Modelling the effect of windows size


Window position, size, and type are variables that affect both visual and thermal comfort. Typically, for better thermal comfort we look to minimize fenestration rate (area of windows on the facade). That is, reduce the size of windows (thermal transmittance is lower for walls, for example, 25 cm brickwall + 5 cm of insulation is about 0.5 W/m2K while double glazed windows can be 1.5 W/m2K). To achieve good visual comfort, multiple factors are taken into consideration: typically we look for large windows (to achieve a high level of daylight availability through the year and to have a connection to the outdoors – well, who doesn’t love having a view) and controllable shadings to prevent glare [1]. Also, operable windows allow a user to control indoor temperature, change air when needed, and connect to the exterior weather conditions. As seen, there are multiple reasons to have large and small windows. Is there an ultimate best size for the windows, their position, and type? No. We are not looking for that. Although we wanted to learn about relations between the size of windows, energy consumption and daylight conditions for the Lisbon Metropolitan Zone. 

Model setup

As with any scientific study that focuses on simulation, we built a virtual model. Such a virtual model of a building is a 3D design that looks to represent typical characteristics of a building typology in a selected region. In our case, it is a detached house in the rather rural part of the metropolitan zone (it means there is no shade cast by neighbouring buildings, and that the urban island effect modifies the site’s climate data to list the most important reasons). The roof ridge is oriented south-north. Windows can be located on all facades, depending on the visual and thermal needs. The house was divided into four rooms, occupying the south, north, east, and west part and each room has only windows belonging to one of the exterior walls.  It essentially means that to comply with visual comfort standards, the fenestration rate for each zone needs to be higher than 0.  Fenestration rate might be a confusing characteristic, it essentially means how much of the facade is transparent. It excludes grills, frames, aprons, etc.The smallest fenestration rate was 30% and the highest 90%, also no shadings for the windows were provided. 


We ran 276 options for different window configurations to check which one is the best from the point of view of daylight and energy. It turned out that the relation between the average fenestration rate and the total energy consumption is linear: with a higher fenestration rate comes higher energy consumption.


Another step was to compare some of the 276 solutions:

A, all the facades have the fenestration rate equal to 30%

B, the north facade has the fenestration rate equal to 60%, other facades 30%

C, all the facades have the fenestration rate equal to 60%

D, all the facades have the fenestration rate equal to 90%

According to the European Norm EN 17037, a view outdoors to a landscape and clearance of a minimum of 6 meters is required. The norm specifies the size of the smallest window for 1.00×1.25 m. 

Option A is also the least energy-consuming, while option D is the most energy-consuming. 

First off in Lisbon daylight availability does not seem to be a problem. For a well designed, not very deep interior, there is always enough light regardless of the size of the windows [2]. Just have a look at Fig.02, made for a different study but it clearly illustrates the problems, for a test room with a 45% fenestration rate facing east. Rooms as deep as nine meters will receive enough light for everyday activities and a depth of seven meters or less would be good for work. 

Fig 02. Top view of a room

The problem is rather the overexposure, as strategies A and B illustrate quite well. When looking at the energy consumption, option D requires 60% more than option A to maintain occupant comfort. Overglazing the facade in Portugal does not elevate visual comfort nor thermal comfort. For all the typologies the facade should be solid with carefully studied windows equipped with shading.



The following study shows that minimization of window size in Portuguese vernacular and contemporary architecture (look at the projects of Souto de Moura or Aires Mateus) is a characteristic that aligns with daylight availability and thermal comfort. It is important to mention and emphasize that view to the outside provides a visual connection with the surroundings, it supplies residents withinformation about the local environment, weather changes and the time of day. Lack of this information can relieve the fatigue associated with long periods of being indoors.

In Scandinavia the lack of light is omnipresent in architectural design, they have been designing to capture as much daylight as possible. Large windows are not a problem anymore since they can be very energy efficient. The tradeoff between the amount of daylight and thermal performance was a topic. Now it is time to tackle this problem in the south when daylight conditions need to negotiate with thermal comfort in another context. 

Adrian Krężlik


[1] Although glare in southern Europe is not as big a problem as in the north. The position of the sun in late autumn, winter and early spring in northern Europe is quite low, and once it is setting it tends to cause glare. While in the south of Europe the position of the sun would not cause glare in the afternoon, and it is not considered as a problem.

[2] Although through my observation and preliminary measurements of visual comfort in the interior in Portugal the amount of daylight is lower than according to international standards. 300 lux for work is rarely achieved.

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