The influence of the placement of roof windows on the natural light in a room

Visual comfort is one of the main design conditions that take into account a healthy indoor environment in buildings. To achieve lighting comfort, it is necessary to ensure not only adequate intensity of natural light in the room and its functionally defined areas but also, if possible, as uniform lighting as possible.
The evenness of lighting plays an important role in the design of lighting openings. Staying in a room with uneven lighting can cause worsened vision. When looking into such a room, the eye is exposed to constant alternation of high and low lighting intensities, which causes a person to recognize fewer details that they would otherwise see under suitable lighting conditions [1, 2].
The uniformity of day lighting depends on the light transmittance of the glazing for roof windows and primarily on their area and location concerning the illuminated room.
The assessment of day lighting in a living room with roof windows and the evaluation of the impact of their positioning on lighting uniformity was carried out in the following example. The room has a floor plan size of 4m x 5m, illuminated by two roof windows measuring 1140mm x 1400mm, with a total window area of 3.2 m² and glazing area of 2.8 m². The windows are installed in a sloped roof with a pitch of 40°.
The assessment of day lighting in the room was conducted for the following design variants:

Fig.: Diagram of the assessed room with roof windows (designed in Variants I, II, and III)


To evaluate the illuminance of the stated room, the calculation of the daylight factor was carried out according to ČSN 73 0580 Daylighting of buildings [3] at selected points on a reference plane 850 mm above the floor. The layout of the room with indicated points (1 to 12) is shown in Figure 1. Calculations were performed using the computer program DEN, DQL (author Ing. Tomáš Maixner) for the following input data: light reflectance of the ceiling p=0.70, walls p=0.55, floor p=0.30, window reveals p=0.60, surrounding terrain p=0.10; light transmittance of the window glazing t=0.72 - double glazing, contamination factor of the glass t_z1=0.90 from the inside, t_z2=0.80 from the outside.
The results of the daylight factor calculation characterizing the level of daylighting on the reference plane in the assessed room and the evaluation of lighting uniformity are compared for individual design variants in Tables 1 and 2.

Table 1: Daylight factor determined on the reference plane in the assessed room:

Note: For the assessed roof window, no external obstacles were considered; therefore, the external reflected component of the daylight factor D_e=0%.

Table 2: Uniformity of day lighting in the room with roof windows:

Note: Values of the daylight factor D[%]: minimum D_min, maximum D_max, average D_avg

An example of a graphical representation of the distribution of daylight on the reference plane in the assessed room is provided for the individual variants in Figure 2.

Variant I:

Variant II:

Variant III:

a)

b)

Fig. 2: Graphical representation of the distribution of daylight in the assessed room, Variants I, II, III
a) Isofote progression (connecting points with the same daylight factor) on the reference plane
b) Spatial graph (D% ... daylight factor)



The basic criterion for establishing requirements for daylight levels in buildings is the classification of visual tasks according to visual difficulty in accordance with the relative observation distance of the critical detail and with the characteristics of visual tasks into 7 classes according to ČSN 73 05080-1 [3] - see Table 4.
The values of the daylight factor specified in the table must be met [3]:

• minimum D_min: at all control points in the internal space or its functionally defined part,
• average D_m: for internal spaces with top lighting or combined lighting, in which the proportion of top lighting to the average value of the daylight factor D_m is at least one half.

Table 4: Required daylight factor values for visual activity classes [3]:
Note to the table:

1. The average value of the daylight factor Dm is determined as the arithmetic mean of values at control points of a chosen regular grid on a horizontal reference plane.
2. The uniformity of daylight is determined as the ratio of the smallest to the largest value of the daylight factor, measured at control points on the horizontal reference plane throughout the entire range of the indoor space or in its functionally defined part.
3. The requirements for the values of the daylight factor increase, e.g., in case of low brightness contrast, for a short observation time period, concerning an older average age or health condition of the inhabitants, during operating conditions interfering with good visibility, and everywhere where the influence of poor visibility could cause errors leading to inaccuracies, accidents, serious injuries, etc.
4. For example, Class IV of visual activity includes visual tasks such as medium-precision production and control, reading, writing, machine operation, ordinary laboratory work, examinations, treatments, coarser sewing, ironing, food preparation, competitive sports.

Internal spaces with permanent human occupancy must have daylighting that meets the requirement for the value D_min of at least 1.5% and for the average daylight factor D_m of at least 3%, even if lower values would suffice for a given visual task.
In residential rooms, a minimum value of the daylight factor of 0.5% is required, which must be met at all control points on the reference plane. The average value of the daylight factor must be at least 2% [4].

Comparing the stated normative requirements with the calculated values of the daylight factor in the assessed room, it is evident that the most appropriate solution in terms of daylighting and ensuring sufficient uniformity is the design of roof windows in Variants II and V.

Roof windows allow for higher illuminance in attic rooms from natural light, as they are typically not shaded by external obstacles. Higher lighting intensities in rooms with roof windows are mainly because the brightness of the sky increases from the horizon to the zenith. This fact also explains the increase in the daylight factor in the assessed room with the window in Variant II compared to the results of the assessment with the window in a lower position - Variant I. The lower illuminance of the room assessed in Variant III is caused by the position of the window concerning the assessed room. The gradient of brightness of the sky can be usefully utilized in designing attic rooms with high requirements for daylight illuminance.
By ensuring a sufficient size of the glazed areas of roof windows and their suitable placement concerning the floor plan arrangement of rooms and the requirements for visual tasks in their individual functionally defined parts, optimal lighting conditions can be achieved for both residential and workspaces. Not least, uniformity of lighting can also be influenced by the interior design of rooms, where light and shiny surfaces reflect light, matte scatter, and dark absorb it.
In residential rooms, a roof window should be installed so that its bottom edge is a maximum of 1100 mm above the floor [6]. It is also possible to combine roof windows with additional side lighting openings to ensure visual contact with the surroundings.

[1] Plch, J. Lighting technology in practice. IN-EL, Prague 1999
[2] Rybár, P. et al.: Daylighting and solar gain of buildings. Era group, Brno 2002
[3] ČSN 730580-1 Daylighting of buildings. Part 1: Basic requirements (1999)
[4] ČSN 730580-2 Daylighting of buildings. Part 2: Daylighting of residential buildings (1992)+ changes Z1 (1997), Z2(1999)
[5] ČSN EN 12665 (ČSN 36 0001): 2003 Light and lighting - Basic terms and criteria for determining lighting requirements
[6] ČSN 73 4301 Residential buildings (2004)