Enhancing visual comfort in classrooms through daylight utilization

Enhancing visual comfort in classrooms through daylight utilization Kleo Axarli and Katerina Tsikaloudaki Laboratory of Building Construction & Physics, Aristotle University of Thessaloniki, Greece Corresponding email: axarli@civil.auth.gr SUMMARY Research has confirmed that the well-being and the school performance of pupils depend significantly on the quality of the luminous environment, which can be achieved through daylight utilization. This paper focuses on the impact of different fenestration systems on the visual comfort achieved in classrooms. Various window locations, clerestories, roof openings and light shelves were examined with regard to indoor daylight conditions. The study was conducted as a parametric analysis, in which models incorporating the above-mentioned systems were generated on the basis of a typical classroom. The outcomes of the analysis enabled the performance evaluation of the examined apertures. The main objective of this study was not only to derive results regarding the performance of each system separately, but also to highlight specific strategies, which would promote daylight admission without increasing the construction and operation costs in school buildings. INTRODUCTION Nowadays the merit of daylight is well acknowledged; research has revealed not only its significant impact on our visual system, but also its supportive role on physical and psychological health, improved productivity and performance of the occupants. These positive effects are highly appreciated in schools, where daylight levels are directly related to student s performance and enable the creation of a pleasant atmosphere for learning and teaching activities. A research conducted in over 20,000 elementary students and 100 schools in three school districts of USA has provided evidence to this statement. It has been found that students progressed 26% faster in reading and 20% faster in math in classrooms with high levels of daylight. Similar findings were observed in classrooms with well-designed skylights and louvers controlling the illumination level [1]. Among the potential mechanisms that may have been responsible for the positive association between daylight and improved performance of students, improved visibility due to higher illumination levels and better lighting quality, mental stimulation, as well as improved mood and well-being are included. This link between increased daylight levels and improved student performance has motivated the present study. It is focused on the analysis and the evaluation of several daylight strategies for the enhancement of visual comfort in classrooms. METHODOLOGY The study was conducted as a parametric analysis, in which models of classrooms incorporating various window locations, clerestories, roof openings and light shelves were generated on the basis of a typical classroom. The fenestration area in the cases under

consideration was assessed in order to comply with the standards provided by the State Organization responsible for the construction of School Buildings in Greece. A typical classroom, presented in Figure 1, is of rectangular shape, usually side lit with unilateral windows that account for 20% of floor area. For the sufficient shading of the classroom, overhangs are incorporated in the design. The geometrical and optical properties of the classroom s transparent and opaque elements are displayed in Figure 1 and Table 1. Sun protection of the openings is a crucial part of the school building design in Greece, where clear skies dominate. It aims not only in the reduction of excessive solar heat gains, but also in the limitation of glare caused by direct solar radiation and extreme differentiation of the luminance between internal surfaces. With respect to the orientation, sun protection is usually offered with overhangs, side fins or semi-transparent curtains and movable internal blinds. On the basis of the typical classroom, the reference model (C 0 ) was created. It is assumed that the reference model is south orientated, side lit, with an unobstructed view to the sky vault. By changing the geometry, the position and the function of the transparent elements, 5 more models were created. The aim was to include fenestration arrangements offering a variety of ways to admit daylight, using the standard building design and construction. The transmittance of the glazing and the internal reflectances were kept unaltered. It must be mentioned that the decision concerning models configuration was based on the initial simulation results and the evaluation of the daylight conditions prevailing in the reference case. A brief description of the models is presented below: Model C 1 (Figure 2): the area of the glazed surfaces remains unaltered (20% of floor area); a light shelf is added and divides each window into a view area below and a clerestory above, projecting towards both the interior and the exterior of the window façade. Light shelf plays a dual role; it enhances daylight in the space, while protecting it from direct sun radiation. Model C 2 (Figure 3): the glazed area is increased to 25% of the floor area. Daylighting is provided by the lateral windows with light shelves (following the configuration of model C 1 ) located on the classroom s façade, and the additional vertical southorientated skylights, which are positioned on the roof at a distance of 3.5m. 7,8 3,7 1,8 0,2 6,6 0,8 a) b) Figure 1. Plan, view and section of a typical classroom -model C 0 -.

Model C 3 (Figure 4): the glazed area accounts for 25% of the floor area. Daylighting is provided by the lateral windows with light shelves (following the configuration of model C 1 ) located on the classroom s façade and vertical, south-orientated skylights, which are positioned on the roof at a distance of 4.6m (accounts for the 2/3 of the classroom s width) parallel to the window wall. In order to improve the daylight distribution, the roof extension beyond the skylight is inclined. Model C 4 (Figure 5): the glazed area accounts for 25% of the floor area. Daylighting is provided by the lateral windows with light shelves (following the configuration of model C 1 ) located on the classroom s façade and additional vertical, north-orientated clerestories positioned on the opposite wall. The roof is inclined across the classroom s width. Model C 5 (Figure 6): the glazed area accounts for 25% of the floor area. Daylighting is provided by the lateral windows with light shelves (following the configuration of model C 1 ) located on the classroom s façade and tilted, north-orientated skylights, which are positioned in two rows on the saw-tooth roof. 3,7 1,2 0,2 0,7 Figure 2. View and section plan of model C 1. 1,2 0,7 0,2 0,2 3,4 3,8 0, 0,7 Figure 3. View and section plan of model C 2. 1,2 0,7 0,2 0,2 0,7 2,3 Figure 4. View and section plan of model C 3.

