A critique of European Standard EN 15251: strengths, weaknesses and lessons for future standards

European Standard EN 15251 has broken new ground. Firstly, it assembles existing information about optimizing thermal, air quality, acoustic and visual comfort so that designers can make energy calculations which embody all the best information about aspects of the environment that impinge on energy use. Secondly, it follows other standards in recognizing the different expectations occupants have of the thermal environment in mechanically and naturally ventilated buildings. Potentially this makes EN 15251 an important standard in the design and the evaluation of new and existing buildings. Therefore, it is essential to be clear about its strengths and weaknesses and to indicate ways in which future updates might improve on it. This paper considers the way in which the Standard is written, its underlying philosophy and purpose, and the ways in which it defines unacceptable thermal conditions and how they might be misunderstood. Questions are raised regarding the use of outdoor temperature to define indoor comfort as the basis for providing a comfortable indoor thermal environment. Suggestions are made for improving the Standard.