**4. Discussion**

*4.1. Thermal Comfort*

The thermal comfort evaluation of the historical residential buildings of Stone Town based on subjective and objective surveys is summarized in Table 5.

**Table 5.** Summary of subjective and objective survey results.


The air temperature obtained from both the subjective and objective surveys was above 26 ◦C, which is higher than the comfort zone for summer conditions (23–26 ◦C). Similarly, the relative humidity, which was around 63% as obtained from the questionnaires and measurements, is also higher than the value in the comfort zone (50–60%). Referring to the results obtained for the PMV and PPD values in the field measurements, neither of the case study buildings are controlled within the specified acceptable thermal comfort range based on the ASHRAE Standard 55, which is [−0.5 < PMV < +0.5] with a PPD < 10%. Indeed, numerous thermal comfort studies carried out worldwide have revealed that the PMV-PPD index cannot evaluate thermal comfort under non-air-conditioned space without modification due to the ignorance of subjects' adaptations. Although at least 20% of occupants are not satisfied with the current thermal environment, the TSV reflecting real thermal sensations indicates that the majority (73.57%) of the respondents' thermal sensation votes are within the central three categories of the 7-point scale. It once again proves that the PMV-PPD index is not applicable to assess thermal comfort in naturally ventilated buildings. Therefore, in Table 5, we add TSV with the purpose of reflecting the actual thermal satisfaction of occupants.

#### *4.2. Passive Cooling Strategies*

Based on the questionnaire and field survey results, passive cooling strategies implemented in the historical residential buildings in Stone Town include the north-south orientation (HRB1), natural ventilation (HRB1 and HRB2), pitched roof (HRB1), light color finishing (HRB1 and HRB2), and various types of shading. The majority (73.57%) of respondents' TSV results are within the central three categories, which indicates that these passive techniques are beneficial for indoor thermal comfort in hot areas.

Similar results have been found in previous research in recent decades. Lapisa et al., and Ozarisoy indicated that the main façade the buildings with large area windows should avoid exposure to direct solar radiation [35,36]. In this study, bedroom 3 in HRB2 is facing south, while the living room is exposed to the worst orientation (west) to solar radiation. This is one of the possible reasons why the indoor temperature in bedroom 3 is much lower than that in the living room. Window shading is an optimal way to compensate for the non-ideal orientation in retrofitting historical buildings. Lapisa et al. confirmed that slope roofs can improve the thermal comfort effect of buildings, while high-slope roofs increase costs and cause potential safety problems in earthquakes [35]. Therefore, the application of pitched roofs to historical buildings in developing countries such as Zanzibar in this study should take the cost, potential safety risk, and thermal comfort into consideration. Previous research found that solar radiation can be effectively reflected by using cool roofs and walls, thereby decreasing the heat gain in the interior [35,37]. The exterior walls of the two case study buildings in this research are painted in white or light yellow, which contributes to thermal comfort inside the buildings. This technique can be further implemented on roofs to improve thermal performance. Natural ventilation is the most efficient way to reduce energy demand and increase thermal comfort in a hot climate by providing a large amount of airflow [36,38,39]. This technique is also implemented in the historical residential buildings in Stone Town, where a large number of openings and cross-ventilation can be found. However, large areas of openings will increase the potential energy demand for heating, so the decision making of the window areas should depend on the heating/cooling demand.

In addition, numerous studies have focused on the energy demand of residential buildings by using passive cooling strategies. Fernandez-Antolin et al. found that northsouth orientation and light color finishing contribute to low cooling energy demand for residential buildings in Spain. This is consistent with the thermal comfort evaluation results obtained in this research, which implemented the same strategies [40]. Pero et al. Proposed four main processes for passive building design, including site planning, building's shape and envelope, window design, and renewable energy sources [41]. These strategies are effective for new buildings. However, many are not applicable to the renovation of historical buildings, such as the orientation and building shape. In some cases, window openings and shadings are also restricted due to the local culture. Therefore, design strategies

for improving the thermal comfort of historical residential buildings are proposed in the following subsections.

