Optimization of External Horizontal Slat Configurations for Enhanced Tropical Daylighting in High-Rise Apartments

Abstract

Tropical regions experience intense and highly variable solar radiation, often resulting in excessive indoor illuminance, uneven daylight distribution, and visual discomfort in high-rise residential buildings. This study investigates the daylighting performance of various external horizontal slat configurations as a shading strategy for east- and south-facing rooms in a typical high-rise apartment under both intermediate sky with sun and overcast sky conditions. Using IESVE simulations, ten shading device (SD) configurations (SD 1–SD 10) were evaluated for their impact on daylight illuminance and distribution uniformity. Results show that high-rise apartment room with a commonly used overhang provided poor daylighting quality, with excessive illuminance and low distribution uniformity. SD 10 and SD 9 achieved the best performance at 09:00 and 12:00, respectively, for east-facing rooms across design days, while SD 10 was optimal for south-facing rooms on both 21 March and 22 December. Under overcast sky conditions, SD 9 demonstrated superior performance. This study proposes a novel adaptive external shading device featuring adjustable horizontal slats that can be reconfigured throughout the day to respond to changing solar positions and sky conditions. This approach overcomes the limitations of fixed shading systems by adapting to dynamic tropical sky conditions, offering a practical solution for enhancing daylight quality in tropical high-rise apartments. The findings provide design guidance for the development of energy-efficient shading, climate-responsive shading systems tailored to tropical environments.

1. Introduction

Daylight in tropical regions represents a highly potential renewable energy resource due to the abundant sunlight available throughout the day [1,2,3,4]. However, many high-rise apartments in tropical regions fail to utilize daylight effectively due to various challenges [5]. Fixed shading devices, such as overhangs, which are commonly used in high-rise residential buildings in Malaysia, provide limited daylight control and are unable to adapt to the dynamic nature of solar radiation across different times of the day and seasons. This often leads to excessively high indoor illuminance, uneven daylight distribution, and visual discomfort. As a result, there is a pressing need to explore more effective external shading designs that can better respond to varying weather conditions and improve daylighting performance.

As this paper focuses on optimizing daylighting in tropical regions through shading devices, recent studies (2019–2024) on daylighting performance of various shading devices in the tropics are reviewed (Table 1). Atthaillah et al. (2022) investigated the interaction between the depth and elevation of the overhang in tropical classrooms with bilateral openings, revealing that shading elevation significantly influences daylight performance while shading depths beyond 2.0 m are generally ineffective in improving daylight metrics [6]. Mangkuto et al. (2019) optimized blinds considering the type of blinds and slat angle for an open-plan office in Indonesia, addressing dynamic sky conditions by evaluating daylight metrics across varying diffuse solar irradiance ranges [7]. Louver systems have also been studied, with findings demonstrating improvements in both daylighting performance and energy efficiency in office settings [8,9]. Mangkuto et al. (2021) emphasized the importance of adaptive louver systems for achieving optimal daylight uniformity and glare control on east- and west-facing facades in tropical high-rise buildings [10].

A range of innovative shading strategies beyond conventional designs have also been explored in tropical climates to improve daylighting performance and energy efficiency. Adaptive light shelves have been proven to enhance visual comfort and optimize natural light penetration under varying tropical sky conditions [2,11,12]. When integrated with other shading elements, such as perforated screens and blinds, light shelves can significantly reduce glare and improve daylight distribution in tropical office environments [1,13]. Furthermore, the integration of horizontal light pipes with other shading devices has been explored to enhance daylighting performance in deep-plan office buildings in tropical climates [4,14,15]. Kinetic façades have been explored in tropical regions, with recent studies exploring biomimicry and origami-inspired designs to develop responsive systems capable of dynamically regulating daylight and contributing to overall energy efficiency [16,17].

However, previous studies on tropical shading devices in relation to daylighting have predominantly focused on office buildings, and their findings may not be directly applicable to residential buildings due to differences in building typology and window configurations. Although a few studies have examined horizontal louver systems for tropical residential buildings [18,19], these are limited to flats and low-rise residences. Moreover, there is limited research on the use of shading devices to reflect daylight and improve uniformity, particularly in the deep portion of the room. This study investigates various configurations of external horizontal slats to enhance the daylighting quality of a typical east- and south-facing room of high-rise apartments in Malaysia under diverse tropical sky conditions. The influence of horizontal slat parameters on daylight illuminance level and distribution uniformity is evaluated. The findings contribute to the development of dynamic external shading devices for residential façades in tropical climates, optimizing daylighting performance under variable sky conditions.

Table 1. Studies on shading devices for tropical daylighting (2019–2024).

Cite No.	Year	Climate	Location	Model	Orientation	Operability	Building	Function
[7]	2019	Am	Bandung, Indonesia	Blinds	W	MSD	O	IDL, RG
[20]	2020	Af	Johor, Malaysia	Horizontal light pipe	S	FSD	O	IDL, EDDU
[12]	2020	Af	Penang, Malaysia	Light shelf	S	FSD	O	IDL, RG, EDDU
[19]	2021	Aw	Surabaya, Indonesia	Folding shutter	N	MSD	R	IDL, REDL, EDDU, REC
[3]	2021	Af	Johor Bahru, Malaysia	Light shelf + Venetian blinds	N, S, E, W	MSD	O	IDL, EDDU, RG
[9]	2021	Aw	Bangkok, Thailand	Horizontal shading slats	N	MSD	O	IDL, EDDU, REC
[10]	2021	Am	Jakarta, Indonesia	Horizontal louvers	N, S, E, W	MSD	O	IDL, EDDU
[15]	2021	Aw	Surabaya, Indonesia	Light shelf + blind + Horizontal light pipe	E	FSD	O	EDDU, REDL
[17]	2021	Am	Ho Chi Minh, Vietnam	Origami-inspired kinetic shading device	N, NE, E, SE, S, SW, W, NW	MSD	O	IDL, EDDU, RG, REC
[4]	2022	Af	Malaysia	Light shelf + Horizontal light pipe	N, S, E, W	FSD	O	REDL, EDDU
[14]	2022	Af	Malaysia	Horizontal light pipe + Venetian blinds	SW	FSD	O	REDL, EDDU
[6]	2022	Af	Lhokseumawe, Indonesia	Overhang	N, S, E, W	FSD	C	IDL, REDL, RG
[11]	2022	Af	Penang, Malaysia	Light shelf	S	FSD	O	IDL, EDDU, RG, REC
[8]	2022	Aw	Jakarta, Indonesia	Horizontal louver	N/A	MSD	O	IDL, REC
[16]	2022	Aw	Bangkok, Thailand	Adaptive biomimetic kinetic façade	N/A	MSD	O	IDL, EDDU, REC
[21]	2023	Aw	Surabaya, Indonesia	Light shelf + blind + Horizontal light pipe	E, W	FSD	O	EDDU
[13]	2024	Aw	Surabaya, Indonesia	Light shelf + Perforated screen	W	FSD	O	IDL, RG, EDDU
[18]	2024	Aw	Surabaya, Indonesia	Horizontal bent louver	N	MSD	R	EDDU

