Where: Tc is the comfort temperature and To is the monthly mean of the outdoor
temperature. In this case, the comfort temperature has a range of ±2.0°C with the possibility of extending it further on the provision of adaptive opportunities (Nicol and Humphreys, 2002).
Both of the aforementioned equations and similar adaptive comfort models are frequently used to approximate comfort temperatures in naturally ventilated buildings. Specifically, it has been acknowledged that the adaptive approach offers building designers a close approximation of the internal temperatures that users will find to be satisfactory within passive buildings (Nicol and Humphreys, 2002).
In a comparative review of adaptive and PMV models of comfort, Orosa and Oliveira (2011) note that the adaptive model gives a more precise estimate for passive buildings. However, they also caution that the adaptive approach may sometimes be lacking, in that it may predict the same neutral temperature for areas with similar outdoor temperature but of different relative humidity levels. Halawa and van Hoof (2012) also note with concern that the over dependence on the outdoor temperature values ignores the fact that there are other variables highlighted in the PMV model that are still quite valid and do not seem to receive similar weighting in this approach. Instead, they suggest that the way forward should be to integrate
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both approaches to arrive at a more comprehensive method of evaluating thermal comfort.
Nevertheless, as explained by Brager and de Dear (1998) in their review of numerous significant field research studies, the fact remains that users of naturally ventilated buildings show a preference of temperature levels that closely followed the outdoor temperature and are more tolerant of temperature swings. Similarly, Aulicems and Szokolay (2007) suggest that significant field investigations and verification of the adaptive model in warm humid regions such as Darwin and Singapore reveal that the static model is not suitable for warm regions.
Given the findings of this review, it would appear that the adaptive method allows for a better understanding and appreciation of warm humid climates. It has been suggested that it would be of good use to building designers to incorporate standards that consider how a building will run within its given climate and take into account locally acceptable comfort as opposed to simply aiming to create an indoor environment independent of this; as seems to be the case with the PMV models (Koch-Nielsen, 2002, Nicol and Humphreys, 2002).
A recent review of adaptive thermal comfort reveals that the model has continued to gain traction across the world. Undoubtedly, the most significant effect of this has been its integration into two international standards: ASHRAE Standard 55 (2004) and later European Standard EN 15251 (2007) (Halawa and van Hoof, 2012). More recently, the ASHRAE Standard 55 has been revised to incorporate findings from the latest research on adaptive comfort (ASHRAE, 2013a). A significant change (2010 version) has included provisions for evaluating the impacts of elevated air speed. In warm regions, countries such as Brazil (where much of its territory may be classified as hot humid) have already had a direct shift towards the adoption of the adaptive approach into local jurisdiction by way of bio-climatic design (Cândido et al., 2011a). As highlighted earlier, a similar approach is planned for Kenya.
Based on the findings of this literature review, a similar adaptive approach was adopted as the main model for this research. This decision was founded on these two findings:
25 1. A significant number of thermal comfort field studies conducted in warm regions have found that building occupants (acclimatised to local climate conditions) tend to be comfortable at higher temperatures.
Therefore, it would be unreasonable to propose the use of PMV methods that tend to recommend small comfort ranges that could only be met by the provision of active ventilation systems.
2. The method promotes the provision of adaptive opportunities to occupants for the restoration of comfort, if need be.
Unlike the PMV method, adaptive comfort models are more responsive to human behaviour and assume that, if changes occur in the thermal environment to produce discomfort, then occupants will generally change their behaviour and act in a way that will restore their comfort. The main effect of such adaptive models is to increase the range of conditions that designers can consider as comfortable, especially in naturally ventilated buildings where the occupants have a greater degree of control over their thermal environment.
It was determined that the PMV model is better utilised when designing air conditioned zones or buildings where conditions are more tightly controlled. Therefore, it was recommended that the PMV ‘traditional’ model be considered as the primary prediction method for buildings that use active conditioning methods, whereas adaptive comfort model be used for naturally ventilated systems.
1.3 Local Climatic Considerations in Kenya
Covering an area of 585,363 km², Kenya extends between latitudes 5° 30’ N and 5° S and longitudes 34° E and 40° E and (Figure 1-9). Kenya is a tropical country with a variety of climatic subzones resulting from differences in topography, the presence of significant water bodies (including Lake Victoria and the Indian Ocean), and extensive vegetative cover in some regions (Figure 1-10). Given its location and prominence in Kenya, the city of Mombasa was selected to provide a focus for the study of the warm humid region of Kenya (denoted as ‘coast’ in Figure 1-10). Mombasa is the second largest city in Kenya (after the capital city of Nairobi) with a
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population of approximately 940,000 people (Kenya National Bureau of Statistics, 2010). It has served as the most significant port city in East Africa for a number of centuries to date (Mombasa Municipal Council and National Museums of Kenya, 1990).
