Lighting performance

A good daylight design could maintain indoor living activities during the daytime when the sun is available. Designing building glazed fenestration for daylight requires considering its impact on visual and thermal comfort, thermal performance, energy consumption and emissions. It is because over glazing for good daylight may cause significant heat loss or overheating issue, thus higher energy consumption and emissions are required to maintain thermal comfort in a space.

The approach to assess sufficient daylight provision through building fenestration is to consider the worst case scenario under overcast sky conditions. The required light level for domestic building is that the illuminance range between 100 and 300 lux for the lounge, from 150 to 300 lux for kitchen, 150 lux for bathroom (CIBSE, 1999).

IES<VE> Radiance tool was used to predict daylight level in the living space, with the sensor located at the centre of the space in the modelling. The living space was selected regarding its occupancy profile of use during weekend daytime, the daylight illuminance readings under overcast sky condition over a year as shown in Figure 5-9.

Figure 5-9: Illuminance performance in the living space.

The illuminance outputs under overcast sky are within the required lighting design for the space, and improved daylight level can be expected under partially cloudy sky or clear sky with the sun. It can be then concluded that the living space is well day lit during the period when it is occupied (including daytime in the weekend).

In order to quantify illumiance level of the room, the average daylight factor becomes a poor representation in spaces under overcast sky conditions as there are high daylight levels near windows and very low at the rear, especially for deep rooms. The daylight illuminance level depends on glazing area, its location in a space and orientation (i.e.

solar access) as well as transmittance value of the glass. The daylight illuminance values in five rooms of the building unit via four different orientations were determined by the use of IES<VE> Gaia tool. This was because the IES<VE> FlucsDL using a radially symmetric overcast sky thus changing building orientation does not have any effect on daylight illuminance. Simulation results of average daylight factor, illuminance uniformity (calculated as minimum illuminance value divided by the average illuminance value) under CIE overcast sky condition for the design date as 21^st September for four different orientations are shown in Table 5-7. The design date was selected as a date half way (i.e. 21^st of September) between mid-summer, the 21^st June and mid-winter, the 21^st December which gives maximum and minimum levels of

daylight (Ward, 1992). A solar noon time (i.e. the time when the sun is at its highest due south) was selected, it could be considered as 12:00 in this case study as for its location.

It also includes illuminance values for three different sky conditions in correspondence with the design time and date.

Table 5-7: Daylight analysis of the building unit under CIE overcast sky conditions Cases Rooms Mean daylight

factor,%

Illuminance uniformity

Illuminance (lux) at

12:00 on September 21^st.

South

Living space 6.7 0.09 640

Bedroom 7.1 0.14 674

Office 4.8 0.19 453

Bathroom 5.1 0.22 485

Hall 1.3 0.02 120

North

Living space 4.8 0.09 454

Bedroom 3.7 0.12 350

Office 1.5 0.23 143

Bathroom 1.6 0.24 148

Hall 3.7 0.01 354

East

Living space 3.9 0.11 370

Bedroom 7.5 0.06 714

Office 2 0.16 192

Bathroom 2.4 0.17 227

Hall 2.3 0.04 218

West

Living space 7.3 0.15 693

Bedroom 3.7 0.13 355

Office 2.4 0.18 231

Bathroom 3.7 0.15 208

Hall 1.8 0.02 169

Visual discomfort (glare) caused by excessive brightness contrast could be annoying or even causes pain however in domestic building it would not be the main issue. It is because the occupant can flexibly move away from the glare or using the blind/shutter to prevent glare.

5.4 SUMMARY

The previous section discussed about heating, cooling, ventilation and lighting when the dwelling was occupied. The space heat load was predicted to be highest on north case at 2495 kWh (or 54.4 kWh/m² pa). This is lower than the requirements for new built dwellings from 2010 and 2016 onwards for similar floor area at 5400 kWh per year for 2010 built and 4000 kWh per year from 2016 (NERA, 2010). Further development for heating improvement will be investigated to explore the potential heating energy savings.

Opening windows as mean of natural ventilation was effective in alleviating overheating problem with reduction in the percentage of annual occupied hours regarding overheating risk criteria for the living space at 1.4 - 2 (%) from 13.2 - 17.9 (%) or bedroom to the level of 0.5 - 1.9 (%) from 4.3 - 9 (%). Natural ventilation supply adequate air change in eliminate overheating risk in the rest of the building. By opening glazed doors to increase air exchange rate, overheating risk disappears in the bedroom but still slightly remains within 1.3 - 1.6 (%). in the living space. In addition, regarding comfort criteria for natural ventilated living spaces of 5% annual occupied hours when the room operative temperature exceeds 25°C, the living space of the building unit does not meet this criteria and the bedroom only meets this when main building facades faces due east or when windows open at lower temperature than the threshold temperature (25°C is acceptable summertime and when it increases, occupant starts to feel hot).

Therefore, interventions for cooling design are sought to reduce overheating risk by controlling solar access with different type of shading devices or by increasing thermal mass of the building envelope combined with effective ventilation strategy. Such development offers opportunities for improved cooling performance to reduce the frequency in window opening or to take account of warmer climate at other locations or under climate change context. Furthermore, it is unlikely that air exchange rate supplied by trickle ventilators could meet the requirement of whole building background ventilation. In summertime this could be easily met by opening windows to enhance ventilation; however it becomes critical in wintertime resulting in ventilation heat loss and draught. Thus, ventilation strategy to provide adequate ventilation rate needs to be sought through. Current building fenestrations ensure the building is well lit for building occupant to work and live in daily activities.