Climate—Am: Tropical Monsoon, Aw: Tropical Savanna, Tropical Wet and Dry, Af: Tropical Rainforest, Equatorial; Orientation—N: North, S: South, E: East, W: West, NE: Northeast, NW: Northwest, SE: Southeast, SW: Southwest; Moveability—MSD: Moveable Shading device, FSD: Fixed Shading Device; Building—O: Office, R: Residence, C: Classroom; Function—IDL: Improve Daylight Level, REDL: Reduce Excessive Daylight Level, RG: Reduce Glare, EDDU: Enhance Daylight Distribution Uniformity, REC: Reduce Energy Consumption.

Table 1. Studies on shading devices for tropical daylighting (2019–2024).

Cite No.	Year	Climate	Location	Model	Orientation	Operability	Building	Function
[7]	2019	Am	Bandung, Indonesia	Blinds	W	MSD	O	IDL, RG
[20]	2020	Af	Johor, Malaysia	Horizontal light pipe	S	FSD	O	IDL, EDDU
[12]	2020	Af	Penang, Malaysia	Light shelf	S	FSD	O	IDL, RG, EDDU
[19]	2021	Aw	Surabaya, Indonesia	Folding shutter	N	MSD	R	IDL, REDL, EDDU, REC
[3]	2021	Af	Johor Bahru, Malaysia	Light shelf + Venetian blinds	N, S, E, W	MSD	O	IDL, EDDU, RG
[9]	2021	Aw	Bangkok, Thailand	Horizontal shading slats	N	MSD	O	IDL, EDDU, REC
[10]	2021	Am	Jakarta, Indonesia	Horizontal louvers	N, S, E, W	MSD	O	IDL, EDDU
[15]	2021	Aw	Surabaya, Indonesia	Light shelf + blind + Horizontal light pipe	E	FSD	O	EDDU, REDL
[17]	2021	Am	Ho Chi Minh, Vietnam	Origami-inspired kinetic shading device	N, NE, E, SE, S, SW, W, NW	MSD	O	IDL, EDDU, RG, REC
[4]	2022	Af	Malaysia	Light shelf + Horizontal light pipe	N, S, E, W	FSD	O	REDL, EDDU
[14]	2022	Af	Malaysia	Horizontal light pipe + Venetian blinds	SW	FSD	O	REDL, EDDU
[6]	2022	Af	Lhokseumawe, Indonesia	Overhang	N, S, E, W	FSD	C	IDL, REDL, RG
[11]	2022	Af	Penang, Malaysia	Light shelf	S	FSD	O	IDL, EDDU, RG, REC
[8]	2022	Aw	Jakarta, Indonesia	Horizontal louver	N/A	MSD	O	IDL, REC
[16]	2022	Aw	Bangkok, Thailand	Adaptive biomimetic kinetic façade	N/A	MSD	O	IDL, EDDU, REC
[21]	2023	Aw	Surabaya, Indonesia	Light shelf + blind + Horizontal light pipe	E, W	FSD	O	EDDU
[13]	2024	Aw	Surabaya, Indonesia	Light shelf + Perforated screen	W	FSD	O	IDL, RG, EDDU
[18]	2024	Aw	Surabaya, Indonesia	Horizontal bent louver	N	MSD	R	EDDU

Climate—Am: Tropical Monsoon, Aw: Tropical Savanna, Tropical Wet and Dry, Af: Tropical Rainforest, Equatorial; Orientation—N: North, S: South, E: East, W: West, NE: Northeast, NW: Northwest, SE: Southeast, SW: Southwest; Moveability—MSD: Moveable Shading device, FSD: Fixed Shading Device; Building—O: Office, R: Residence, C: Classroom; Function—IDL: Improve Daylight Level, REDL: Reduce Excessive Daylight Level, RG: Reduce Glare, EDDU: Enhance Daylight Distribution Uniformity, REC: Reduce Energy Consumption.

2. Methodology

This study employed IESVE as a simulation tool to evaluate the daylight performance of various horizontal slat configurations [22]. IESVE utilizes a Radiation-based engine that applies a highly validated ray-tracing calculation to simulate daylight performance. Previous studies have demonstrated that IESVE is a powerful tool for daylighting simulation, offering accurate, dynamic, and integrated analysis suitable for research. These studies have examined various shading and daylighting systems, including light pipes [4,21,23], light shelves [2,3,24], perforated screens [13], venetian blinds [14], anidolic daylighting systems [25]. The findings of these existing studies corroborate the reliability of IESVE in simulating the daylight performance of various systems under tropical sky conditions.

2.1. Validation

Validation of the simulation method was carried out by comparing the simulated data between the verification simulation model and the referenced simulation model from a previous study of light shelves in tropical office buildings.