The visual environment prevailing in each case was estimated with the help of the simulation program ADELINE, an integrated lighting analysis tool for building design purposes, developed by the IEA Solar Heating and Cooling Task 12. It uses both radiosity and ray tracing techniques for the estimation and the visualization of daylighting conditions [2]. Details regarding the input data for the geometrical and optical characteristics of the transparent and opaque elements of the reference model C 0 and the models C 1 to C 5 are presented in Table 1. The working plane was regarded at a height of 0.8m above floor level. In order to validate the input data used for the simulations with regard to the local climate, in situ measurements were also conducted. A test cell with southern orientation, located in Thessaloniki, Greece (40 o N), was used for this purpose [3]. For the evaluation of the daylighting conditions prevailing in classrooms, where the adequacy of daylight is the main objective, the daylight factor was considered as the most appropriate parameter for indicating the quantity of admitted daylight and consequently the efficiency of the daylighting design. In bibliography, values of daylight factor ranging from 2% to 5% are reported as satisfactory. Furthermore, the homogeneity of the luminance distribution contributes to the rating of the ability of the daylighting system to attenuate glare. The quality of daylighting can be depicted by the visualization of each classroom s interior, since the possibility of glare occurrence is associated to the excessive contrast of luminance between the various indoor surfaces. Illuminance ratios, such as minimum-to-maximum can be used in order to quantify lighting uniformity; i.e. ratios between 1:3 and 1:8 are acceptable [4]. RESULTS The distribution of daylight factor on the working plane of the reference case C 0 and the models of the parametric study C 1 to C 5 are presented in Figures 7 to 12. The display of daylight factor contours provides a clear interpretation of daylight penetration in the classrooms. The visualization of daylight conditions is shown at figures 7 to 12. The appearance of excessive difference in luminance distribution is an indication of glare. 0,7 0,2 0,7 1,2 Figure 5. View and section plan of model C 4. 1,2 1,2 60 1,1 3,4 Figure 6. View and section plan of model C 5.

Table 1. The geometrical and optical characteristics of the transparent and opaque elements of the reference model C 0 and the models C 1 to C 5. Model C 0 C 1 C 2 C 3 C 4 C 5 Transparent elements Daylight technique Side windows light shelf & skylights light shelf & skylights light shelf & skylights light shelf &clerest.. & skylights View windows No of windows 3.0 3.0 3.0 3.0 3.0 3.0 Length (m) 2.1 2.1 2.1 2.1 2.1 2.1 Height (m) 1.8 1.2 1.2 1.2 1.2 1.2 Position Front wall Front wall Front wall Front wall Front wall Front wall Window sill height 1.0 1.0 1.0 1.0 1.0 1.0 Clerestories - 3.0 3.0 3.0 3.0 - Length (m) - 2.1 2.1 2.1 2.1 - Height (m) - 0.6 0.6 0.6 0.6 - Sun protection Overhang Light shelf Light shelf Light shelf Light shelf Overhang Length 2.1 2.1 2.1 2.1 2.1 2.1 Ext. projection (m) 0.8 0.5 0.5 0.5 0.5 0.8 Int. projection (m) - 0.7 0.7 0.7 0.7 - Skylights & clerestor. - - No of windows - - 3.0 3.0 3.0 6.0 Length (m) - - 2.1 2.1 2.1 1.0 Height (m) - - 0.5 0.5 0.5 0.5 Position - - 3.5m from front wall 4.6m from front wall Back wall Saw-tooth in 2 rows Orientation - - South South North North Inclination - - 90 o 90 o 90 o 60 o Sun protection - Overhang Overhang - - Glazing, T v Double, 0.8 Double, 0.8 Double, 0.8 Double, 0.8 Double, 0.8 Double, 0.8 Opaque elements Reflectance Roof: 80% 80% 80% 80% 80% 80% Floor: 30% 30% 30% 30% 30% 30% Sidewalls: 50% 50% 50% 50% 50% 50% Light shelves: - 90% 90% 90% 90% - In reference model C 0, daylight reaches very high levels in areas near the lateral windows and is reduced across the classroom s length, ranging from 16.8% to 2.8%. The mean daylight factor is equal to 5.4%; however, on approximately 40% of the working surface daylight factor is lower than 3.0%. Moreover, the contrast between daylight levels in the interior of model C 0 often exceeds 1:9. It is therefore indicated that although the average levels of daylight are satisfactory, uniformity of daylight can not be achieved with conventional windows, since adequate daylight can not be admitted deeply in the room. In order to enhance visual comfort, it is recommended to both moderate illuminance in areas near the windows and promote daylight penetration deep in the room. Towards this direction, light shelves were incorporated in the classroom s openings, in a way that a view window and a clerestory are created, without altering the total area of transparent elements. Their primary function is to redirect daylight to deep areas of the classroom s interior, while providing sunshade. Simulations showed that the light shelves of model C 1 reduced the amount of light received in the interior relative to the conventional windows of model C 0. Daylight factor varies from 15.3% in areas close to the windows to 1.8% in remote zones and is on average

