Windows combined with a heat source (e.g. a fireplace) are one of the oldest methods of achieving thermal comfort in buildings during cold periods. Today the simplest way to achieve thermal comfort is to install a system that can adjust the parameters. Most houses have a heating system installed and, in warm climates, possibly a cooling system. However, windows can cool down a building on a warm summer day.

Draught and temperature asymmetry can be caused by windows, as mentioned earlier. It can be difficult to determine whether the sensation of coldness is caused by draught from the windows or by cold radiation. A leaky window can be fixed by replacing the gasket and/or pane – or the whole window could be replaced. To some extent, cold radiation can be limited with the use of an internal blind that will increase the inside surface temperature.

3.4.1 Blinds and shutters

Blinds and shutters block solar radiation and thus reduce the amount of heat entering a room. Overheating during summer can be efficiently reduced, and even eliminated, by the use of proper solar shading. It can also improve the thermal insulation of windows in winter. This can reduce thermal discomfort from cold radiation and temperature asymmetry. Even better, when applied at night, this extra insulation can decrease the demand for heating. In terms of energy, shading should only be used at night during winter, because the solar gains are often of greater importance than the heat loss (see section 5.6.3).

Ventilative cooling refers to the use of natural or mechanical ventilation strategies to cool indoor spaces. The use of outside air reduces the energy consumption of cooling systems while maintaining thermal comfort. The most common technique is to use increased ventilation airflow rates and night ventilation. Ventilative cooling is applicable in a wide range of buildings and may be critical to realising low energy targets for renovated or new Nearly Zero-Energy Buildings (NZEBs) (venticool.eu).

Natural ventilative cooling by opening windows is a very direct and fast method of influencing the thermal environment. An open window will cause increased air motion, and if the outdoor temperature is lower than indoors the temperature will fall. Even when the outdoor air temperature is slightly higher than the indoor, the elevated air speed due to increased airflow will increase the cooling of the body and reduce the thermal sensation. For ventilative cooling, a division could be made between two strategies in terms of natural ventilation – day ventilation and night ventilation.

Night cooling makes use of the fact that the outdoor temperature is lower during the night than during daytime. When windows are opened during the night, the temperature in the house is reduced to e.g. 21°C in the morning. During the day, the indoor temperature will increase, but the temperature in the afternoon will be lower than if night cooling had not been used. Often, indoor daytime temperatures below the outdoor temperature can be maintained.

Example: night cooling in Southern Europe.

The VELUX Energy and Indoor Climate Visualizer is used to find the effect of night ventilation in a house in Rome. The ventilation flows achieved per window are in the range of 50-100 l/s when 8 roof windows are used for night cooling, and the ventilation rate of the house is in the range of 4-6 ACH.

The results in the table show that without night cooling, overheating will occur for 12% of the occupied hours of a year; with night cooling the problem is reduced to 9%, which could be further reduced with solar shading. Using natural ventilation for night cooling thus improves the thermal environment in the house.

An automatic control system for thermal comfort includes those dynamic elements that have an influence on the thermal environment: electric window openers, external shading and/or internal blinds. The most reliable solution is sensor-based control. Time control can also achieve good performance.

The advantage of an automatic control system is the ability to adjust the window and its accessories to match the actual needs of the occupants. If solar gain causes overheating, external shading is used; when it makes sense in relation to energy and comfort, the shading is deactivated.

VELUX ACTIVE Climate Control andEnergy Balance are good examples of automatic controls. Energy Balance is a time-controlled feature available in all VELUX Integra and Solar products controlled by io-homecontrol. VELUX ACTIVE Climate Control is a sensorbased control that can also be used with all VELUX electrical products compatible with io–homecontrol®.

The VELUX ACTIVE Climate Control algorithm has been validated by the French building research institute, CSTB, for both German and French locations (Couillard, 2010). Its findings are that dynamic shading control can reduce the experienced temperature by up to 7°C in summer and, in most cases, eliminate overheating (or reduce the cooling demand by up to 90%).

Example: Use of external solar shading in Model Home 2020 project Sunlight house

The VELUX ModelHome 2020 project Sunlighthouse is used as an example of how external, dynamic solar shading (awning blinds) is used to prevent overheating. The solar shading was controlled automatically, based on external solar radiation and indoor temperature. Figure 3.4.4 from the living room in Sunlighthouse shows when solar shading is used and the thermal comfort category. The overall result is that category 1 or 2 was achieved practically all year; a very good performance. The dark green indicates inactive solar shading, light green indicates Active solar shading. Solar shading was used intensively during mid-summer and also often used in spring and autumn. Solar shading played an important role in maintaining good thermal comfort (Foldbjerg and Asmussen, 2013B).