In the referenced study [2], an open-plan office room with various internal light shelf designs under the tropical climate of Malaysia was simulated to evaluate daylight performance. The simulation setup in IESVE adhered to the referenced work, including the dimensions of the office room and light shelves (LS), material properties, and sky conditions. In addition, four light shelf designs with significant configuration differences were selected from the referenced study for validation, including base case, LS 1, LS 3, and LS 6 (See Figure 1).

Figure 2 illustrates the comparison of the average Daylight Ratio (DR), defined as the ratio of indoor work plane illuminance to exterior global horizontal illuminance, between the referenced study and our verification model across different light shelf designs during the three different times for north-facing and south-facing office rooms. The relationship between the two datasets was then analyzed using Pearson Correlation. As shown in

Table 2. Pearson correlation analysis of simulated DR between the referenced study and the verification model.

Orientation	Pearson Correlation	Sig. (2-Tailed)	N
North	0.998 1	0.000	36
South	0.997 1	0.000	36

¹ Correlation is significant at the 0.01 level (2-tailed).

, the Pearson correlation analysis indicates that the simulation model in this study closely aligns with the referenced research.

2.2. Daylighting Simulation

Overhangs, balconies, and openings with a moderate window-to-wall ratio are common façade elements in high-rise apartment buildings in Malaysia. However, due to regulatory, economic, climatic, and cultural factors, high-rise apartments without balconies are also widely prevalent. Moreover, the presence of deep balconies could interfere with the accurate evaluation of daylight performance in configurations with external horizontal slats. Consequently, this study focuses on a typical façade configuration commonly found in Malaysian high-rise apartments, characterized by short overhangs and the absence of balconies. A reference model was constructed in IESVE with dimensions of 6.60 m (depth), 3.06 m (width), and 0.29 m (height), based on a typical living room and dining room of a high-rise apartment in Selangor, Malaysia (Figure 3). The model features an opening of 2.06 m × 1.90 m, with a window-to-wall ratio of 0.44 and an overhang with a protruding depth of 250 mm above the window, representing a typical high-rise apartment façade. The visible transmittance (VT) of the clear glazing is 0.75. The reference model served as the base case. Based on the reference model, ten different shading device (SD) configurations (SD 1–SD 10) with varying horizontal slat designs were modeled, as shown in Figure 4. These ten configurations, along with the base case, were evaluated through daylight simulation.

The ten shading devices were divided into two categories based on clerestory height: SD 1, 3, 5, 7, and 9 have a clerestory height of 1300 mm, while SD 2, 4, 6, 8, and 10 have a clerestory height of 1000 mm. The geometry of the shading devices varies between pairs. Specifically, SD 1 and 2 feature horizontal slats positioned at the middle height of the opening with a protruding depth of 250 mm, whereas SD 3 and 4 have horizontal slats with a protruding depth of 500 mm. Additionally, SD 1 and 2 differ from SD 5 and 6 in the number of horizontal slats. The former pairs (SD 1 and 2) have a single horizontal slat, whereas the latter pairs (SD 5 and 6) feature two horizontal slats, with one slat of the same size positioned at the bottom. A similar difference exists between SD 3 and 4 and SD 9 and 10. SD 7 and 8 share a similar design to SD 5 and 6, but the bottom slats extend to a depth of 500 mm. All slats have a thickness of 50 mm.

The tropical climate is characterized by dynamic sky conditions, where cloud formations vary rapidly, predominantly featuring intermediate skies [2]. According to Zain-Ahmed et al. [26], the sky conditions are predominantly intermediate comprising 85.6% of the time, while overcast sky account for 14.0%, and clear blue sky are virtually non-existent (0%) in Subang, Malaysia. Lim and Heng [2] stated that clear skies are virtually absent in Malaysia, suggesting that daylighting research in tropical regions should account for the variable cloud patterns typical of intermediate sky conditions. Therefore, the simulation employed two types of sky conditions: intermediate sky with sun and 10 k lux overcast sky for three design days (21 March, 22 June, and 22 December), at three different times (09:00, 12:00, and 15:00).

The properties of materials used for interior surfaces significantly impact daylight performance, as the reflectance of materials affects the direction of incident light [12,27]. The internal surface properties, including reflectance, specularity, and roughness, used in the IES-Radiance modeling are shown in

Table 3. Internal surfaces properties of base case and 10 test cases.

Components	Reflectance (%)	Specularity	Roughness	Visible Transmittance
Floor	20	0.03	0.20	N/A
Ceiling	80	0.03	0.03	N/A
Wall	70	0.03	0.03	N/A
Horizontal slat	90	0.05	0.03	N/A
Glazing	N/A	N/A	N/A	0.75

. These properties align with the recommendations of the IESNA lighting guides for indoor reflectance in residential spaces [12], and represent typical residential designs [28,29,30].

2.3. Criteria of Analysis

IESVE employs several sky components suitable for tropical sky conditions. It utilizes International Commission on Illumination (CIE) sky models to generate global or outdoor illuminance. However, CIE sky models do not accurately represent the tropical sky condition due to their lower global illuminance in comparison with the tropical sky condition which can reach up to 120,000 lux [1,2]. While daylight metrics such as Daylight Autonomy (DA) and Useful Daylight Illuminance (UDI) were widely adopted in daylighting research, ratio-based metrics were often prioritized in tropical studies because they better account for high ambient illuminance and the variability of tropical sky conditions [1,2,3,4,7,14,15,20,21,23,31,32]. Consequently, this study employed the Daylight Ratio (DR) to evaluate daylight performance under intermediate sky conditions and use the Daylight Factor (DF) to assess daylight performance under overcast sky condition, representing a worst-case scenario. The calculations of DR and DF are shown in Equation (1):

DR or DF = Work Plane Illuminance/Exterior Global Horizontal Illuminance × 100%

(1)

The test room was divided into two zones, each instrumented with a 9 × 6 grid of horizontal illuminance sensor points (total 54 sensors per zone), to evaluate daylight performance in specific areas, as illustrated in Figure 5. Rows 1–9 correspond to the living room area, while rows 10–18 represent the dining room. All sensor points are placed at a height of 0.75 m from the floor. The estimated average indoor illuminance was calculated using Equation (2):

