3.4 Thermal comfort with roof windows and solar shading


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).


Example: solar shading reduces experienced temperature for different glazing and accessories under strong solar radiation.

The measured values are the results of a small experiment. The operative temperature was measured behind a glass unit with different shading accessories to illustrate the effect of different types of shading.

! Remember
Expectations of the thermal environment in naturally ventilated buildings are dependent on the outdoor temperature.


Example: Solar shading as cooling

A study from CSTB in France made for an attic room investigated how solar shading could be used to assist or replace a mechanical cooling system. Simulations were made for Hamburg, Munich and Stuttgart in Germany, and Paris, Lyon and Marseille in France (Couillard, 2010). The conclusion was that the experienced temperature could be lowered by up to 7°C when using a solar shading device for locations in both Germany and France. Energy for cooling was eliminated in all locations except Marseille, where it was reduced by 90%. The figure shows the experienced temperature on a typical hot and sunny summer day in Paris with and without solar shading.

​Roller blind.
Example: Solar shading as cooling

A study from CSTB in France made for an attic room investigated how solar shading could be used to assist or replace a mechanical cooling system. Simulations were made for Hamburg, Munich and Stuttgart in Germany, and Paris, Lyon and Marseille in France (Couillard, 2010). The conclusion was that the experienced temperature could be lowered by up to 7°C when using a solar shading device for locations in both Germany and France. Energy for cooling was eliminated in all locations except Marseille, where it was reduced by 90%. The figure shows the experienced temperature on a typical hot and sunny summer day in Paris with and without solar shading.
​Figure 3.4.1 Experienced temperature on hot and sunny day in Paris, France (Couillard, 2010). 


3.4.2 Ventilative cooling

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.

• Ventilation during the day removes excess heat from the building by creating high air movements by natural ventilation.
• Night ventilation (also referred to as night cooling) will cool down a building's thermal mass at night by using cool outdoor air. The following day, less cooling energy (or none at all) is needed in the building, as the thermal
mass has already been cooled down. Buildings with high thermal mass soak up more heat during the day, that needs to be removed – an ideal situation for night cooling strategy (see Figure 2).

​Figure 3.4.2 Passive cooling at night (night cooling). 
Night cooling is an aspect of ventilative cooling, see section 3.4.3.

Example from MH 2020:

In the French Model Home, Maison Air et Lumière (MAL), the airing rates and resulting indoor temperature were studied during the summer of 2012. Through a combination of measurements and detailed simulations, the effects of ventilative cooling on indoor temperature were determined by the Institute Armines (Favre, 2013). The first step was to verify the simulation model against the measured values based on the chosen control of the building. The next was to simulate variants of the control using actual weather data. This made it possible to determine the effects of e.g. ventilative cooling (window openings) compared to closed windows. When ventilative cooling was used as intended, the indoor temperature was typically 5-8°C lower than if it had not been. It was even possible to keep the indoor temperature below the outdoor during daytime (especially when the control system in MAL was used), only opening windows when the net effect on thermal comfort was positive

​Maison Air et Lumière. 

​Figure 3.4.3 Ground floor bedroom in MAL. Blue curve shows the simulated indoor temperature when windows are constantly closed. Dark grey curve shows the simulated indoor temperature when windows are constantly open. Red curve shows the measured indoor temperature when windows are controlled by the MAL control system. Light grey curve shows the outdoor temperature. 
Example: Ventilative cooling in northern Europe
The VELUX Energy and Indoor Climate Visualizer is used to find the effect of ventilative cooling in a house in Stockholm. The ventilation flows achieved per window are in the range of 40-70 l/s when the windows are used to maintain a pleasant temperature and the ventilation rate of the house is in the range of 5- 8 ACH where 15 windows are opened.

The results in the table show that without ventilative cooling, overheating will occur for 3% of the occupied hours of a year; with ventilative cooling the problem is eliminated. Using natural ventilation thus improves the thermal environment during the summer.

The results in the table show that without ventilative cooling, overheating will occur for 3% of the occupied hours of a year; with ventilative cooling the problem is eliminated. Using natural ventilation thus improves the thermal environment during the summer.
! Remember
Opening of windows reduces overheating efficiently.


3.4.3 Night cooling

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.

​LichtAktiv Haus. 
Example: night cooling in ModelHome 2020 project LichtAktiv Haus 

The use of windows for ventilative cooling and particularly night cooling has been investigated in the VELUX ModelHome 2020 projects. The window openings were controlled automatically to maintain an indoor temperature within category 1 or 2. Figure 3.8 from the kitchen-living room in LichtAktiv Haus shows when this was achieved and how windows were used. The overall result is that category 1 or 2 was achieved almost all year, with the exception of approx. 10 afternoons; a very good performance. The dark green indicates closed windows, light green indicates open windows. It is clear that windows were open intermittently during daytime in the spring and autumn, and almost permanently during daytime in summer. It is also seen that during the summer, windows were also open during the night, which means that night cooling was part of achieving the good thermal environment (Foldbjerg et al., 2014).
LichtAktiv Haus
​Figure 3.4.4 Temporal map for kitchen-living room in LichtAktiv Haus showing open or closed window in combination with thermal comfort category according to Active House specification. 
3.4.4 Automatic control

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%).
! Remember
Automatic control of windows and shading can reduce overheating and the need for mechanical cooling.
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 3.9 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).

LichtAktiv Haus

​Figure 3.4.5 Temporal map for Living room in Sunlighthouse showing active or inactive solar shading (awning blinds) in combination with thermal comfort category according to Active House specification. 
​Sunlighthouse.
Couillard, N. (2010) Impact of VELUX Active Sun screening on Indoor Thermal Climate and Energy Consumption for heating, cooling and lighting. Case study for Germany Research project, Centre Scientifique et Technique du Batiment.
Favre, B., Cohen, M., Vorger, E., Mejri, O., Peuportier, B. (2013) Evaluation of ventilative cooling in a single family house (pp. 1–131)
Foldbjerg, P., Knudsen, H. N. (2014) Maison Air et Lumière a case from model home 2020 project. REHVA Journal (June), 55–57.
Foldbjerg, P., Rasmussen, C., Asmussen, T. (2013B) Thermal Comfort in two European Active Houses : Analysis of the Effects of Solar Shading and Ventilative Cooling. In Proceedings of Clima2013.