This structure also enables a 'natural ventilation' through the selective opening of windows. The 50 cm gap between the 17 cm thick floors and the glass encasing also allows for vertical ventilation, if needs be helped by opening glass hatches in the roof. And the glass allows for maximum entry of sunlight. Peutz reported after one year of use of the building, that experience had shown that the 50 cm gap was more than enough. In the early years, the gap has even been reduced in size in some places. In winter only the window flaps in the glass encasing need to be opened and the roof hatches can remain closed. During summer, on the sunny side the curtains should be closed and the hatches opened (note, though, that the south side is the one without glass). The result is a 'double wall' of glass on the outer side and curtains on the inner side, with an upward draft caused by the Sun's heat. This suffices for ventilation and the two surface ducts (as shown on the drawings) were partly intended for mechanical ventilation (with an exhauster fan), but this has turned out to be unnecessary. During winter, heating is done with a hot water installation (of course fuelled with coal) with natural circulation. The radiators along the windows now provide the heat for the circulation. The installed pumps have turned out to be unnecessary. The airtight glass walls have turned out to be better insulators than thought. Also, the mass of the concrete is such a good heat accumulator that extra time heating up the building in the morning has turned out unnecessary and there have never been any major temperature changes. This technology is being rediscovered and used, for example, by Canadian architect Chris Holmes in his energy-saving designs.