#### *4.3. Design Strategies for Historical Residential Buildings*

Three factors should be considered for the improvement of thermal comfort in historical residential buildings: (1) improve the thermal comfort conditions; (2) respect the local culture and history; and (3) reduce energy consumption.

Sun radiation is the main source of heat gain inside buildings; therefore, one of the best means by which to improve the thermal comfort of buildings is to create a quality building envelope using construction materials that help reduce the sun's heat. For historical buildings, traditional building materials, such as lime plaster and lime wash, are suitable for solving the problem.

#### 4.3.1. Lime Plaster

Lime plaster is generally made by heating lime rock (rock made of calcium carbonate, such as limestone or marble). Pure lime is white, lighter in weight than the original rock, and reacts violently with water. The lime plaster has the benefits of protecting masonry against weathering conditions, improving the thermal and acoustical performance.

Historical buildings in Stone Town were built hundreds of years ago, and damage is commonly detected nowadays. Lime plaster can improve the appearance of walls by hiding the imperfections of rough work, giving it an attractive texture compatible with the local environment.

The climate in Stone Town is hot and humid. Excessive humidity may enter the masonry in different ways and forms, such as the penetration of rainwater, fog, rising damp, and condensation, thereby leading to damage to the building's structure. Thus, lime plaster has an important role in allowing the walls of the property to breathe by letting out excess humidity.

#### 4.3.2. Wash Lime

White and light colors are beneficial in reducing cooling energy [35,37]. Although the two case study buildings already have light-colored paint on the walls, as presented in Section 3.2.1, the indoor temperatures and thermal comfort are still under the requirements indicated by the ASHRAE; thus, wash lime is proposed. Wash lime is mostly applied to stone and brick walls and adobe. It is widely used due to its availability and low cost, making it a good choice for developing countries. In addition, wash lime can brighten up the exterior walls and form calcite crystals, which reflect the sun rays striking on the walls. A light color is preferred for wash lime, as indicated earlier.

#### **5. Conclusions**

A thermal comfort study of historical residential buildings in a tropical hot–humid climate was conducted in Zanzibar. A subjective survey was carried out among 159 occupants living in the historical site of Stone Town to investigate the passive cooling techniques used in historical residential buildings and the occupants' subjective evaluations of indoor environmental parameters. The objective indoor environmental parameters were collected by field measurements from two case study buildings over six continuous days to evaluate the thermal comfort in the historical buildings.

Even with the use of passive cooling technologies, such as the use of sun shading devices and more openings to create better ventilation, thermal comfort was still not acceptable, according to international ASHRAE Standard 55. In the questionnaire survey, 73.57% of the participants voted within the three central categories of the thermal sensation vote, and 68.55% of the occupants stated that they would prefer to be cooler. This indicates that their thermal environment is not acceptable. In the field survey, the indoor temperatures and relative humidity, as well as the PMV–PPD values, of the two case study buildings were found to be higher than the acceptable limits for thermal comfort zones, which indicates

the necessity of improving the thermal environment in these historical residential buildings that already take passive cooling into consideration.

Based on the analysis of passive cooling technologies (north-south orientation, natural ventilation, pitched roof, light color finishing, and window shading) and their positive and negative effects on the thermal environment in historical residential buildings, design strategies are proposed to improve the local thermal environment. The design strategies include the application of lime plaster and wash lime, which have the potential to reduce energy consumption, improve thermal comfort, and respect the local culture and history.

This research provides guidelines to assist architects in designing energy-efficient residential buildings while also taking into account cultural heritage and thermal comfort. The retrofitting of historical buildings should avoid the large destruction of historical buildings, construct as much as possible with local materials, and preserve coordination with the local environment. Further studies concerning thermal comfort should be performed in suburban areas over a longer period, or more historical residential buildings, in order to obtain a broader picture of thermal comfort phenomena in areas with different cultural and behavioral patterns.

**Author Contributions:** Conceptualization, H.X. and H.M.A.; methodology, H.X. and H.M.A.; writing—original draft preparation, C.L. and H.M.A.; writing—review and editing, C.L., H.X. and J.L.; supervision, H.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Science and Technology Research Program of Chongqing Municipal Education Commission, grant number KJQN201900113.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