equal to 4.3%. Moreover, for almost half of the working surface, the daylight factor is lower than 3.0%. However, the luminance distribution is better and the contrast between minimum and maximum values is limited to 1:8 (fig. 8a & 8b). Such a behavior has also been observed in similar studies for light shelves [5]. With the intention of increasing daylight levels without decreasing uniformity of illuminance distribution, south-orientated vertical skylights were introduced in two models (C 2, C 3 ). The minimum and maximum values of daylight factor were found to be approximately equal in both models (2.4% and 16.5% respectively), but the average daylight factor was slightly higher in model C 2 (5.1%) than in model C 3 (4.9%). It is therefore obvious that the skylights of model C 2, located across the middle of the model s width, performs better as regards daylighting, when compared with those located at the 2/3 of the classroom width. Since in Mediterranean climates, such as that of Greece, thermal performance is also very important especially during the hot months, north orientated clerestories placed on the back wall were also examined, given that south orientated apertures are associated with increased solar gains. The illumination in model C 4 ranges in high levels -the average daylight factor reaches 17%-, but the zone formed from values of daylight factor between 2.0% and 3.0% Daylight factor (it is applied to the diagrams of Figure 7a to Figure 12a): 0,0- -2,0 2,0-3,0 3,0-4,0 4,0-5,0 5,0-6,0 6,0-7,0 7,0-8,0 8,0-9,0 9,0-10,0 10,0-1 1-12,0 12,0-13,0 13,0-14,0 14,0-15,0 15,0-16,0 16,0-17,0 17,0-18,0 model C 0 Figure 7. The distribution of daylight factor and the visualization of indoor daylight conditions prevailing in reference model C 0. model C 1 Figure 8. The distribution of daylight factor and the visualization of indoor daylight conditions prevailing in reference model C 1.

appears to be wider than the ones in models with south orientated skylights. The decrease of daylight factor in remote areas and the simultaneous increase of maximum values near the windows indicate a less uniform distribution of daylight on the working plane and consequently a probable discomfortable visual environment (Figures 11a and 11b). On the other hand, when the north orientated apertures are tilted and arranged as skylights in the form of a saw-tooth roof, daylighting is significantly enhanced, despite of the absence of model C 2 Figure 9. The distribution of daylight factor and the visualization of indoor daylight conditions prevailing in reference model C 2. model C 3 Figure 10. The distribution of daylight factor and the visualization of indoor daylight conditions prevailing in reference model C 3. model C 4 Figure 11. The distribution of daylight factor and the visualization of indoor daylight conditions prevailing in reference model C 4.

Figure 12. The distribution of daylight factor and the visualization of indoor daylight conditions prevailing in reference model C 5. the light shelves. Not only there is an overall increase of daylight factor on the working plane, but also the uniformity of its distribution is considerably intensified. The daylight factor ranges from 3.1% to 17.3%, representing a ratio of minimum to maximum equal to 1:6. CONCLUSIONS The above analysis has showed that many alternatives regarding the location of openings exist for the enhancement of daylighting in classrooms. It is found that a combination of façade and roof apertures performs better than advanced façade systems (e.g.light shelves). Also, skylights, clerestories and double side openings are better to be used instead of unilateral façade windows. From the examined models the north orientated skylights arranged in a saw-tooth roof combined with south windows appear to have the best performance. They bring more daylight in the rear areas, where illumination levels are low, daylight distribution becomes balanced and glare is avoided to a great extent as well. The construction of such a configuration is neither complicated nor expensive; on the contrary its benefits encompass visual comfort, sustainability and quality of life. REFERENCES 00 1,450 2,600 3,750 4,900 6,050 model C 5 1. Heschong Mahone Group 1999. Daylighting in schools: An investigation into the relationship between daylighting and student performance. Report submitted in The Pacific Gas and Electric Company on behalf of the California Board for Energy Efficiency Third Party Program. 2. Erhorn H. and Stoffel J. 1996. Adeline 2.0, Documentation of the software package Adeline. IEA Solar Heating and Cooling Task 12. 3. Karanikoloudis G. 2005. Study of design strategies for the improvement of daylighting in a typical classroom. Diploma thesis submitted in the Department of Civil Engineering, Aristotle University of Thessaloniki. 4. IEA 2000. Daylight in Buildings: a source book on daylight systems and components. A report of IEA SHC Task 21/ ECBCS Annex 29. 5. Antoniou K., Axarli K. and Meresi A., 2005. Insolation and daylighting in classrooms: the influence of shading devices and light shelves. Proceedings, 8 th national conference organized by the Institute of Solar Energy Techniques on alternative forms of energy, Thessaloniki, Greece.