The Window Size in Residential House Facades After the Current and New CEN Standard

Nowadays daylight apertures in buildings are designed on the bases of overcast sky conditions in wintertime in Slovakia and Czech Republic. The criterion is the relative value of the Daylight Factor in its average 0.9% in two reference points in the middle depth of the room. Commonly, in practice as a consequence are designed windows roughly 1.5 - 1.7 m high in living rooms. The proposed European standard FprEN17037 is introducing a new criterion median exterior horizontal illuminance with the required indoor illuminance 300 lx in the middle of the room depth or 100 lx as minimum close to the rear wall. This study discusses problems of the design of window size in residential buildings and consequences of the new median criterion as well as the required interior illuminance in absolute lux values.