Library study area with large roof windows and modern furnishings.

1.5 Parameters influencing daylighting performance

1.5.1 Climate

The prevailing climatic conditions of a building site define the overall preconditions for the daylighting design in terms of sunlight availability, visual comfort, thermal comfort and energy performance. Figures 1.5.2 to 1.5.4 show the effect of climatic conditions on the sky luminous distribution and intensity.

Example

Figure 1.5.1. shows an overview of the monthly sky conditions for 3 European locations: Rome, Paris and Oslo. Within working hours (8-17), cumulative data of daylight availability show that a horizontal illuminance of 10 klx might be available for 60 to 85 % of working hours and 20 klx for around 30% of working hours. By contrast, the global illuminance (total of sunlight and skylight) varies significantly with latitude. A global horizontal illuminance of 30 klx is exceeded for 35% of working hours (8-17) in Oslo, but 65% of the time in Rome.

Bar chart comparing weather frequency in Oslo, Paris, and Rome.

Figure 1.5.1 Frequency of weather in % for three different European cities.

Dual image of sky with sun and light intensity analysis.

Figure 1.5.2 Luminance map of a clear sunny sky. The image above describes a clear sky luminance distribution. Under clear sky conditions, the sky luminance is about ten times brighter at the horizon than the zenith. In addition to the sky luminance is the sun luminance. The sun acts as a dynamic light source of very high intensity.

Fisheye view of cloudy sky and thermal analysis of the same scene.

Figure 1.5.3 Luminance map of an intermediate sky. The image above describes an intermediate sky luminance distribution. In this particular case, the sun energy has been scatteredby the clouds, which results in a softer transition between the very intense luminance of the sun and the luminance of the sky. It is possible to observe that the clouds (illuminated by the sun) have higher luminance values than the sky.

Comparison of cloudy sky and luminance analysis with color-coded map.

Figure 1.5.4 Luminance map of an overcast sky. The image above describes an overcast sky luminance distribution. Under perfect overcast sky conditions, the sky luminance is the same in all orientations, and the zenith isabout three times brighter than the horizon.

1.5.2 Latitude

The latitude of a building site determines the solar altitude for a given time of day and year. The summer and winter solar altitude properties for a specific location are important design inputs for the control of direct solar radiation. Latitude will also determine the length of daytime and solar availability at different seasons of the year. Maximum and minim solar elevation will depend on the latitude of the site; on moving away from the equator towards north or south, the difference between summer and winter becomes greater as latitudes increase. Figure 1.5.5 show the difference in outdoor illuminance between northern and southern European locations.

Global illuminance chart for Kiruna, Sweden, showing seasonal daylight variation.

The highest peak of global illuminance is during the summer (for the northern hemisphere) when the sun is at its highest level, and about two and a half times greater than the lowest peak during the winter, when the sun is at its lowest level.

Chart showing global illuminance levels in Rome, Italy, throughout the year.

Figure 1.5.5 Global illuminance in northern and southern European locations.

1.5.3 Obstructions and reflections on site

External reflections and obstructions from surrounding elements on the building site (buildings, vegetation, ground surface etc.) will influence the amount of daylight reaching the interior of a building.

Roof windows and skylights are generally less affected by obstructions from surrounding elements and have more generous views of the sky than facade windows, as illustrated in Figure 1.5.6 and Figure 1.5.7.

Attic room with VELUX roof window, clear sky view, obstructed view diagram.

Figure 1.5.6 Components of view – roof window situation.

Diagram showing attic room with VELUX roof window and clear sky view.

Figure 1.5.7 Components of view – facade window situation.

Example

The following figure shows the effect of obstruction on daylight availability in a simple room with a vertical facade window, and the effect of adding a flat-roof window to deliver daylight deeper into the obstructed room. The results show that obstruction can greatly affect the amount of daylight that will reach the building interior, and how adding an unobstructed window on the roof can provide much more daylight.

Figure 1.5.8 Comparison of daylight levels in a room without (left) and with external obstruction. (Center and right).

1.5.4 Building design

Geometry:

The geometry of a building influences its capacity to deliver adequate levels of daylight to the interior. When the building is deep, daylighting solely by facade windows has its limitations. No matter how much glass there is in the facade, it will only be possible to achieve an adequate daylight distribution (DF > 2%) a few metres from the facade, as shown in Figures 1.5.9 and 1.5.10.

Measures like light shelves and reflective ceilings can improve the light distribution from the facade slightly, but these solutions are often associated with visual discomfort. The most effective way to bring daylight deeper into buildings is to use light from the roof with products like VELUX roof windows and sun tunnels.

Example: daylight in deep buildings

The simulations below demonstrate the daylight performance of a deep room with three different window configurations installed. Room dimensions: 8m (d) x 4m (w) x 3m (h) Pane visual transmittance (τv): 0.78 Surface reflectance: 0.35 (floor), 0.66 (wall), 0.90 (ceiling).

Office space with daylight factor analysis, showing light distribution.

Figure 1.5.9 Luminance and daylight factor simulations of scenario 1.

Lighting analysis of attic room with VELUX roof windows and sloped ceilings.

Figure 1.5.10 Luminance and daylight factor simulations of scenario 2 and 3.