Figure 1-9 (a) Map of Africa showing location of Kenya. (b) Topographical map of Kenya showing the location of Mombasa (Author-modified from Maps for Design, 2013, Maps of the World, 2013).
(a)
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Figure 1-10 Climatic zones in Kenya (Hooper, 1975, p.3).
Previously in section 1.1, it was established that Mombasa experienced relatively high temperatures and relative humidity levels for majority of the year. This was coupled by instances of high solar radiation. In this section, a more in depth analysis is presented. To begin with, a climate graph was prepared to present the basic climate data for Mombasa Figure 1-11.
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Figure 1-11 (a) to (f) Climate graphs for the warm humid city of Mombasa, Kenya (author-generated with Meteonorm Version 7 using weather data from U.S Department of Energy (2013).
Mean daily max. Mean daily min. Mean monthly min. Mean monthly max.
(a) M o nt hl y T em p e rat ur e (b) D ai ly T e m pe rat ur e (c) M o nt hl y Radi at ion (d) D ai ly Gl obal Radi at ion (e ) S uns hi n e D ur at ion (f) P re ci pi tat ion
29 Following this, the thermal comfort limits for Mombasa were extracted based on the findings of section 1.2. Using Equation 1-3 and Equation 1-4, the predicted comfort limits for the warm humid city of Mombasa were calculated and the results presented in Table 1-4, Figure 1-12 and Figure 1-13.
Table 1-4 Calculated comfort limits for Mombasa.
Month Ta/To Tn LTc01 UTc01 Tc LTc02 UTc02
Jan 27.4 26.1 23.6 28.6 28.3 26.3 30.3 Feb 27.8 26.2 23.7 28.7 28.5 26.5 30.5 Mar 28.5 26.4 23.9 28.9 28.9 26.9 30.9 Apr 27.5 26.1 23.6 28.6 28.4 26.4 30.4 May 26.1 25.7 23.2 28.2 27.6 25.6 29.6 Jun 25.0 25.4 22.9 27.9 27.0 25.0 29.0 Jul 23.9 25.0 22.5 27.5 26.4 24.4 28.4 Aug 24.0 25.0 22.5 27.5 26.5 24.5 28.5 Sep 25.0 25.4 22.9 27.9 27.0 25.0 29.0 Oct 25.8 25.6 23.1 28.1 27.4 25.4 29.4 Nov 26.9 25.9 23.4 28.4 28.0 26.0 30.0 Dec 27.4 26.1 23.6 28.6 28.3 26.3 30.3
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Figure 1-13 Comfort limits for Mombasa based on Nicol and Humprey’s Tc equation.
The choice to compare both comfort limit equations was informed by a similar study conducted by Nicol et al. (1999) where they aimed to derive the thermal comfort limits of office buildings in various climatic regions of Pakistan. Their study findings for the warm humid region indicated that the temperature limits derived from both equations were fairly similar with the temperatures extracted from the Tc equation
being slightly higher than those derived from the Tn equation. In addition, they
reported that subjects were generally comfortable at temperatures of between 20° to 30°C when using fans. When temperatures became unbearable, mechanical fans would be employed to enhance air movement.
A similar approach was adopted for this study. Using both the Tn and Tc equations, it
was predicted that the upper comfort limits for Mombasa could be extended to 29°C and 31°C, respectively (both based on results extracted for the month of March). It is noted that, although the local climate conditions are fairly constant all year round, March is the warmest month of the year and therefore the most prone to overheating. Consequently, it was suggested that the predicted upper comfort limits of March could be used to define the comfort extreme for the entire year. It was suggested that if the ambient temperatures exceeded these limits, there would be the need to restore the indoor comfort conditions. To help maintain the comfort conditions indoors, it would be necessary to apply design strategies for regulation
31 purposes. These measures could either be passive methods (preferable) or mechanical/active methods (last resort). To help determine the passive methods that might be considered, a psychrometric chart analysis was conducted for Mombasa using the climate analysis software Climate Consultant 4.0 and presented in Figure 1-14. A psychrometric chart is a graphical representation of the psychrometric processes of air which include physical and thermodynamic properties such as dry bulb temperature, wet bulb temperature, humidity, enthalpy and air density (Aulicems and Szokolay, 2007). These charts provide a rapid overview of air conditions as they relate to the selected climate and occupant comfort. Further, they can be used to chart the environmental design strategies that can be used to extend comfort and the period for which this can be effective.
Figure 1-14 Psychrometric chart with environmental strategies overlays for Mombasa.
