Designing carbon neutral schools: The Victor Miller Building, a critical review Daniela BESSER 1 Lucelia RODRIGUES 1 Benson LAU 1 1 School of the Built Environment UK ABSTRACT: Nowadays, one of the UK Government s ambitions to reduce carbon emissions is to improve the design, environmental performance and energy efficiency of school buildings, aiming that all new schools in England become zero carbon by 2016. This paper uses the Victor Miller Building, a school building designed to achieve this goal, as a research vehicle critically analysing its environmental performance. It focuses on the role of the atrium as a passive design strategy, and its useful environmental contribution to the classrooms. Daylighting, thermal performance and ventilation strategy were qualitatively and quantitatively assessed within the atrium and typical classrooms, using different analytical tools which include on site monitoring, surveys and computer simulation. In this study, comfort conditions were critically evaluated and the performance analysis showed contradictions between the users perceptions and simulation results. Whilst staff and students showed a positive response to the building, simulation results indicate that the passive environmental strategies employed may not perform as expected, and comfort conditions might be heavily reliant on artificial lighting and heating systems. Hence, the users perception of the resultant conditions in the space might be primarily due to highly efficient active systems and well controlled indoor environment, and only secondarily due to its passive environmental design features. Keywords: Zero carbon schools, comfort in schools, environmental performance 1. INTRODUCTION umber of governmental initiatives aiming to improve the des energy efficiency of school buildings have been implemented in the UK in an attempt to reduce schools carbon emissions. Building Schools for the Future (BSF) was a programme to rebuild or renew every secondary school in England over a 15 years period aiming that all new school buildings become zero carbon by 2016. The Primary Capital Programme mirrored the BSF was entrusted to refurbish at least half of the country s primary schools by 2022 [1]. England s school building programme is currently under review by the new Government [2]. A zero carbon building is the one that produces net carbon dioxide emissions as much power as it uses over the course of a year. key step towards zero carbon buildings is to reduce their energy use. In this sense solar design strategies play an important as they encompass features which support passive heating as well as making the best use of daylight. In addition it becomes crucial not only to enhancing the but also to use efficient equipment and renewable sources of energy. Victor Miller Building is a recently built teaching block UK. It was designed to have the lowest possible carbon dioxide emissions. This paper reports on the passive environmental strategies used in the on the role of the atrium in relation to the classrooms. The aim is to analyse how the use of an atrium can positively contribute to the day passive solar heating and natural ventilation of its and it relationship with user comfort. These three environmental aspects are evaluated through both qualitative analysis based on the post occupancy evaluation of the building by quantitative analysis by on-site spot measurements and computer simulations. The double height atrium space and two typical classrooms were chosen in the ground and first floor to be evaluated. 2. ATRIUM BUILDINGS 2.1. space There are several definitions about atrium: the Latin word atrium alluded to the open central court & Aizlewood [3] state that the courtyard has been used successfully for thousands of years to bring air and light to the heart of the building. The development of (...) panels of glass allowed the courtyard to be glazed over, and transformed into the modern atrium -lit space which organizes a building. contributing to the environmental performance of its adjacent atria may act as buffer spaces with the ability to support a relative stable indoor environment. An atrium could thus contribute provide daylight to its adjacent spaces. It is therefore a space with a high energy saving potential and g maximum use of passive energy flows and seasonal climatic variations. EXAMPLES OF SUSTAINABLE ARCHITECTURE AND URBAN DESIGN 471