1.5.4 Building design

Situation with 10% glazing to floor area ratio (facade window only). The results from scenario 1 show that a 10% glazing to floor area ratio will only achieve a DF of 2% a few metres from the facade and leave the back of the room with very low light levels. Even though the average DF of the room is equal to 1.9%, only a small work plane area achieves values above 2%, and only one of the three workplaces represented can be considered daylit.
The situation with 30% glazing to floor area ratio (facade window only). The results from scenario 2 show that a 30% glazing to floor area ratio will achieve a DF facade of 2% approximately 4.5 meters from the facade. The DF average is equal to 5.1%, but it is highly nonuniform and not well distributed over the work plane area, with very high values near the window and low values at the back, a luminous environment likely to cause visual discomfort and glare. In this scenario, two of the three workplaces represented can be considered daylit.
The situation with 20% glazing to floor area ratio (11% facade window + 9% roof window). The results from scenario 3 show that a combination of facade and roof windows with a 20% glazing to floor area ratio provides generous and useful DF levels over the entire work plane, within average DF of 6.4%. The results demonstrate that the use of roof windows means better daylighting performance and a luminous environment not as likely to cause glare and visual discomfort. In this scenario, all of the three workplaces represented can be considered well daylit. Simulations performed with the VELUX Daylight Visualizer. CVP VELUX roof windows are used in scenario 3.

Material properties

The colour and reflectance of room surfaces are part of the lighting system. Dark surfaces reflect less light than bright surfaces, and the result is likely to be an unsatisfactory luminous environment in which there is little in direct reflected light. Bright vertical surfaces inside the room are generally preferred to dark ones, but shading devices used to control sunlight should use darker materials in order to limit the risk of glare (e.g. grey awning blinds).

Lighting simulation in attic with VELUX roof windows, showing daylight distribution.

Figure 1.5.11 Luminance simulations showing the effect of surface reflectance on daylight levels.

1.5.5 Windows and skylights

Orientation

The orientation of windows influences the availability and qualities of daylight in the interior. In the northern hemisphere, the light coming from the north is mainly composed of diffuse skylight and provides the interior with a functional and comfortable light that is pretty stable throughout the day. Light coming from south, east and west orientations will, in many cases, provide the interior with direct sunlight and light levels that vary significantly throughout the day as the sun pursues its course around Earth. Note that roof windows and skylights installed in low-pitched roofs and flat roofs are likely to receive direct sunlight

Diagram showing sun's path around a house with VELUX roof windows.

Figure 1.5.12 A diagram showing the sun's paths on the winter solstice (shortest day), the equinox (day and night almost equal) and the summer solstice (longest day).

Four shading options for VELUX roof windows: Venetian, roller, pleated, awning.

Figure 1.5.13 Different shading solutions.

Glazing dimensions

The amount of daylight entering a room is linked to the total glazing area of windows in that room.

Glazing transmittance

The amount of daylight transmitted through a windowpane is reduced by the number of glass layers it has to penetrate. As a rule of thumb, double glazing(with no coating) lets in approx.80% of the light, while triple glazing(with no coating) lets in approx. 70%of the light (compared to an open window). Coloured or coated glass can reduce the visible transmittance of a windowpane to values as low as 20% and significantly modify the spectral quality of the transmitted light, as well as the perception of surface colours in the interior.

"It is impossible to 'optimise' buildings for good daylight performance with static glazing alone, as daylight intensity varies dramatically"

Shading

Shading and sun-screening are just as important to good daylighting performance as the window itself. Pleated blinds and Venetian blinds can be used to adjust the amount of daylight entering spaces and to reduce window luminance to control glare. The Venetian blind can also be used to redirect the light into the room. The most efficient shading solution to prevent direct solar radiation into the building is to use external shading. Examples of external shadings are roller shutters and awning blinds. A dark grey screen (VELUX awning blind 5060) will reduce the illuminance and luminance levels significantly to a level where the risk of glare is avoided.

Fact

As a rule of thumb, double-layer glazing transmits approx. 80% of the light and triple-layer glazing transmits approx. 70% of the light. Coloured or coated glass can reduce the visible transmittance of a windowpane to values as low as 20%.

Position

The positioning of windows will influence the distribution of daylight in the room and determine the amount of 'useful' daylight. Window position should also take into account the relation between the view to the outside and the eye level of the occupants.

Linings

The geometry and depth of window linings will influence the amount of daylight entering the room and can be used to soften the luminance transition between the high luminance values of the window and the surfaces of the room.

Example

The figure below shows the effect of different window position in an attic with four roof windows. The results show that the average DF values vary in the room, but not as much as median DF values, which are a better representation of the useful amount of daylight in the room. It is also worth noting the effect of window placement on the uniformity of daylight in the room and taking it into consideration in the building design and window layout.

Attic room with sloped ceilings and VELUX roof windows, daylight analysis shown.

Figure 1.5.14 Different window positions.

1.5.6 Sun tunnels

Orientation

Orientation is a crucial factor influencing Sun Tunnel's performance. These products are intended to transport intense sunlight to diffuse it into useful daylight in deep areas of buildings or areas where a window is not necessary but daylight is wanted. Sun Tunnels should be oriented to maximise their exposure to direct sunlight.

Length and configuration

The length of a Sun Tunnel influences the number of inter-reflections needed for sunlight to reach the interior of a room. While shorter Sun Tunnels will deliver more light, the very high reflectiveness of the metal material used in them allows sunlight to be efficiently transported over long distances up to 6m. Rigid Sun Tunnels will deliver more light than flexible Sun Tunnels.

Diffuser transmittance

The transmittance and optical properties of the diffuser influence both the amount and distribution of daylight from Sun Tunnels. As the name suggests, the diffuser takes the direct sunlight coming down the Sun Tunnel and diffuses it to achieve a good distribution of daylight in the room.