of the top most floor compared to the floors below. However, this is significant enough if seen at the whole buildings energy consumption pattern too.

Studies by Akbari et al, (2003), Bozonnet et al., (2011), Hernández-Pérez, I., et al., (2014) reveal that reduction in roof temperature and other building envelope components reduces the heat gain in the building leading to considerable savings through reduced overheating periods. In the study done by Susanti, L., et al., (2011) comparisons have been made between a cavity roof and a simple single roof in factories in Japan. Results showed that the cavity roof was better than the single roof in lowering the operative temperature by about 4.4°C and enhanced the life time of air conditioning system by reducing the cooling load.

Alvarado et.al (2009) studied the impact of the polyurethane insulation and reflector made of aluminium, and the results strongly prove that the heat flux is reduced by 88%. Due to the reflection of the light component of solar radiation by the white colored roof surface, the amount of heat received through solar radiation over the roof is cut down multi folds. Studies by Amer shows that high reflective surfaces over the roof has an impact on room temperature which drops by 6.5°C on average.

3. Passive cooling strategies in Roof design:

Knowing the importance of the roof design in order to reduce the heat gain into the buildings, a lot of measures have been incorporated in buildings over the decades. One of the basic strategies in order to reduce the heat gain through the roof is to reduce the incident solar radiation on the roof and there by cutting down the overall heat received on the roof surface. Next important strategy is to reduce the conductive heat gain through the roof assembly by introducing insulation to the roof assembly (layers). Further, ventilation is seen to be an additional feature to dissipate the accumulated heat over the roof and the interiors of the building.

Current ECBC codes specified prescriptive criteria only for two important parameters of the roof assembly. One is the overall U-Value (transmittance coefficient) of the roof assembly which needs to be

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as minimum as possible to ensure the minimal heat gain into the building through the roof layers. Second important feature prescribed by ECBC is to have highly reflective surfaces over the roof surface in order to reflect back the solar radiation. Materials with high solar reflective Index (SRI) are prescribed for better thermal performance of roofs.

The passive strategies in roof design which impacts the thermal performance of the roof assembly include Roof shading through canopy/ pergola, ventilated double roofs, roof shading through solar panels and Green Roofs. The following is the detailed literature review on such passive roof design features.

3.1 Roof Shading - Pergola/ Canopy

Oakley (1961) found that parasol roof could be one of the best solutions to address the heat ingress in the buildings, especially in hot humid climates. Parasol roof serves as a large canopy towards the spaces and allowing air movement in sides between roof and ceiling. Parasol roofs provide dual benefit in controlling the heat gain through the roof. Roof is the surface of the building which is highly exposed to the solar radiation for most of the day, compared to walls of any orientation. There are two main objectives of this roof for energy performance. One is to shade the roof from the harsh solar radiation over the roof and second is to allow the ventilated air carry out the heat gained over the surface through convective cooling. Thus the ventilated double roofs/ parasol roofs are one of the key designs features considered for this study which has huge impact on the buildings heat gain.

Nahar et al., (1999) studied various roof shading strategies and found that about 50% of the heat gains for single-story buildings is received via roof. Though the diurnal variation of solar intensity over the roof surface is varying drastically, the overall radiation intensity received on the roof is the highest because of continuous exposure to the sun during the day time. Sometimes, if the roof area needs to be used for any functional purpose, the roof is often protected from sun and rain by providing a covering over the roof in the form of canopy or a pergola. Though the intent of the provision of pergola or canopy over the roof is largely to cover the roof space for functional use, the shading helps in reducing the insolation and thereby the heat gain into the building through the roof membrane. Roof shading can be seen as an alternative to the provision of insulation to the roof membrane, to meet the reduced heat gain in the floors below. This would typically show impact on the energy consumption of the top most floor compared to the floors below. However, this is significant enough if seen at the whole buildings energy consumption pattern too. El-Razek et al., (2003) found that shaded roofs supported by ventilation improves the shading effect and further, movable shading helps in nocturnal radiation in night hours.

3.2 Double roof – Ventilated

Another form of parasol roofs is double roof which is ventilated. The specific feature of double roof is that the gap between two layers of the roof is optimized to enhance the ventilation and not to be used by the occupants of the building. Studies by Omar et al. (2017), indicates that such roofs have a potential of energy saving in an air conditioned buildings at a range of 3-7 % compared to a similar building with conventional roofing systems. Some of the parameters related to such parasol roofs which


impacts the energy consumption in buildings include the area of the roof that is covered, gap between the structural roof and the parasol roof, air velocity, the direction of the slope of the parasol roof in case of sloped roofs, and materials used for the roofing system. Villi et al. (2009) tried to correlate the characterization of the airflow and heat transfer phenomena in the ventilation cavity of ventilated double roofs. Based on the study of the thermo-fluid dynamic behavior of the air within the ventilated roof and the heat fluxes through ventilated roofs, Gagliano et al. in 2012 concluded that the ventilation of roofs can reduce significantly the heat fluxes (up to 50%) during summer season.

Takahashi (2001) has developed a double roof, of which the upper roof is thermally insulated to provide the function of shading over the roof, and the lower roof, namely the ceiling is composed of glass-fiber cloth of 1mm and zinc board of 0.5mm so that it functions as a radiant cooling panel., on which the rainwater is sprayed for evaporative cooling. The thermal comfort of interiors is achieved through the combined effect of natural ventilation and shading offered by the double roof system. However, the testing is being done for residential buildings.

Madi Kabore et al, have explored the potential of architectural and non-architectural passive cooling solutions to improve the thermal performance of the steel roof. The specific study on the thermal barrier In the form of attic has been explored and found that Ventilation improvements by increasing areas and height between the opposite openings show insignificant effects on the ceiling and the occupied zone air temperatures.

