2.2.2.1. Ventilation Proper ventilation is essential and is clearly defined under Belgian law (via the energy performance regulation, EPB). However, proper ventilation requires a large amount of energy, as the air from outside needs first to be warmed, cooled or humidified to meet thermal comfort requirements, without speaking of the energy required for the fans. Mechanical ventilation thus consumes a large amount of electricity, with the result that a lot of effort used to be put into limiting air flows in low-energy buildings. This tendency has however since been reversed as a result of better knowledge of the health effects of proper ventilation, with a number of reference works41 now recommending higher ventilation flow rates, especially for residential buildings. This does not of course make the problem any easier. A. Ventilation flow rates in residential buildings Flow rates are dependent on a building's use. For residential buildings, the norm NBN D50-001 dating back to 1991 still applies. This sets fresh air input flow rates in living areas and output flow rates in wet rooms (bathrooms, kitchens, toilets, etc.). It is obvious that mechanical ventilation (system D) with high-quality heat recovery is best in any passive building. As such a building is also airtight, it is a good idea to do everything to properly balance flow rates, avoiding local over- or underpressure through sufficiently sized transfer openings. B. Ventilation flow rates in commercial buildings For non-residential buildings, norm NBN EN 13779 applies. The category "medium indoor air quality" (IDA2) is increasingly being given preference over the minimum legal requirements stipulated in the building energy performance (EPB) or working environment (RGPT) regulations. The norm sets a flow rate of 40-45 mÂł/h per person, thereby limiting the concentration of CO2 in indoor air to around 400 - 600 ppm above that of outdoor air. Certain flexi-programmes such as offices delivered in a semi-finished (casco) state may have to cope with varying levels of occupancy during the day (or a year), and it may be a good idea to dimension the system somewhat larger in order for its capacity to keep pace with the developing
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