3.5.3 Low-energy buildings
Overheating can occur in most residential and office buildings if no ventilation and solar shading strategy has been implemented from the start. In many buildings, overheating is handled by air conditioning, but natural ventilation (passive cooling) is a good substitute as it saves energy compared to air conditioning.
There have been many cases in the past few years of overheating in low-energy houses, where the main goal has been to achieve a low heating consumption. In these cases, passive technologies, such as solar shading and natural ventilation, are often not fully utilised (Larsen et al., 2011). Learnings from these cases have been to better implement natural ventilation and openable windows into the design of the building to prevent overheating, instead of installing mechanical cooling systems. Buildings built to “Active House” principles focus primarily on user well-being by creating a good indoor climate. In the design of Active Houses, solar shading and natural ventilation allow the full potential of passive cooling to be utilised.
3.5.4 Schools and kindergartens
There may be legislative requirements for the maximum temperature in schools and kindergartens. The following considerations can be used to prevent overheating:
• In summer, opening windows has a good effect on both thermal comfort and indoor air quality and should be done frequently
• Dynamic external solar shading efficiently reduces solar gains
• Automatically controlled natural ventilation allows for the full potential
of solar shading and natural ventilation and is recommended in schools. If the schedule of lessons and breaks is rarely changed, a schedule-based control of ventilation may be sufficient (Dhalluin et al., 2012).
Performance can be verified by a simulation in VELUX Energy and Indoor Climate Visualizer (see section 2.7.1
), which determines temperatures and includes the effects of solar shading, ventilative cooling and solar protective glazing.
See figure 3.5.2
for an example of measured thermal comfort in a kindergarten. VELUX roof windows perform well in schools. In larger rooms, VELUX Modular Skylights perform very well.
3.5.5 Commercial buildings
It is becoming a de facto standard in office buildings to include mechanical cooling (air conditioning) in the design, also in buildings in northern Europe. Some office buildings are designed without mechanical cooling, using natural or hybrid ventilation instead.
VELUX Modular Skylights are designed for use in commercial buildings and perform well as extract openings in an atrium roof. The solar shading that can be integrated in VELUX Modular Skylights, as well as the opportunity to open every second module, provides good opportunities to prevent overheating.
The control of shading and opening of VELUX Modular Skylights will often be performed by the building’s BMS system in a control setup that integrates all systems of the building.
3.5.6 Effects of climate change and urban heat islands
The risk of overheating in buildings will increase as outdoor temperatures increase due to climate change (Orme, 2007). Another effect influencing the risk of overheating is the “urban heat island” effect. Large and densely populated urban areas have a higher temperature than the surrounding countryside, most likely caused by the increased use of energy in urban areas. During the 2003 heat wave in London, temperature differences between the city and the surrounding rural areas at times exceeded 9°C (Carmichael et al., 2011). These two effects underline the importance of not only designing buildings to perform well under today’s outdoor conditions, but also considering the conditions that can be expected in the future at the building's location.
Example from the VELUX Model Home 2020 project, Maison air et Lumière
The thermal environment in Maison Air et Lumière has been evaluated according to Active House specifications (see section 3.6.4). The high daylight levels in the house increase the risk of overheating, so its prevention has been a top priority. The result is seen in figure 3.5.2. The house achieves category 1 (corresponding to EN 15251 category I (CEN, 2007)). This excellent performance is achieved by designing the house to take maximum advantage of natural ventilation, and to use shading and window openings to their full potential (Foldbjerg and Knudsen, 2014).