Estimated Indoor Illuminance (EII) = DR × Estimated Outdoor Illuminance

(2)

According to field measurements, the average exterior global horizontal illuminance values were 26,895 lux at 09:00, 84,086 lux at 12:00, and 75,117 lux at 15:00. Additionally, the quality of daylight distribution was evaluated using the illuminance uniformity ratio [2,33], which is the ratio of minimum illuminance to average illuminance within the room, as specified in Equation (3):

Illuminance Uniformity Ratio = Emin/Eavg

(3)

3. Results

3.1. Daylight Illuminance Level

Figure 6 shows the comparison of mean EII at rows 1–9 and rows 10–18 between base case and SD 1 to SD 10 under the east orientation at three different times for three design days. Generally, the base case exhibited the highest mean EII values at rows 1–9 across all periods. The results indicate that SD 1 to SD 10 reduced the EII values at rows 1–9 to varying degrees during all evaluated periods. The top four shading devices in reducing excessive illuminance at rows 1–9 across all three dates were SD 3, 4, 9, and 10. Only minor differences were observed between them, as detailed in

Table 4. Mean estimated indoor illuminance of base case and 10 SD cases for intermediate sky.

Selection Criteria				Cases Under the East Orientation
				Base Case	SD 1	SD 2	SD 3	SD 4	SD 5	SD 6	SD 7	SD 8	SD 9	SD 10
Mean Estimated Indoor Illuminance (lux)	21 Mar	9:00	Entire room	3903	3420	3424	3087	3094	3447	3436	3459	3461	3125	3138
			Rows 1–9	9136	8038	7963	7008	6962	8066	8030	8078	8058	7050	7064
			Rows 10–18	386	350	366	331	349	362	370	367	384	352	366
		12:00	Entire room	2362	1417	1382	1012	1306	1501	1472	1601	1580	1198	1447
			Rows 1–9	5584	2474	2397	2009	2115	2520	2470	2666	2676	2357	2388
			Rows 10–18	209	202	212	208	226	232	239	261	269	262	261
		15:00	Entire room	580	462	462	383	402	454	454	446	445	369	383
			Rows 1–9	1177	923	895	744	769	912	868	882	867	716	739
			Rows 10–18	104	94	97	89	93	93	93	88	90	85	87
	22 Jun	9:00	Entire room	3387	2944	2919	2626	2600	2960	2936	2973	2956	2662	2636
			Rows 1–9	7312	6265	6243	6052	5216	6316	6253	6329	6346	6031	5301
			Rows 10–18	320	285	295	280	295	295	301	300	319	293	303
		12:00	Entire room	2298	1477	1462	1176	1355	1562	1548	1644	1637	1322	1471
			Rows 1–9	5661	2729	2699	2336	2432	2883	2825	3027	3020	2602	2697
			Rows 10–18	244	235	248	244	265	261	275	290	298	280	289
		15:00	Entire room	764	614	613	514	533	606	602	599	596	500	515
			Rows 1–9	1555	1236	1192	1004	1027	1204	1173	1208	1158	980	983
			Rows 10–18	135	119	122	115	117	115	116	116	117	109	110
	22 Dec	9:00	Entire room	3160	2765	2735	2468	2451	2776	2752	2792	2771	2505	2474
			Rows 1–9	6232	5597	5322	5115	4542	5664	5341	5565	5382	5090	4569
			Rows 10–18	301	276	280	260	284	279	285	286	300	273	286
		12:00	Entire room	2366	1571	1556	1267	1430	1644	1636	1723	1715	1404	1537
			Rows 1–9	5689	2943	2881	2490	2570	3074	3020	3201	3178	2781	2817
			Rows 10–18	266	250	268	259	284	276	288	303	314	295	304
		15:00	Entire room	841	677	678	570	591	672	671	667	663	556	573
			Rows 1–9	1683	1352	1320	1113	1124	1334	1289	1337	1294	1080	1093
			Rows 10–18	145	130	133	125	128	126	132	124	126	116	124
Selection Criteria				Cases Under the South Orientation
				Base Case	SD 1	SD 2	SD 3	SD 4	SD 5	SD 6	SD 7	SD 8	SD 9	SD 10
Mean Estimated Indoor Illuminance (lux)	21 Mar	9:00	Entire room	670	562	562	494	505	575	575	589	591	524	535
			Rows 1–9	1363	1135	1107	960	971	1154	1139	1184	1167	1040	1027
			Rows 10–18	114	106	107	104	114	110	112	113	117	114	119
		12:00	Entire room	927	729	734	671	715	748	744	821	826	681	719
			Rows 1–9	1931	1490	1464	1308	1398	1520	1469	1669	1622	1338	1393
			Rows 10–18	143	134	136	149	158	139	138	161	167	158	169
		15:00	Entire room	977	779	783	702	741	793	784	853	853	720	746
			Rows 1–9	2013	1575	1528	1375	1419	1604	1542	1710	1675	1412	1434
			Rows 10–18	150	141	144	152	161	144	146	162	168	160	170
	22 Jun	9:00	Entire room	521	426	425	363	371	424	421	424	421	356	359
			Rows 1–9	1059	614	536	494	518	572	569	570	572	568	565
			Rows 10–18	87	68	68	66	61	54	61	56	57	50	51
		12:00	Entire room	763	611	608	507	531	599	599	594	589	493	507
			Rows 1–9	1556	1224	1187	1005	1019	1207	1178	1194	1153	971	987
			Rows 10–18	133	119	122	116	119	118	118	115	119	108	110
		15:00	Entire room	801	643	643	540	557	634	634	630	626	527	544
			Rows 1–9	1627	1280	1248	1069	1070	1278	1240	1261	1229	1029	1033
			Rows 10–18	137	124	128	119	125	122	123	118	119	114	119
	22 Dec	9:00	Entire room	1472	1226	1183	1026	969	1242	1199	1256	1218	1061	1004
			Rows 1–9	2924	2495	2264	2025	1623	2508	2297	2535	2308	2092	1690
			Rows 10–18	166	150	154	144	153	155	157	156	166	155	168
		12:00	Entire room	3824	2632	2617	1700	2207	2704	2704	2791	2786	1888	2400
			Rows 1–9	9893	8999	8487	5876	5818	9184	8842	9349	8935	6109	6188
			Rows 10–18	333	319	329	303	334	341	350	365	378	363	386
		15:00	Entire room	3579	2550	2546	1710	2121	2606	2610	2674	2679	1864	2284
			Rows 1–9	8262	7488	7133	4797	4735	7619	7368	7675	7392	5015	5036
			Rows 10–18	319	301	322	310	329	326	344	351	356	348	358

