Summertime Thermal Insulation. The Daylight Factor

Summertime thermal insulation is recommended for transparent fagades in buildings with natural ventilation in order to avoid the possibility of overheating. The recommendations are as follows: The product of the total energy transmission factor (g) (- (25)) x the solar protection factor (z) (- (27)) x the window surface component (f) on the fagade, i.e. g x z x f, should have a value of 0.14-0.25 for strongly constructed buildings, and a value of 0.12-0.17 for those of lighter structure (see (26)).

Extensive solar shading precautions – (28) should be critically evaluated, since wide-ranging visual effects may result and the view may be permanently impaired – (28).

The interplay of natural surroundings, physical laws and the development of constructional styles in specific materials means that each case requires accurate, individual analysis – (29).

Explanation of Figure (29). Outside and fagade - (1)
- Shadows and cooling due to vegetation (trees, shrubbery, etc.)
- Light-coloured pathway (width approx. 1 m), e.g. pebbles, in front of the house
- Sun or anti-glare protection (b = 35°) installed, extent approx. 900 mm
- Fagade in bright reflecting materials (pastel colours)
- Adequate window size (with insulating glass) for incident light and heat, with white internal frames

Inside - (2)
- Consideration for house plants, if present
- Light- or medium-coloured floor covering
- Flexible heating system (a combination of air and hot water)
- Light-coloured curtains as anti-glare protection to diffuse direct solar radiation (particularly during transition periods)
- Light matt colours (pastel and natural colours for furniture) on surrounding areas, particularly the ceiling
- Cross-ventilation via tilting flaps
- Simple mechanical ventilation, if required

The measurement and evaluation of daylight in internal areas with light admission from the sides and above.

The daylight in internal areas can be evaluated according to the following quality criteria: illuminance and brightness; uniformity; glare; shadow.

Basis: daylight in internal areas, the illuminance of a clouded sky (i.e. diffuse radiation) is taken as the basis. Daylight admitted to an internal area through a side window is measured by the daylight factor D. This is the ratio of the illuminance of the internal area (Ei) to the prevailing external illuminance (Ea), where D = Ei/(Ea x 100)%. Daylight in internal areas is always given as a percentage. For example, when the illuminance of the internal area is 500 lx and the external illuminance is 5000 lx, then D = 10%.

The daylight factor always remains constant. The illuminance of an internal area varies only in proportion to the external illuminance prevailing at the time. The external illuminance of a clouded sky varies from 5000 lx in winter to 20000 lx in summer – (30), and depends on the time of year and the time of day.

The daylight factor at a point P – (31) is influenced by many factors. D = (DH + DV + DR) x t x k1 x k2 x k3, where DH is the component of light from the sky, DV is the effect due to neighbouring buildings, DR is the contribution from internal reflections, and the following reduction factors are taken into consideration: t, the light transmission factor for the glass; k1, the scatter effects due to the construction of the window; k2, the scatter effects due to the type of glazing; k3, the effects of the angle of incidence of the daylight.

The reference plane for the horizontal illuminance of daylight in an internal area is as shown in – (32). It can be taken as 0.85m above floor level, and is separated from the walls of the room by 1 m. The points EP used for the horizontal illuminance are fixed on this reference plane. The corresponding (to be determined) daylight factors can then be represented in the form of a daylight factor curve – (32). The shape of the curve on the section provides information about the horizontal illuminance on the reference plane (at the corresponding points), and then Dmin and Dmax can be established (see also uniformity). The curve of the daylight factor also provides information on the variation of daylight in the room.

Required daylight factors D%. The relevant, currently valid requirements are laid down in regulations relating to daylight in internal areas and in the guidelines for work areas. Since no other relevant data are available at present, the required variation in daylight can be determined and checked from the uniformity (see later).

On the assumption that living rooms are comparable in terms of their dimensions with work rooms, the following values for the required daylight factors should be adhered to:
- Dmin > 1% in living rooms, reference point the centre of the room – (33);
- Dmin > 1% in workrooms, reference point the lowest position in the room – (33);
- Dmin > 2% in workrooms with windows on two sides;
- Dmin > 2% in workrooms with light coming from above, with the minimum mean daylight factor (Dm) > 4%.

Note: With side windows, the associated maximum daylight factor should be at least six times greater than the minimum requirement, and in the case of light from above in workrooms, Dm should be twice as large as Dmin. Several examples for different internal area illuminance requirements as a function of external illuminance are shown in – (34).