Assessment of daylight in a living room with skylights

Fig. 2: Efficiency of sky light utilization at windows depending on the slope of the glazing

The question remains regarding the dimensions of roof windows that can be used for sufficient room lighting. The requirements for daylighting in buildings are summarized in the Czech standard ČSN 730580 "Daylighting of buildings". For living rooms, daylighting for medium accurate visual tasks is required, corresponding, according to ČSN 730580-1 "Basic requirements" (1999), to visual task class IV - that is, tasks such as reading, writing, coarser sewing, ironing, food preparation, operating machines, common laboratory work, etc. For this class of visual tasks, the aforementioned standard requires that:

• the minimum daylight factor must be equal to at least D=1.5 % at all control points on the reference plane in the functionally defined part of the room with the required daylighting for the expected visual task,
• for spaces with top lighting, the average value of the daylight factor calculated from all values on the reference plane should not be less than D=5 %.

According to ČSN 730580-2 "Daylighting of residential buildings" (1992), the minimum daylight factor for residential rooms must be at least D=0.5 %. This value must be met at all control points on the reference plane in the entire residential room.

Furthermore, sufficient uniformity of daylighting is also required, which is defined as the ratio between the minimum and maximum value of the daylight factor at the assessed working plane. For visual task class IV, the uniformity of daylighting should not be less than 0.2. For comparison, the uniformity value defined as the ratio between the average and maximum value of the daylight factor in the assessed space can also be listed.

The assessment of daylighting through a roof window in comparison to a conventional vertical window with the same glazed area and the same sill height was carried out for a selected residential room measuring 3.2 m x 4.0 m, with a clear room height of 2.5 m.
This room was assessed for daylighting in the following variants:

• VARIANT I: Room illuminated by a VELUX roof window measuring 1140 mm x 1400 mm, sill height 900 mm. The window is placed on a roof slope of 45°.
• VARIANT II: Room illuminated by a vertical window in the facade, window dimensions 1140 mm x 1400 mm, sill height 900 mm.

Additionally, the assessment of the indicated room illuminated by two windows measuring 940 mm x 1400 mm, sill 900 mm, was conducted in two other variants:

• VARIANT III: Room illuminated by two roof windows (roof slope 45°).
• VARIANT IV: Room illuminated by two vertical windows in the facade.

The illumination of the room characterized by the daylight factor D [%] was determined for all mentioned variants for a grid of control points on the reference plane at a height of 850 mm above the floor, assuming uniformly overcast CIE sky and dark terrain. The evaluation was carried out using the computer program DEN – DQL (auth. Ing. Tomáš Maixner).

The calculations of the daylight factor were performed with the following input data:

• ceiling reflectance p=0.70, wall reflectance p=0.55, floor reflectance p=0.30, window reveal reflectance p=0.70, surrounding terrain reflectance p=0.10,
• light transmittance of the window glazing (insulating double glazing t=0.80),
• pollution factor of the window from the inside t_z1=0.90, from the outside t_z2=0.80.

Graphical representation of the distribution of isophotes with the same daylight factor D [%] on the reference plane and comparison of their minimum, maximum, and average values and uniformity of lighting in the assessed room are given in Table 1 for VARIANT I and II, and in Table 2 for VARIANT III and IV.

Tab. 1: Comparison of daylighting in a room with a roof window and with a vertical window:

VARIANT I	VARIANT II
Roof window	Window in the facade
Window dimensions 1140 mm x 1400 mm, sill height 900 mm
Total daylight factor D=Ds+Di+De [%]
Minimum Dmin = 0.93 % Maximum Dmax = 11.38 % Average Davg = 4.12 %	Minimum Dmin = 0.54 % Maximum Dmax = 2.81 % Average Davg = 1.32 %
Sky component Ds [%]
Minimum Dmin = 0.39 % Maximum Dmax = 10.75 % Average Davg = 3.54 %	Minimum Dmin = 0.25 % Maximum Dmax = 2.47% Average Davg = 1.01%
Interior reflectance component Di [%]
Minimum Dmin = 0.54 % Maximum Dmax = 0.63 % Average Davg = 0.58 %	Minimum Dmin = 0.29 % Maximum Dmax = 0.34 % Average Davg = 0.31 %
Uniformity of lighting r [-]
r1 = Dmin/Dmax = 0.08 r2 = Dmin/Davg = 0.23	r1 = Dmin/Dmax = 0.19 r2 = Dmin/Davg = 0.41

Note:

Daylight factor D = E/E_h . 100 [%]

where

E … illuminance at the control point in the room on the reference plane [lx]

E_h … reference illuminance of the outdoor horizontal unshaded plane [lx]

Components of the daylight factor D=D_s+D_e+D_i [%]

where

D_s … sky component of the daylight factor [%]

D_i … interior reflectance component of the daylight factor [%]

D_e … exterior reflectance component of the daylight factor [%]

(if there are no external shading obstacles D_e=0 %)

Tab. 2: Comparison of daylighting in a room with two roof windows and with vertical windows:

VARIANT III	VARIANT IV
Two roof windows	Two windows in the facade
Window dimensions 940 mm x 1400 mm, sill height 900 mm
Total daylight factor D [%]
Minimum Dmin = 1.48 % Maximum Dmax = 13.31 % Average Davg = 5.72 %	Minimum Dmin = 0.84 % Maximum Dmax = 3.83 % Average Davg = 1.89 %
Sky component Ds [%]
Minimum Dmin = 0.59 % Maximum Dmax = 12.26 % Average Davg = 4.75 %	Minimum Dmin = 0.39 % Maximum Dmax = 3.29 % Average Davg = 1.40 %
Interior reflectance component Di [%]
Minimum Dmin = 0.89 % Maximum Dmax = 1.05 % Average Davg = 0.97 %	Minimum Dmin = 0.45 % Maximum Dmax = 0.54 % Average Davg = 0.49 %
Uniformity of lighting r [-]
r1 = Dmin/Dmax = 0.11 r2 = Dmin/Davg = 0.26	r1 = Dmin/Dmax = 0.22 r2 = Dmin/Davg = 0.44

From the results of the daylight factor calculations in the assessed room, it is clear that the roof window allows for higher interior illumination compared to a vertical window. The lower uniformity of illumination across the entire room can be compensated for by appropriately placing individual roof windows in accordance with the layout of the room and the requirements for expected visual tasks. It is optimal to design roof windows with a lower sill height, which allows for a view of the landscape, or to use new systems of roof windows combined with a lower vertical section.

[1] Rybár, P. et al.: Daylighting and Solar Heating of Buildings. Era group, Brno 2002
[2] ČSN 730580-1 Daylighting of Buildings. Part 1: Basic Requirements (1999)
[3] ČSN 730580-2 Daylighting of Buildings. Part 2: Daylighting of Residential Buildings (1992) + amendments Z1 (1997), Z2(1999)