. At 09:00, SD 4 achieved the highest reduction in EII, with reductions of 23.8% on 21 March, 28.7% on 22 June, and 27.1% on 22 December. At 12:00, SD 3 performed best, reducing EII by 64.0%, 58.7%, and 56.2% on the respective dates. At 15:00, SD 9 achieved the greatest reduction, with reductions of 39.2%, 37.0%, 35.8%. Additionally, all of the SD cases reduced the EII value at rows 10–18 at 09:00 and 15:00 across the three design days. SD 3 exhibited the highest reduction at 09:00, with values of 14.3% (21 March), 12.4% (22 June), and 13.7% (22 December). However, EII values at rows 10–18 at 09:00 for all cases remained within the recommended range of the MS 2680:2017 benchmark for residential daylighting [34]. SD 9 exhibited highest reduction at 15:00, with reduction of 17.9%, 18.7%, and 19.7%, respectively. In contrast, at 12:00, all SD cases, except for SD 1, increased the EII of the base case at rows 10–18 in three design days. The top 4 shading devices in this regard were SD 7, 8, 9, and 10. SD 8 achieved the highest increase, with values of 28.7% on 21 March, 22.1% on 22 June, and 18.0% on 22 December. SD 7, 9, and 10 showed only minor differences compared to SD 8, with variations ranging from 2.6% to 6.1%.

Figure 7 illustrates the mean EII values at rows 1–9 and 10–18 across three design days under the south orientation. Similarly to the east orientation, the base case exhibited the highest mean EII values at rows 1–9 across all time points and dates. The mean EII values on 22 December were the highest compared to those on 21 March and 22 June. Similarly, SD 3, 4, 9, and 10 exhibited the highest performance in reducing illuminance level at rows 1–9 across all three dates, with only minor differences among them, as detailed in Table 4. SD 3 achieved the greatest reduction in EII on both 21 March and 22 June, with values of 29.5% (09:00), 32.3% (12:00), and 31.7% (15:00) on 21 March, and 53.4%, 35.5%, and 34.3% on 22 June, respectively. On 22 December, SD 4 exhibited the highest reduction, with values of 44.5%, 41.2%, and 42.7% at 09:00, 12:00, and 15:00, respectively. Additionally, a widespread decrease in EII values at rows 10–18 was observed on 22 June, with SD 9 exhibiting the highest reduction of 42.5%, 18.8%, and 17.1% at three different times. In contrast, a general increase in EII values at rows 10–18 was observed on 21 March and 22 December. SD 8 and SD 10 increased EII values at three different times for both dates, with SD 10 demonstrating a highest improvement.

Figure 8 compares the average Daylight Factor (DF) between the base case and SD 1 to SD 10 under overcast sky for both east-facing and south-facing room. The DF values displayed similar trends across these two orientations. The base case achieved an average DF of 3.27% in both orientations. All the SD cases exhibited lower average DF values compared to the base case. SD 3 produced the lowest average DF in both orientations (2.37%), followed by SD 4, 9, and 10. In contrast, SD 7 and SD 8 achieved the highest DF values (2.92%), followed by SD 5 and SD 6. Only minimal differences were observed among all SD cases. Notably, the average DF of all shading devices fell within the recommended range of MS 2680:2017, that is between 1.0% and 3.5%. Additionally, all SD configurations reduced the maximum DF compared to the base case (Figure 9). SD 4 exhibited the lowest maximum DF, 13.24% for east orientation, and 13.31% for south orientation, as shown in

Table 5. Summary of 10 test cases performances under overcast sky.

Selection Criteria		Cases Under the East Orientation
		Base Case	SD 1	SD 2	SD 3	SD 4	SD 5	SD 6	SD 7	SD 8	SD 9	SD 10
Daylight Factor (%)	Minimum	0.34	0.32	0.3	0.3	0.33	0.31	0.33	0.32	0.34	0.35	0.34
	Maximum	24.83	22.9	20.13	18.18	13.24	23.25	20.43	23.3	20.84	18.68	14.28
	Average	3.27	2.72	2.71	2.37	2.45	2.81	2.79	2.92	2.92	2.56	2.63
Percentage of points achieving DF range (%)	<1	45	47	46	47	46	46	45	44	43	45	43
	1 < X < 3.5	29	29	29	31	29	30	29	31	30	31	31
	>3.5	26	24	25	22	25	24	26	25	27	24	26
Uniformity Ratio		0.105	0.118	0.111	0.127	0.135	0.109	0.117	0.11	0.116	0.138	0.131
% of Improvement in Uniformity		/	12.4	5.7	21	28.6	3.8	11.4	4.8	10.5	31.4	24.8
Selection Criteria		Cases Under the South Orientation
		Base Case	SD 1	SD 2	SD 3	SD 4	SD 5	SD 6	SD 7	SD 8	SD 9	SD 10
Daylight Factor (%)	Minimum	0.32	0.33	0.32	0.31	0.34	0.32	0.34	0.32	0.34	0.36	0.33
	Maximum	25.02	22.86	20.04	17.92	13.31	23.07	20.32	23.28	20.76	18.62	14.35
	Average	3.27	2.72	2.71	2.37	2.46	2.8	2.79	2.92	2.92	2.56	2.63
Percentage of points achieving DF range (%)	<1	44	44	44	44	44	44	44	44	41	42	40
	1 < X < 3.5	28	30	29	30	28	30	28	28	31	32	32
	>3.5	28	26	27	26	28	26	28	28	28	26	28
Uniformity Ratio		0.099	0.121	0.117	0.129	0.136	0.113	0.12	0.11	0.123	0.139	0.126
% of Improvement in Uniformity		/	22.1	18.2	30.3	37.4	14.1	21.2	11.1	24.2	40.4	27.3