The analysis indicated that if passive strategies were considered, it would be possible to meet thermal comfort requirements for 85% of the entire year by keeping ambient temperatures within the predicted comfort range of 22°C to 29°C (based on
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the Tn equation results). It was also predicted that, even if passive methods were
applied, ambient temperatures would lie above 29°C for 11% of the year – signalling potential for occupant discomfort during this period. This analysis was deemed to be particularly significant as it indicated the amount of time that a building could be run in Mombasa without the input of mechanical or active measures.
The following is a checklist of passive design guidelines recommended for the extension of comfort as derived from this analysis:
Sun-shading designed for the specific latitude to reduce or eliminate the need for air conditioning.
Minimise or eliminate west facing glazing to reduce heat gain in warmer periods and in the afternoons.
Use of natural ventilation, where windows are well shaded and oriented to prevailing breezes.
Use of light coloured materials for walls and roofs (with high emissivity) to reduce heat gain through conduction.
Use of high thermal mass indoor walls to provide coolth and provide time-lag. Use of passive design strategies developed by vernacular type architecture
(where suitable).
On particularly warm days (applies to the period when temperatures are above 29°C – 11% of the hours in the year) air conditioning may be required where fan-forced ventilation does not make conditions suitable for occupants.
Of the guidelines recommended, both sun shading (39% hours of the year) and natural ventilation (15.1% hours of the year) were found to be the most beneficial passive interventions in the extension of the comfort range. Specifically, sun shading is particularly suitable for mitigating solar heat gain whereas natural ventilation is suitable for removing warm air build up and providing psychological cooling (Koenigsberger et al., 1973, Olgyay and Olgyay, 1963). Thermal mass and fan-forced ventilation were recommended for 0.7% and 15.5% hours of the year, respectively. The suitability and application of these strategies was examined in greater detail in
33 Chapter 2. It was also recommended that a review of local vernacular architecture (which has inherently tried and tested over long periods of time) might be worth considering when seeking solutions for local buildings. Similarly, a review of the local vernacular types was examined in greater detail later in section 1.4 and Chapter 3.
Further to determining the period of time when passive strategies could facilitate comfort all year round (85%), Climate Consultant version 5.4 software was used to derive the proportion of this period that falls within typical working hours (set from 8am to 6pm as would mainly be the case in office buildings). The results of this review were illustrated in Figure 1-15. The findings indicated that ambient temperatures would lie between 22°C and 29°C for 74% of the year and greater than 29°C for 26% of the year if passive design strategies were applied during working hours.
Figure 1-15 External temperature range during working hours (8am to 6pm).
For comparison purposes, psychrometric chart analysis using the PMV model (ASHRAE Standard 55-2004) was conducted using Climate Consultant version 5.4. In this method, an experimentally derived algorithm considers dry bulb temperature, humidity, air velocity and metabolic activity. It has two comfort zones for summer and winter clothing and the slightly sloped temperature limits account for the fact that in dryer air people are more comfortable at slightly higher temperatures. With this model the mean radiant temperature (MRT) is assumed to be roughly equal to dry bulb temperature.
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From this, it was determined that the PMV comfort limits lay between 20.3°C and 26.7°C for 90% percentage people satisfied, PPS (PPS = 100 – PPD). A review of the ambient temperatures with respect to these limits indicated that the passive measures would only be able to restore thermal comfort for 67% of the year, leaving 38% of the year reliant on active measures. This was significantly decreased to 34% (comfort facilitated by passive means) and 66% (comfort facilitated by mechanical or active means) when considering working hours only.
A comparison of these findings indicated that the adaptive comfort model offered a suitably higher upper comfort limit that predicted the provision of comfort via passive means for 40% more time than the PMV method. Similarly, a 40% reduction in the use of active means was also recorded. Currently, the emission factor for the Kenyan electricity grid is 0.33kgCO2 per kWh generated (Ecometrica, 2011, IEA,
2015). Considering the impact of applying adaptive comfort measures to commercial buildings such as offices in Kenya, this would translate into significant annual cooling energy cost savings of approximately KShs.10bn ($107m at the current rate of Ksh.1 to $0.009), and significant carbon emission reductions of 17,902,269kgCO2 (representative of a 15% cut in current national emissions).
1.4 Local Architectural Responses
In this section an initial review of the vernacular and contemporary architectural responses in the warm humid region of Kenya was undertaken. This included an introduction to vernacular Swahili architecture (a typology identified to be predominant/ typical of the local precedent) and a comparative analysis of the environmental responses of the local Swahili vernacular and typical contemporary office buildings.
