2.4.1 Driving forces of natural ventilation

Natural ventilation is driven by stack effect (temperature differences) and wind pressure.

Stack effect (temperature difference)

Warm air is lighter than cold air. That causes the stack effect, which means that warm air inside a building will rise. The warm air will leave the building at the top through leakages, stack ducts or open windows and be replaced by cold air entering the building at ground level. The higher the building, the more powerful the stack effect. For the stack effect to work efficiently, there must be air passages through the building.

These can be stairways in combination with windows at both ground level and roof level that can be easily opened at the same time. Due to their position in the roof, VELUX roof windows maximise the ventilation potential of the stack effect. See section 2.4.3 for an example of stack effect.

Wind (wind pressure)

When a building is exposed to wind, air will enter the building at the windward side and leave through openings at the leeward side. The wind pressure is higher on the windward side than on the leeward side. This will drive air from the windward side of the building through the building to the leeward side. The shape of the building and the surrounding landscape or buildings have an impact on the air flow. The magnitude of the pressure difference generated by wind pressure is determined automatically as part of a simulation in tools like the VELUX Energy and Indoor Climate Visualizer. Typical values can be found in standards (e.g. BS5925:1991, DIN1946­6:2009). See section 2.4.3 for an example of wind driven natural ventilation.

The living room is also shown as a temporal map, illustrating that, for the vast majority of hours when roof windows are open, CO2 levels below 1 150 ppm (green), are achieved, which, in this case, is considered as satisfactory indoor air quality. There are no periods with open windows and high CO2 levels.

All the rooms in Maison Air et Lumière (MAL) were evaluated and achieved category 1-2 (<1 150 ppm) in summer for 95% of the time and category 1-2 for the entire year. The results show a satisfactory indoor air quality for the house. See Figure 2.4.3 below.

2.4.2 Background ventilation with the VELUX ventilation flap

The ventilation flap on VELUX roof windows can be used to provide a continuous flow of fresh air into the building.

Example: Background ventilation with ventilation flap

The example investigates the background ventilation rate that can be achieved with different numbers of roof windows per floor area. Two ratios of windows to floor area are used, i.e. 10% and 20%. The house is in Berlin, Germany. Figure 2.4.4 shows the ventilation flows on 7 January, with outflows in the range of 2­-6 l/s per window.

2.4.4 Optimal winter ventilation strategy for existing buildings

During activities like cooking, cleaning and showering airings should be used. A combination of VELUX roof windows and facade windows provides efficient airings with stack effect and cross-ventilation. Additionally, this is a recommended strategy to achieve good air quality at a reasonable energy demand, as short airings are more efficient than continuous ventilation (Heiselberg and Perino, 2010; Perino and Heiselberg, 2009).

Airings can cause draughts, but by making short and efficient airings, the problem can be minimised.

See design advice for specific building types in section 2.5.1.

2.4.5 Summer ventilation

In warm summers, natural ventilation can be used to maintain a comfortable indoor temperature. In this situation there is no heat loss to consider – on the contrary, there is a potential for saving energy for cooling, with air conditioning, if it is installed in the building.

Increased ventilation rates in summer prevent overheating and the increased air motion is pleasant when it is warm. See section 3.5.3 in the Thermal Comfort chapter for an example of use of both solar shading and natural ventilation to maintain thermal comfort. The large air flow rates provided by natural ventilation during summer facilitate very low CO2 levels in the indoor air – at no energy cost.

The use of natural ventilation to improve thermal comfort in warm periods is explained in more detail in the Thermal Comfort chapter.

See design advice for specific building types in section 2.5.1.

2.4.6 Automatic window opening with VELUX roof windows

The VELUX INTEGRA and SOLAR windows can be programmed to open automatically. This can be very helpful in a busy daily routine, where there might not always be time to do the required airings. Windows in selected rooms of the house can be programmed to open for, say, 10 minutes in the morning and the afternoon, and at midday on weekends. The ventilation flap can be programmed to open when the occupants are at home, or during the entire day or night, to provide background ventilation.

Windows installed in bedrooms can be programmed to open the flap during the night. If the occupants accept that the window is opened during the night, a few automatic airings can be programmed during the night as well.