3.2 Roof Shading - Solar Panels

Provision of solar panels has become a mandatory aspect of future buildings in order to make them energy efficient, especially in making net zero or nearly zero energy buildings. Best face of the building to install solar panels is roof for its high exposure to sun compared to other vertical surfaces in any given day. As discussed earlier for buildings in India, being a tropical country and near to the equator, the solar exposure to the roof is considerably high. The solar panels installation over the roof is typically tilted to certain angle for different locations in India based on latitude of location, for optimised performance of the solar panels. This makes the solar panels array over the roof act similar to the parasol roof in controlling the heat ingress into the buildings.

The roof top solar panels always provide dual benefit – first by generation of electricity and by adding to the thermal performance of the roof through shading. Since the intent of installing solar roof top systems is majorly to generate electricity, the benefits of shading by the solar PV modules is not typically considered during the design and installation. A study by Ahmed Bilal Awan (2019) indicates that very limited research is done on impact of shading the roof by solar PV systems, while much research has been observed on integration of solar panels for windows and wall systems. Awan has conducted a study to evaluate the performance of the rooftop PV system and its shading benefits. The study is done for three types of installation approaches: “(1) a flat PV system, (2) a monthly tilt angle adjusted PV system, and (3) a dual-axis sun tracking PV system.” The typical parameters of the roof shading through solar panels include the tilt angle, monthly variation in tilt angle, roof coverage by solar panels, latitude of the place and distance at which the solar arrays are placed with respect to the tilt angle. Awan’s study indicates that the energy savings in the cooling load in buildings vary in the range of

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53.4% for fully shaded rooftop. The energy savings seems to be reduced till 10% based on the, arrangement, tilt angle and spacing between the solar arrays.

3.3 Green Roofs:

Besides the thermal insulation which is offered by the oil layered of roof gardening, the shading of the roof offered by the plant foliage and moreover, the transpiration cooling rendered by the natural activity of the plants are additional benefits and thus considered for this literature. Many researchers have carried out simulations to bring in a methodology to consider the effect of green roofs in the simulation tools for an effective assessment of the green roof’s performance for energy consumption in buildings. Jaffal et al (2012) has classified the green roofs into two major categories based on the substrates the green roofs include. One is the extensive green roofs where the roof assembly has shallower substrates which is less than 200 mm and the other category is the intensive green roofs with substrates more than 200 mm. The second category generally adds on to the structural load of the roof, while it may offer better resistance to the heat gain through the roofs.

Ran & Tang, (2017) compared the performance of green roofs with wall insulation, both along with the night ventilation, and identified that green roofs can reduce the average indoor temperatures up to 2.3°C compared to that of wall insulation. A comparative study done by Pandey et al., (2012) for Indian city indicates that the green roofs always perform better in summer season when compared to the conventional RCC roofs. Also, the peak temperatures and the diurnal variations are found to be reduced to a considerable extent. Yang et al., (2015) have compared various roof types and found that green roofs outperformed the other roofs in terms of thermal performance. The study highlighted the comparison of exposed roof to that of green roofs, whereby green roofs provided a cooler indoor air temperature 0.9–1.0◦C as compared to that of exposed roof in summers.

Himanshu et al (2018) studied the performance of green roofs by simulating the green roofs in

summer months to check the variation in indoor temperature due to the provision of green roofs over a conventional roof. The study reveals that the variation in indoor temperatures ranges from 0.85 °C to 6.31 °C. Niachou A, et al studied the roof temperatures of a conventional building with and without the

green roofs. The results indicate that the surface temperatures of the conventional building are varied

from 42 to 480C while the surface temperatures of the buildings with green roof are in the range of 28 to

400C which is much lower than the conventional case.

3.4 Roof Ponds:

Evaporative cooling has been an effective cooling strategy in tropical climates, which was an integral part of the architecture in tropical climates centuries ago evident through Mughal architecture in India (Bowen, A.B., 1981). Roof ponds, typically the use of evaporative cooling as a provision over the roofs to handle the heat gain through roof. The research studies on this technique dates back to 1920s and various types of roof ponds have been evolved over the period (A. Spanaki, 2007). A comparative study by A. Spanaki (2007) on ‘different type of roof ponds for cooling purposes’ highlights different roof ponds classified by the parameters such as presence of covering, sprays, containment of water within paparet etc,. Also other roof pond types such as coolpool (open pond shaded by sloping louvers), coolroof (with floating insulation), walkable pond with night water circulation (insulation embedded


within the pond), wet gunny bags (with a “floating” wetted cloth) and ventilated roof pond are explored. The study concludes that wetted gunny bags and movable insulation as the most efficient type of roof ponds. Also, large air-conditioning energy savings were estimated, reaching 100% in a variety of locations.

Kharrufa and Adil (2008) observed that the forced ventilation over a roof pond can impact the energy savings by 29% reduction in cooling load when compared to that of conventional roof system. The requirement for water is identified to be the main disadvantage of various types of roof ponds and that too in tropical climates. Nahar et al. (1999) estimated that about 50 litres of water would be required for an effective evaporative cooling. In warm and humid places, roof ponds fail to perform due to the presence of higher relative humidity levels.

4. Conclusion: The study reveals that the roof design strategies have significant impact on the energy

performance of buildings. Passive strategies in roof design for energy efficiency in building were evolved since decades and some even centuries through traditional architecture. Different types of roof design strategies perform at their best based on the type of climatic conditions they are used in. The energy savings from these strategies are envisaged in wide range based on various parameters the designs are associated with. Also, it is observed across the studies that optimizing the parameters of roof design is very important to maximize the impact on energy performance of buildings.

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a study on roof design strategies for energy conservation

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