2.2. The Victor Miller Building The Victor Miller Building is one of the four buildings which compose the Bowbridge Primary School the most recently built in the scheme (2007-2008). It was designed to be a sustainable and eco-friendly educational able to support a 21 st century curriculum. In this e as carbon neutral as possible but also to become the core of a sustainable community which aspires to boost people s quality of life and well-being [5]. The building is a two-storey timber-frame construction and accommodates approximately 300 10 new classrooms and a large multi-propose atrium space. Its passive design strategies intended to make full use of daylight and boiler and rainwater harvesting were included. Furthermore the choice of the building materials and the construction method aimed to minimize environmental impact. The result is a compact rectangular shape eastwest oriented in order to take the most of solar energy through the south faç hich is not overshadowed by the surrounding buildings and is fully exposed to sunlight. A large double height atrium space is located along this faç all the classrooms are placed towards the north (Fig. 1). The atrium was designed to capture a vast acting as a buffer space between the classrooms and the external environment. The classrooms do not receive direct solar radiation overheating and glare. Timber was chosen due to its low embodied the foundations and the ground floor. The building s structural insulated panels (SIP) in order to achieve low U- efully studied in order to achieve an air-tight envelope. Figure 1: The Victor Miller Building's layout. Source: Nottinghamshire County Council According to the Display Energy Certificate (which assesses the energy performance of public highly A in a rating regard to the annual energy consumption of the 35 KWh/m 2 /year and electricity 21 KWh/m 2 /year. Comparing these values to the typical school s energy usage provided in the consumption is below 23%. 3. THE VICTOR MILLER BUILDING POST OCCUPANCY EVALUATION (POE) The comfort conditions within the building were qualitatively evaluated using surveys through questionnaires. Through satisfaction degree with the indoor environmental conditions was recorded and analysed. Two different surveys were applied to the users: a simpler one to quality comfort; and a more complex one to the s Trust (see www.usablebuildings.co.uk). The surveys were answered by 8 students and 12 staff members. Temperature 12% 62% 25% Winter 33% 33% 25% very cold neutral very hot Summer Light 12% 75% 12% Winter 33% 50% 12% very dim neutral very bright Summer Air Quality 12% 88% Winter 12% 33% 50% Summer stuffy fresh Figure 2: Students' POE survey results* Temperature 17% 17% 58% 8% Winter 10% 10% 10% 40% 30% Summer too cold too hot Natural Light 17% 17% 42% 8% 8% All year too little too much Air Quality 8% 8% 50% 17% 17% Winter 30% 10% 30% 20% 10% Summer stuffy fresh Figure 3: Staff's POE survey results* * Percentage of answers given by the users on each rating scale By comparing the results of both surveys (Fig. 2 and Fig. similar. ially in winter. Most of the answers are within the comfort rarely given. issues related to overheating and poor air quality during warm weather conditions. This is also 472 EXAMPLES OF SUSTAINABLE ARCHITECTURE AND URBAN DESIGN
supported by the students answers related to would prefer to open the windows more often during summer [6]. It can also be noticed that daylight was perceived lower than the benchmark by some members of the staff. 4. THE VICTOR MILLER BUILDING ENVIRONMENTAL PERFORMANCE 4.1. Lighting strategy and benchmarks The building was designed to be mainly day- receiving direct sunlight within the atrium space and diffuse light within the classrooms through the north windows. The first floor classrooms were also provided with roof lights in order to improve the amount of daylight into these spaces. designed when rooms are unoccupied. According to the Department for Education and Skills (DfES) [7] daylight should be the main source of light within schools. It states that a space is likely to be considered well lit if there is an average daylight factor of 4-5%. the maintained illuminance of teaching accommodation shall be not less than 300 lux on the working plane. When this cannot be achieved, the daylight will need to be supplemented by electric light. With regard to the uniformity ratio of the daylight it should be in the range 0.3 to 0.4 for side-lit rooms. Where spaces are top-lit, e.g. atria, then higher uniformities should be expected of the order of 0.7. CIBSE [8] recommends a minimum of 300 lux on which would be the case of the atrium. because this space i was considered it should comply with the same illuminance as the classrooms. 