. The second lowest maximum DF was achieved by SD 10 (14.28% on east and 14.35% on south), followed by SD 3 (18.18% east and 17.92% south), and SD 9 (18.68% and 18.62%). Regarding minimum DF, SD 9 performed best in increasing minimum DF compared to the base case, from 0.34% to 0.35% for the east orientation, and from 0.32% to 0.36% for the south orientation.

Sensor points analysis of the Daylight Factor was conducted at 108 points within both east-facing and south-facing rooms for the base case and the ten proposed shading devices. Figure 10 illustrates the percentage of sensor points with DF value falling within the recommended range of MS 2680:2017 (1.0–3.5%). In the east-facing room, SD 3, 7, 9, and 10 exhibited the highest percentage of DF points within the recommended range, each achieving 31%. In the south-facing room, SD 9 and SD 10 performed best, with 32% of points falling within the benchmark range. Additionally, SD 8 and SD 10 demonstrated the lowest percentage of points with DF values below 1.0%.

3.2. Daylight Distribution Uniformity

Figure 11 shows the daylight distribution uniformity analysis for SD 3, 4, 9, and 10 under intermediate sky conditions for both orientations. These shading devices were selected for comparison with the base case due to their optimal performance in daylight illuminance analysis for both orientations. In the east-facing room, SD 4, 9, 10 generally demonstrated the greatest improvements in daylight distribution uniformity at three different times for three design days. Specifically, SD 10 ranked highest at 09:00 for all three dates, with improvements of 84.6% on 21 March, 60.0% on 22 June, and 40.5% on 22 December, as detailed in

Table 6. Distribution uniformity analysis for intermediate sky.

Selection Criteria			Cases Under the East Orientation
			Base Case	SD 3	SD 4	SD 9	SD 10
21 Mar	9:00	Distribution Uniformity Ratio	0.026	0.035	0.04	0.047	0.048
		% of Improvement in Uniformity	/	34.6	53.8	80.8	84.6
	12:00	Distribution Uniformity Ratio	0.034	0.071	0.075	0.088	0.081
		% of Improvement in Uniformity	/	108.8	120.6	158.8	138.2
	15:00	Distribution Uniformity Ratio	0.072	0.086	0.089	0.081	0.079
		% of Improvement in Uniformity	/	19.4	23.6	12.5	9.7
22 Jun	9:00	Distribution Uniformity Ratio	0.035	0.045	0.049	0.048	0.056
		% of Improvement in Uniformity	/	28.6	40	37.1	60
	12:00	Distribution Uniformity Ratio	0.036	0.09	0.079	0.097	0.09
		% of Improvement in Uniformity	/	150	119.4	169.4	150
	15:00	Distribution Uniformity Ratio	0.066	0.079	0.084	0.078	0.072
		% of Improvement in Uniformity	/	19.7	27.3	18.2	9.1
22 Dec	9:00	Distribution Uniformity Ratio	0.042	0.047	0.044	0.052	0.059
		% of Improvement in Uniformity	/	11.9	4.8	23.8	40.5
	12:00	Distribution Uniformity Ratio	0.042	0.083	0.072	0.088	0.083
		% of Improvement in Uniformity	/	97.6	71.4	109.5	97.6
	15:00	Distribution Uniformity Ratio	0.063	0.068	0.072	0.064	0.068
		% of Improvement in Uniformity	/	7.9	14.3	1.6	7.9
Selection Criteria			Cases Under the South Orientation
			Base Case	SD 3	SD 4	SD 9	SD 10
21 Mar	9:00	Distribution Uniformity Ratio	0.047	0.062	0.68	0.071	0.075
		% of Improvement in Uniformity	/	31.9	44.7	51.1	59.6
	12:00	Distribution Uniformity Ratio	0.06	0.077	0.08	0.079	0.082
		% of Improvement in Uniformity	/	28.3	33.3	31.7	36.7
	15:00	Distribution Uniformity Ratio	0.046	0.076	0.076	0.075	0.086
		% of Improvement in Uniformity	/	62.2	65.2	63	87
22 Jun	9:00	Distribution Uniformity Ratio	0.047	0.042	0.057	0.056	0.044
		% of Improvement in Uniformity	/	−12	21.3	19.1	−6.8
	12:00	Distribution Uniformity Ratio	0.057	0.063	0.066	0.058	0.057
		% of Improvement in Uniformity	/	10.5	15.8	1.8	0
	15:00	Distribution Uniformity Ratio	0.053	0.061	0.065	0.054	0.057
		% of Improvement in Uniformity	/	15.1	22.6	1.9	7.5
22 Dec	9:00	Distribution Uniformity Ratio	0.039	0.065	0.058	0.06	0.066
		% of Improvement in Uniformity	/	66.7	48.7	53.8	69.2
	12:00	Distribution Uniformity Ratio	0.032	0.066	0.063	0.074	0.076
		% of Improvement in Uniformity	/	106.3	96.9	131.2	137.5
	15:00	Distribution Uniformity Ratio	0.04	0.068	0.059	0.072	0.077
		% of Improvement in Uniformity	/	70	47.5	80	92.5

. At 12:00, SD 9 exhibited the highest improvements, increasing the distribution uniformity ratio by 158.8%, 169.4%, and 109.5% on 21 March, 22 June, and 22 December, respectively. At 15:00, SD 4 performed best, with increases of 23.6%, 27.3%, and 14.3% on the same dates. However, the proposed shading devices demonstrate significantly lower improvement at 15:00 compared to their performance at 09:00 and 12:00. Under the south orientation, SD 3, 4, 9, and 10 showed better performance in improving distribution uniformity on 21 March and 22 December. SD 10 ranked highest, with improvements of 59.6% (09:00), 36.7% (12:00), and 87.0% (15:00) on 21 March, and 69.2% (09:00), 137.5% (12:00), and 92.5% (15:00) on 22 December. In contrast, on 22 June, some proposed shading devices reduced the uniformity ratio, while others showed only minor improvements.

