Daylighting in classrooms - the daylight factor as a performance criterion

INTRODUCTION: Recent research has shown that environmental conditions in classrooms, and namely daylighting conditions, can influence students health, well-being and performance. In the last years several studies, dealing with the effects of environmental conditions in classrooms in the learning process, have been published (Winterbottom, & Wilkins, 2009; Barret, Zhang, Moffat, & Kobbacy, 2013). The daylight factor (DF) is the most used parameter in the characterization and quantification of daylight in buildings. The DF at a point of a plane inside a room is defined as the ratio (expressed as a percentage) between the daylight illuminance at that point in the interior of the room and the simultaneous exterior horizontal global illuminance due to a hemisphere of a sky of known or assumed luminance distribution (usually, a CIE overcast sky luminance distribution is considered). The DF reflects the effectiveness of daylight penetration in a particular room or space. The exterior daylight conditions may vary, but the DF remains constant, since the interior illuminances change proportionally to the simultaneous changes in the exterior daylight conditions (Santos, 2006). OBJECTIVES: Daylighting, like artificial lighting has advantages and disadvantages. However, taking into account the need for energy consumption reductions, we believe that, in Portugal, and in other Southern European countries, the conscious use of daylight in schools, new or rehabilitated, has a great potential for improving the comfort and the academic performance of users, contributing, simultaneously for the rational use of energy in buildings. This paper presents the results of part of a research project, of a broader scope, which aims to assess, in an integrated way, several aspects that contribute to indoor environmental quality (IEQ) in classrooms. The project comprises 8 schools of different levels of education (from kindergarten to college level) located in urban and peri-urban areas of the city of Viseu (roughly in the centre of Portugal at approximately 500 m of altitude), installed in buildings of different types and dates of construction. A total of 32 classrooms are involved, with different orientations and sunlight exposures. The classrooms have an approximate average area of 50 m 2 . MATERIALS AND METHODS: This part of the project was held in December 2013 and January 2014 and included the assessment of the DF at 8 schools, comprising 2 rooms in each school, both located in the top floors. However, due to differences in the nebulosity conditions, there were only considered valid tests in 6 schools. Some of the characteristics of the classrooms used in the study are summarized in Table 1.