1.4.1 Vernacular Architecture
The East African coastal region is steeped in a history that is said to go back as far as 100 A.D (Ghaidan, 1975). Settlements in this region, including the key city of Mombasa, emerged from a period that was heavily influenced by transoceanic trade and an amalgamation of diverse cultures (Kiamba et al., 2014, Mombasa Municipal
35 Council and National Museums of Kenya, 1990). With this came the development of a distinct architectural type which is now referred to as ‘vernacular Swahili architecture’. Consisting mainly of two to three storey coral stone buildings reminiscent of the hot dry Arab world, this predominant type is distinguished by buildings with thick walls that are punctuated by relatively heavily shaded fenestration openings and ornate timber balconies (Table 1-5).
In addition to the seemingly heavyweight Swahili coral stone buildings, other common typologies included the lightweight Swahili mud and wattle houses with corrugated iron sheet roofs, Mijikenda ‘Kayas’ made of a wattle frames covered with palm leaf fronds and Mijikenda mud and wattle houses with palm frond roofs (Table 1-5). These lightweight typologies were more common with the local indigenous population consisting mainly of a collection of nine ethnic groups collectively referred to as the Mijikenda people; and later among those who could not afford to build coral stone houses (Ghaidan, 1975, Mombasa Municipal Council and National Museums of Kenya, 1990).
Table 1-5 Predominant vernacular architecture in the coastal region of Kenya.
Type Main characteristics a) Swahili coral stone building
(vernacular Swahili architecture)
Laid out in dense urban layouts.
Two to three storey buildings with thick coral rag
walls, flat coral rag ceilings and a palm frond pitched roof - replaced by corrugated iron sheet roofing.
Heavily shaded by balconies, screens and
neighbouring buildings.
Typically mixed-use building in the Mombasa region;
ground floor - commercial, first floor - residential.
b) Swahili mud and wattle house Layouts were similar in concept to the residential
floor of the stone houses (built for occupation by those considered to be of lower class/slaves).
Single storey buildings with wattle frames and mud
and coral aggregate infill. Roofs were pitched and made of palm frond thatch with open gable ends - replaced by corrugated iron sheet roofing.
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c) Mijikenda ‘Kaya’ house Entirely made of wattle frame and palm frond
thatch.
Open-plan layout.
In the mid-19th century, most of these settlements
were abandoned for various reasons (mainly migration to more ‘permanent’ settlements due to increased social and economic empowerment).
d) Mijikenda mud and wattle house Single storey buildings with wattle framed walls of mud and coral aggregate infill and palm frond thatch roofs with open gable ends.
Layouts typically consisted of two rooms for living
and sleeping areas.
The review of the vernacular architectural types found that they tended to be strictly for residential use or mixed residential and commercial use (Kiamba et al., 2014, Hoyle, 2001). Despite the fact that this study is focused on solutions suitable for office buildings, it was suggested that an examination of the local precedent (irrespective of their primary use) presented the opportunity to identify and examine local design solutions with the aim of identifying suitable transferable design strategies.
It has been noted that Mombasa is located in a primarily warm humid climate zone, where heat and humidity are the main environmental issues of concern for building designers. With this in mind, an initial review of the environmental design strategies of the selected vernacular types - taking into account the potential causes of climatic stress on indoor conditions including relatively high temperatures, direct solar radiation, humidity and glare – was undertaken and presented in Table 1-6. For purposes of clarity, the review was divided into two sections, ‘heavyweight’ and ‘lightweight’, as per the main construction materials employed in each architectural type.
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Table 1-6 Comparison of the environmental design principles of the vernacular architectural types found in the coastal region of Kenya.
Heavyweight Lightweight (a) H o u se typ e s
a) Swahili coral stone building b) Swahili mud and wattle house (SM) c) Mijikenda ‘Kaya’ house (MK) d) Mijikenda mud and wattle house (MM) (b ) Or ie n tat io n & Layou t
Are laid out to fit into
dense urban layouts and to channel incoming breezes.
Tend to be oriented along the east-west axis (to avoid solar
gain through the east and west facades).
Open layouts to encourage breezes around all buildings
within residential compounds.
(c ) Co n str u ction an d m ate ri al s
Thick coral rag walls with
potentially high thermal capacity (increased time lag).
Ventilated attic space (to
reduce the conduction of heat into lower floors).
Screened timber window
shutters and balconies for shading
SM and MM: Permeable roof with open gable ends (to
reduce the conduction of heat into lower floors).
38 (d ) Ven til ation
Compact plans with
window placement to encourage cross ventilation.
Ventilation at full body
level is encouraged by windows of up to 1.8m height with a cill level of 0.3m.
SM and MM: A central corridor with rooms on either side to
encourage cross ventilation. This is sometimes replaced by an internal courtyard which serves the same role.
MK: One main opening and the permeable walls area used to