Under overcast sky conditions (refer to Table 5), the shading devices on the east orientation exhibited similar effects on the distribution uniformity ratio as those on the south orientation. The base case recorded distribution uniformity values of 0.105 for the east and 0.099 for the south. SD 9 achieved the highest percentage improvement, with an increase of 31.4% (east) and 40.4% (south). SD 4 ranked second, with an improvement of 28.6% and 37.4%.

4. Discussion

The simulation results revealed that the base case, featuring a short overhang in a typical high-rise apartment, exhibited excessively high indoor illuminance and uneven daylight distribution under both east and south orientation. Specifically, in east-facing room, the mean estimated indoor illuminance at rows 1–9 peaked at 9:00 across all three design days under intermediate sky conditions, reaching 9136 lux, 7312 lux, and 6232 lux on 21 March, 22 June, and 22 December, respectively. These values remained elevated at 12:00, with EII values of 5584 lux, 5661 lux, and 5689 lux on the same dates. Under the south orientation, the base case exhibited excessive sunlight at rows 1–9, particularly at 12:00 and 15:00 on 22 December, with EII values of 9893 lux and 8262 lux, respectively. According to MS2680:2017 [34], these illuminance levels exceed the recommended useful daylight range, leading to glare as well as visual and thermal discomfort. Furthermore, the uniformity ratio of the base case was consistently below 0.1, indicating poor daylight quality. These findings demonstrate that the current shading device design of high-rise apartments in tropical regions often results in poor daylight conditions, prompting occupants to reject daylight use (e.g., closing curtains during the day) and rely entirely on electric lighting.

The performance of the base case underscores the importance of controlling daylight penetration in tropical high-rise apartments by both reducing excessive daylight and improving its distribution to enhance visual comfort. As a general observation, external horizontal slats were found to effectively reduce excessive sunlight in both east- and south-facing rooms under intermediate sky with sun, particularly in the front portion of the room. Among the evaluated configurations, SD 3, SD 4, SD 9, and SD 10 consistently ranked as the top four shading devices in reducing the EII values at rows 1–9 across all three design days. This indicates that deeper horizontal slats positioned at mid-window height (SD 3, SD 4, SD 9, and SD 10) outperformed shorter configurations (SD 1, 2, 5, 6, 7, and 8) in reducing over-illuminance in the front area of the room. These findings highlight the critical influence of the slat depth at the mid-opening height on the overall performance of external shading devices.

The proposed shading devices were categorized into five groups based on their clerestory height variations: Group1 (SD 1 and 2), Group2 (SD 3 and 4), Group3 (SD 5 and 6), Group4 (SD 7 and 8), Group5 (SD 9 and 10). In each group, the first shading device features a clerestory height of 1300 mm, while the second has a height of 1000 mm. As shown in Table 4, there was no significant difference in overall illuminance or the illuminance at rows 1–9 between the two shading devices in each group under intermediate sky conditions. This indicates that clerestory height has minimal impact on these daylight metrics. However, in the presence of direct sunlight (at 09:00 and 12:00 on all three design days in the east orientation, and on 21 March and 22 December in the south orientation), shading devices with a 1000 mm clerestory height exhibited higher illuminance levels in the rear area of the room (rows 10–18) compared to those with a 1300 mm clerestory height. The most significant improvement was observed in SD 3 and SD 4 from Group 2 at 12:00. SD 4 increased the EII of SD 3 by 8.7% on 21 March, 8.6% on 22 June, and 9.7% on 22 December under east orientation, and 6.1% on 21 March and 10.2% on 22 December under south orientation. These findings demonstrate that lower-positioned horizontal slats at mid-opening height more effectively reflect sunlight towards the deeper areas of the room, thereby enhancing rear-zone illuminance. This effect was particularly pronounced during times of higher solar altitude. Under overcast sky conditions, shading devices with lower horizontal slats in the middle achieved lower maximum DF values than those with higher slats within the same group, indicating that clerestory height significantly influences maximum DF under such sky condition.

The SD cases were divided into four groups based on the presence of a bottom horizontal slat: Group A (SD 1 and 5), Group B (SD 2 and 6), Group C (SD 3 and 9), and Group D (SD 4 and 10). In each group, the second shading device has a bottom horizontal slat, while the first does not. Under direct sunlight conditions, shading devices with a bottom horizontal slat effectively increased the EII in the rear area of the room (rows 10–18) for both orientations. In east-facing rooms, the EII increasement at rows 10–18 was observed at both 09:00 and 12:00, with greater improvements occurring at noon. For instance, on 21 March, SD 5 increased the EII of SD 1 by 14.9% at 12:00, compared to only 3.4% at 09:00. Similarly, SD 9 increased the EII of SD 3 by 26.0% at 12:00, compared to 13.2% at 09:00. In south-facing rooms, the presence of bottom horizontal slats elevated EII levels in rear room across three different times on 21 March and 22 December, with greater enhancements at noon (12:00) and afternoon (15:00) than in the morning (09:00). These findings indicate that the bottom horizontal slats can effectively reflect sunlight deeper into the space when there was direct sunlight penetration, thereby increasing illuminance in the rear zone of the room. The extent of improvement is influenced by the solar altitude angle.