4.2. Daylighting performance prediction In order to assess the classrooms and atrium s using Ecotect and simulated using Radiance. One typical classroom on the ground floor () and one on the first floor () plus each level of the double- working plane was considered at 700mm from the finished floor level (students desks height). Table 1 shows the simulation results of each space with regard to daylight factor (DF) and the illuminance levels under different sky conditions. The uniformity ratio (minimum DF / average DF) was also calculated. The results showed the building achieves good daylight factors in all the analysed excepting the atrium if considering its use for group work. In relation to the distribution of the day this ratio is very low. The atrium does not achieve the given uniformity ratio benchmark as well - the value achieved on the first floor is considered as acceptable. On the ground floor is still low. With regard to the illuminance levels achieved on benchmark under all the analysed sky conditions. Only when it is sunny do the desks near to the windows achieve more than 300 lux [9] artificial light may be needed almost through the whole year in this space in order to achieve satisfactory illuminance levels. A similar situation was detected overcast sky conditions the benchmark is not also achieved. In contrast the room becomes very glare issues. Table 1: Daylight prediction results for each analysed space and under different sky conditions. Daylight Factor (%) Uniformity Ratio Illuminance (lux) [overcast] Illuminance (lux) [intermediate] Illuminance (lux) [sunny] 4.05 5.51 9.64 3.65 0.54 0.14 0.26 0.53 119 184 379 101 102 204 2809 605 291 1649 7936 1901 Figure 4: Daylight factors in section If the atrium is if it is considered as a the illuminance levels under overcast sky conditions are low on the but good on the ground floor. On the other the atrium receives a large be n the ground floor. when analysing the contribution of the at should be pointed out that the layout does not directly connect the atrium with the classrooms except for a small 1m 2 window on each classroom a significant amount of light into the classrooms. Th. 4. This is further confirmed by the low uniformity ratio which indicated an uneven daylight distribution. EXAMPLES OF SUSTAINABLE ARCHITECTURE AND URBAN DESIGN 473
4.3. Heating strategy and benchmarks The building s passive heating strategy relies on the highly insulated and air- reduces the active heating requirements of the values were calculated as 0.15W/m 2 0.27W/m 2 K for the walls and 0.24W/m 2 K for the floor. All the windows are double glazed with a U-value of 1.6 W/m 2 was designed to prevent overheating within the classrooms during summer. Moreover is supplied with a biomass boiler for active space heating. All the classrooms with a thermostat. The atrium space is heated by the atriums are usually designed to be unconditioned. The set temperatures are 19 o C when the building is o C when is not. DfES [10 18 o C shall be the minimum temperature during normal hours of occupation in areas where there is normal level of physical activity associated with 11] recommends 25 o C as an acceptable indoor temperature in non-air conditioned school buildings. As the indoor temperature rises from this design value an increasing number of people may become uncomfortable and there may be a decline in the productivity (...) of learning in schools. given overheating benchmark is 28 o C (peak of the annual occupied hours are above this peak temperature. 4.4. Thermal performance prediction The thermal performance of the building was assessed by both on site monitoring where the air and surfaces temperature were measured and recorded ynamic simulation using TAS by EDSL (see www.edsl.net). The same spaces previously analysed were assessed against different factors likely to affect their performance. The spot measurements taken on-site on an overcast mid-season day revealed that all internal air temperatures were within the comfort range while external air temperature was 9 o C. The spot readings showed a gradual air temperature rise from the north the atrium being slightly hotter than the classrooms. The internal surfaces of the building s envelope were slightly colder than the between 17 o and 21 o C [12]. The dynamic thermal simulations aimed to artificial heating. The evaluation was based on the annual percentage of hours where indoor