Like overheating a building, ventilation is considered an inefficient action that leads to significant loss of thermal energy in winter and cooling energy in summer, while not contributing to the regulation of indoor comfort levels – heating a room when it is already warm enough, ventilating when the incoming air is no fresher than the indoor air.

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Ventilation is the most complex and invisible problem of building energy efficiency, simply because a person with their internal sensory systems cannot detect typical air pollutants in living spaces. Excessive CO2 levels are only recognized by the human brain in the blood, where it is possible to react with blood pressure. Meanwhile, the CO2 level in the air is imperceptible, even if pollution exceeds 10,000 ppm (parts per million) and becomes life-threatening. Similarly, a person cannot perceive that a room has been ventilated and the pollution inside is balanced with the outdoor air level, where pollution also exists, of course. For instance, the CO2 gas level in outdoor air exceeds 400 ppm throughout our atmosphere. The human body replaces things it cannot perceive with perceivable ones, so we unconsciously measure air quality incorrectly by temperature. In winter, a cooled room seems ventilated, a heated room feels stuffy. This is not a criterion for air composition. Air composition should be regulated with temperature-affecting instruments, like radiators, whereas appropriate sensors should be used for ventilation criteria. Mathematical guidelines can also serve as a criterion for ventilation, for example, fully opening windows in a workspace every four hours for 15 minutes. The rationale for such a recommendation is that within four hours, the CO2 level in the room is unlikely to exceed 2000 ppm, while 15 minutes is sufficient to completely ventilate the indoor air. Using appropriate sensors allows for much more precise regulation of ventilation intervals and volumes, preventing over-ventilation.

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