The study further investigated the influence of the bottom slat geometry on sunlight penetration into the deeper zones of the space. The shading devices (SD 5, 6, 7, 8) were grouped based on slat depth: Group I (SD 5 and SD 7) and Group II (SD 6 and SD 8). In each group, the first shading device features a shorter bottom slat (250 mm), while the second has a deeper one (500 mm). In the presence of direct sunlight, deeper bottom slats provided higher EII at rows 10–18 compared to the shorter slats in both orientations. In east-facing rooms, EII increasements at the rear zone were observed at both 09:00 and 12:00 across all three design days, with more pronounced improvements at 12:00. Under the south orientation, EII increases occurred at all three time points on both 21 March and 22 December, with the most significant improvements seen at 12:00 and 15:00. These findings demonstrate that the depth of bottom horizontal slat has a substantial impact on sunlight penetration into the rear of the room, with the degree of improvement influenced by the solar altitude angle.

In the absence of direct sunlight under intermediate sky condition (at 15:00 in east-facing rooms and throughout the day on 22 June for south-facing rooms), the proposed shading devices not only excessively reduced the mean EII at rows 1–9, but also caused significant reductions at rows 10–18. Furthermore, only a few proposed shading devices demonstrated slight improvements in distribution uniformity compared to the base case. These findings indicates that external horizontal slats offer limited benefits for improving daylight performance in high-rise apartment buildings when direct sunlight is absent under intermediate sky conditions.

In summary, for the east orientation, SD 3, SD 4, SD 9, and SD 10 exhibited the best overall performance in term of daylight illuminance levels at both 09:00 and 12:00 across all three design days under intermediate sky condition. Among these, SD 9 and SD 10 achieved higher mean EII in the deeper zone of the room compared to SD 3 and SD 4. SD 10 outperformed the others in improving distribution uniformity at 09:00 across all three dates, while SD 9 demonstrated the highest uniformity ratio at 12:00, making it the best-performing option at that time. However, at 15:00 on all three design days, all proposed shading devices excessively reduced the already low daylight illuminance in the front zone of the room and further significantly reduced illuminance in the rear zone. In terms of distribution uniformity, only a few shading devices provided minor improvements. This indicates that, as east-facing rooms do not receive direct sunlight in the afternoon, the use of external horizontal slats negatively affects the daylighting performance of apartment buildings. For the south orientation, SD 3, SD 4, SD 9, and SD 10 also exhibited the best overall performance in daylight illuminance on 21 March and 22 December under intermediate sky condition. SD 10 outperformed the other three in improving distribution uniformity across all three time points on both dates. However, on 22 June, similar to the east-facing orientation at 15:00, all proposed shading devices also reduced the daylight illuminance of the entire room. only a few SD cases showed minor gains in uniformity. Hence, the application of horizontal shading devices is not recommended on 22 June. Under overcast sky condition, daylight performance of proposed shading devices was similar between east- and south-facing rooms, indicating that building orientation has minimal influence on daylight performance under such conditions. The average DF of all cases fell within the recommended benchmark range of 1.0–3.5% for both orientations. Among the shading devices, SD 9 achieved the highest percentage of sensor points with DF value falling within the recommended range (1.0–3.5%). SD 9 also achieved highest increasement of the minimum DF compared to the base case and exhibited the highest improvement in daylight distribution uniformity, identifying it the best-performing device under overcast condition.

5. Conclusions

Static shading devices are insufficient to provide optimal daylighting performance throughout the day in high-rise apartments located in tropical regions, where dynamic sky conditions are prevalent. This study investigated the effect of external horizontal slat configurations on daylight illuminance and daylight distribution uniformity for east- and south- facing rooms of tropical high-rise apartments. Based on the findings, this paper proposes a movable shading device with adjustable horizontal slats to optimize daylighting performance for both east and south orientations in high-rise apartments under dynamic tropical sky conditions, as illustrated in Figure 12. For east-facing rooms under intermediate sky conditions, SD 10 and SD 9 are recommended as optimal configurations at 9:00 and 12:00, respectively, due to their superior performance in enhancing both illuminance levels and distribution uniformity throughout the room at the corresponding time points. The use of any external horizontal slats at 15:00 is not recommended duo to their negative impact on daylight performance. For south-facing room, SD 10 provides the optimum design on both 21 March and 22 December, while the use of horizontal slats on 22 June is not advised. Under overcast sky condition, SD 9 is identified as the optimal configuration.

The daylight performance of external horizontal slats is significantly influenced by the availability of direct sunlight and the solar altitude angle. Under intermediate sky conditions with direct sunlight penetration, shading devices with deeper horizontal slats positioned at the mid-height of the window opening demonstrated superior performance in reducing over-illuminance and enhancing daylight uniformity. While the difference in clerestory height had minimal impact on overall illuminance and front-area illuminance level, lower clerestory heights contributed to increased illuminance levels in the rear areas of the room, particularly during periods of high solar altitude. The presence of a bottom horizontal slat also played a critical role in enhancing daylight penetration into the rear areas of the room. Shading devices with a bottom slat consistently achieved higher effective illuminance levels in the deeper zones, particularly around midday when solar altitude is elevated. This effect was further amplified by deeper bottom slats, which provided greater improvements in rear-zone illuminance compared to shorter slats. These findings highlight the importance of horizontal slat geometry and placement, including clerestory height, slat depth, and the inclusion of bottom slats, in optimizing daylighting performance. However, in the absence of direct sunlight under intermediate sky conditions, the application of external horizontal slats was found to reduce both daylight illuminance levels and distribution uniformity, thereby negatively affecting the daylighting performance of high-rise apartment buildings.

This study is limited to east-facing and south-facing rooms in high-rise buildings; other orientations, such as northeast and southeast, should be prioritized in future research due to their prevalence in tropical high-rise designs and their distinct daylighting challenges. Furthermore, additional investigations into the number of slats and slat tilt angles are recommended, as these parameters are easily adjustable in both design and simulation processes and may significantly affect daylighting outcomes in tropical settings. Future research should also evaluate the trade-off between daylighting and thermal performance, as tropical daylighting is often accompanied by significant solar heat gain.