air zone defined by the given benchmarks (18-25 o C). The same typical classrooms on each floor and both atrium levels were analysed. The assumed infiltration rate was 0.25ach. Three different theoretical cases were assessed: CASE 1: analyse the effectiveness of the building envelope s insulation an air-tightness. CASE 2: Unoccupied but naturally ventilated the ventilation strategy. to study the influence of internal gains due to the occupants (30 students per appliances Subsequent 3 (Case 3-A occupied time only (Monday to Friday between 9 considering a normal British primary school annual calendar). CASE 1 CASE 2 CASE 3 Ext. Temp. Class. Class. Class. Class. Class. Class. 67.4 60.3 45.7 46.3 73.0 66.8 52.1 52.4 58.4 54.8 46.7 46.8 92.6 7.1 0.3 32.6 32.0 7.8 41.1 13.2 40.3 13.3 26.9 32.9 47.1 46.7 41.4 44.9 52.3 52.1 <18oC (below) 18-25oC (comfort zone) >25oC (above) 0.1 0.3 0.8 0.9 0.2 0.4 1.0 1.2 Figure 5: Annual percentage of hours below, within and above thermal comfort CASE 3-A Ext. Temp. Class. Class. 24.1 27.7 34.8 33.5 90.5 75.3 71.7 64.3 65.5 9.5 <18oC (below) 18-25oC (comfort zone) >25oC (above) 0.6 0.6 0.9 0.9 Figure 6: Annual percentage of hours below, within and above thermal comfort (occupied time) Fig. 5 and Fig. 6 summarize the temperature prediction results for the different cases. When only time the indoor air temperatures are below the from 32 to 41% of the year within the analysed remove almost all the surplus heat due to solar gains inside the building but still remaining a small percentage of overheated hours in every analysed space. This percentage slightly increases when from 41 to 52% of the year temperatures within the comfort zone and remains 47 to 58% of the year if accounting only for the as in Case 3-474 EXAMPLES OF SUSTAINABLE ARCHITECTURE AND URBAN DESIGN
comfort zone between 64 to 75% of the time. building would rely in the active heating system for around a quarter of the occupied time in the approximately one third of the time in the atrium. In addition e coldest and the hottest day for Case 3 were compared (Fig. 7). ll the spaces are below temperature difference between the atrium and the o C of difference with the o C with the classroom (3pm). This temperature difference could be useful in order warmer air from the atrium could be transferred to 2 window placed in the partition wall between these spaces is the classrooms do not share a considerable wall surface because there are storage rooms between those spaces (Refer to Fig. 1). Figure 7: Daily temperature variation on the coldest and the hottest day of the year (Case 3) Table 2: Overheating risk assessment (Case 3-B) Occupied hours above 25 o C (%) Occupied hours above 28 o C (%) 0.60 0.60 0.95 0.95 0.00 0.00 0.09 0.26 D all the analysed spaces are above comfort during almost all the occupied only exceeds 25 o C during one hour (1pm). surplus heat gains might not be effectively removed by the passive cooling strategy. With this regard assessed adding the active heating system to Case 3-A -to-day use of the building (Case 3-B). Table 2 shows the annual percentage of time when the building is occupied and temperatures are above comfort. It also shows the percentage of the time where temperatures exceed the peak temperature given as the overheating as simulated here does not present overheating issues because none of the spaces is over the peak temperature more than 1% of the occupied time. 4.5. Ventilation strategy and benchmarks The building is naturally ventilated with automated openings. A computerised system opens and closes the windows ents based on the internal and external temperatures wind speed direction and precipitation. The system also monitors CO 2 levels opening the windows if the air quality is poor. Figure 8: Natural ventilation strategy. Source: Nottinghamshire county council As shown in Fig. 8 each space is ventilated independently. In the classrooms the fresh air linked to a damper which opens to allow the exhausted air to rise in a vertical duct behind a grille and to discharge at roof level by stack effect. allow the and low level windows open automatically to n buoyancy and stack effect. According to DfES natural ventilation is the preferred method of ventilation in schools [13]. With systems in all teaching accommodations shall provide 8 litres per second of fresh air per person to ensure air quality. 