Classrooms in Southern Europe are traditionally ventilated with manual opening of windows. This is an energy-sparing and perfectly appropriate way of ventilating classrooms when weather conditions are warm, however, as outdoor air temperatures drop, teaching staff and students tend to leave windows closed and, as a consequence, ventilation rates fall leading to poor indoor air quality. To safeguard classrooms' indoor air quality and promote energy conservation, understanding 5 the conditions for which manual window-airing is appropriate is of great relevance. Yet, given the stochastic nature of window-airing, it is difficult to get hold of this understanding. The main objective of this paper is to find out when manual window-airing of classrooms is appropriate. To achieve this objective, four free-running classrooms of a Portuguese public secondary school were monitored during a two-year period. Ventilation rates were determined and it is concluded that 10 manual opening of windows provides appropriate ventilation for outdoor running mean temperatures larger than 16 0 C; and, for the studied classrooms, this translates into appropriate ventilation for approximately a quarter of the academic year. Because of the significance of this result, the paper concludes with a review of the ventilation strategy used in the studied classrooms.1 Keywords. Natural ventilation; free-running classrooms; manual window-airing; window opening 15 patterns; indoor air quality; thermal comfort.
Highlights-Ventilation rates in four free-running classrooms are determined during two full academic years.-Manual window-airing of classrooms is appropriate with outdoor running mean temperatures 20 larger than 19 0 C.-For outdoor running mean temperatures between 16 and 19 0 C manual window-airing is still appropriate, but depends on indoor air temperature.-When outdoor running mean temperatures are lower than 16 0 C, regardless of indoor air temperature, manual window-airing is inappropriate.
25-For the studied classrooms, manual window-airing provides appropriate indoor air quality and thermal comfort for 25% of the academic year. 45 sociated with undesirable effects on students productivity (Wargocki and Wyon, 2007a,b;Haverinen-Shaughnessy et al., 2011;Bakó-Biró et al., 2012;Petersen et al., 2015), absenteeism (Shendell et al., 2004 and increased health symptoms (Fraga et al., 2008;Bartlett et al., 2004;Rudnick and Milton, 2003;Mendell and Heath, 2005). Still, in spite of these concerns, along the academic year there are conditions for which natural ventilation is perfectly appropriate, and, from the point of view of 50 users, better than mechanical ventilation alternatives.In fact, mechanical systems have problems too; if not properly balanced and maintained, they are frequently a source of thermal discomfort, noise and can even increase indoor air pollution, as discussed in