4.6. Natural ventilation performance prediction The natural ventilation strategy was assessed through computer calculations using Optivent software. The aim was to evaluate whether the strategy is able to achieve the required air flow rate for indoor air quality. In order to provide enough fresh the analysed classrooms would require a minimum air flow rate of 48 litres per second (approximately 0.48m 3 /s). The atrium would need at least 24 litres of fresh air per second (approximately 0.24m 3 /s). cooling achieved by the stack ventilation was assessed in all the analysed spaces [14]. Fig. 9 summarizes the natural ventilation analysis results. It can be said that during winter and mild weather conditions the ventilation strategy performs indoor air quality and being able to remove internal on buoyancy and stack effect. Nonethe spaces become overheated remove the surplus heat and cooling is therefore not can affect the users comfort and their productivity. EXAMPLES OF SUSTAINABLE ARCHITECTURE AND URBAN DESIGN 475
being critical within the classrooms. Air flow rate (m 3 /s) 14 12 10 8 6 4 2 0 classroom classroom w eq s w eq s w eq s required for fresh air required for cooling air flow achieved Figure 9: Achieved and required air flow rates through natural ventilation in winter(w), equinox(eq) and summer(s). 5. CONCLUSION The highly commended by its users through the performed post occupancy evaluation. Most of the staff and students feel comfortable or just slightly dissatisfied with the percentages of disconformities increase in achieving a positive outcome. The amount of light within the building was also positively although daylight was perceived slightly insufficient by some users. environmental performance through computer simulation for a considerable amount of time the building may need artificial means in order to provide comfortable. The daylight predictions showed that even though the spaces achieved adequate daylight for the tasks of reading and writing nt of light on the working plane would be lower than required within the classrooms throughout great part of the year. The relatively low uniformity ratio also indicates critical in the first floor classroom which may potentially leads to discomfort glare. F the atrium as major architectural feature does not seem to provide useful daylight benefits to the adjacent classrooms. the building would rely in the heating system for around a quarter of the annual occupied hours within the clas time s heating energy consumption may be higher than that of the classrooms due to its volume and high glazing ratio. it can be noticed the absence of a workable strategy to transfer the solar heat gains from the atrium to the connections between these two spaces. becomes clear that the solar heat gains from the atrium are not contributing to the passive heating of the classrooms. With regard to the ventilation performance provides enough fresh air within the analysed spaces during cold and mild weather conditions. In contrast during warm weather none of the analysed spaces is able to meet the air requirements neither for fresh air nor for cooling. The stack effect achieved is not sufficient to remove the surplus heat within the spaces during summer. In common with the contribute to the ventilation of its adjacent spaces. requirements during summer at times. In considering the potential role of atria as concluded that the strategy in this building is underused as it does not benefit the classrooms with regard to daylight performance or thermal comfort either in terms of moderating heat gains and losses or by encouraging ventilation. Significantly achieve good energy efficiency rating and is positively rated by its users. 6. ACKNOWLEDGEMENTS The authors would like to acknowledge Bowbridge Primary School staff and Nottinghamshire County Council architects department. 7. REFERENCES [1] Ventilation and Daylight in Schools. High Wycombe: Monodraught. [2] Department for Education. Website: www.education.gov.uk [Accessed October 2010] [3] in atrium buildings. Watford: Construction Research Communications. [4] York: McGraw-Hill. [5] Primary colour green: Bowbridge Primary [online] Available at: http://bsf.ncsl.org.uk/news.aspx?id=41 [Accessed October 2010] [6] (unpublished). Victor Miller Building: The atrium s influence on the environmental performance of classrooms [Case Study Project report for MArch in Environmental 2010] p10. [7] DfES Building Bulletin 87: Guidelines for Environmental Design in Schools. 2 nd ed. Version 1 pp 18-20. [8] guide A. 7 th p 1- -9. [9] p13. [10] -9. [11] pp 1-11 -12. [12] pp 14-15. [13] 15. [14] p20. 476 EXAMPLES OF SUSTAINABLE ARCHITECTURE AND URBAN DESIGN