List of fi gures
2.1.1 Heat flows
2.1.1.1 Heat gain minimisation
A key issue in the tropics is reduction of heat gain from the solar irradiation. This can be achieved by appropriate orientation and using existing and/or planted vegetation for shading roofs and walls.
Details of these solutions are presented in Part Three.
Furthermore, wind incidence upon the building should be taken into account. The tropics are quite different from moderate and cool climates in that winds actually improve microclimate for most of the time. Thus, the air movement should be one of planning and design objectives beginning with a choice of appropriate site – exposed to winds prevail- ing in a given location – and selective use of vegeta- tion and other landscape elements, such as earth contouring, for wind redirecting and channelling.
To some extent landscaping can even induce air movement.
As mentioned above, the fundamental climate control principle that applies to the tropics is to ex- clude heat gain from external sources, and this should be understood chiefly as solar gain minimisa- tion. Where solar radiation strikes the building enve- lope (roofs and walls) it will be partially reflected, and partially absorbed and subsequently transmitted into the structure. Utilising its thermal properties can control the amount of radiation transmitted through
the building fabric. However, the most effective way to minimise solar gain is by shading the surfaces, so that direct radiation does not reach them.
The quantity of energy falling on a horizontal surface due to solar radiation, sometimes called the
‘energetic exposure’, is a function of the irradiance, measured in watts per square metre. The global irra- diance G has three components: the direct compo- nent (beam irradiance Ib), the reflected component (reflected irradiance Ir) and the diffuse component (diffuse irradiance Id). The latter two can be consid- ered jointly as the indirect component (indirect irra- diance Ii):
G¼Ibþ ðIrþIdÞ ¼IbþIi ð2:3Þ The beam irradiance depends on the angle of inci- dence between the sun’s rays and the normal – a line at right angle to the surface. Tilting a surface towards the mean position of the sun during any given period of time increases the irradiance on that surface for that period. For instance, in Cairns, Australia, in October, when the sun is at zenith, total daily irradiation on a horizontal surface is 6952 Wh/m2, which is the maximum across the year, but can be further increased in December, when the Earth is in perihelion, to over 7100 Wh/m2 on a surface slightly inclined to the south at around 6.5. The opposite also holds true and all surfaces tilted at other angles receive less solar energy. It is important to consider both direction and timing factors jointly to minimise exposure to the sun through the shape, orientation and tilt of the building’s irradiated elements (Figure 2.11).
In the tropics, the greatest quantities of solar ra- diation are received by surfaces close to horizontal, followed by vertical surfaces facing west and east.
Greater solar heat gains at west than at east walls can be explained by higher temperatures and usually lower moisture content in the air in the afternoon as compared with mornings. Curved surfaces, such as vaults and domes, in effect appear to ‘spread’ solar radiation over a larger area of which only a small part is perpendicular to the direction of the radiation.
Some would argue that uneven surfaces, such as cor- rugated sheets or brick walls with alternate courses recessed, have an ability to ‘dilute’ effects of the solar irradiance. The amount of radiation received remains in fact the same, but the total effect can be dimin- ished through increased dissipation by convection, on top of self-shading and the deflecting characteris- tics of such surfaces. This is a debatable question as, for instance, such surfaces are able to support heated Figure 2.10 Cooling strategies in thermal environment
control.
air filling the recesses, thus contributing to increased heat flows (Figure 2.12).
Indirect irradiance is the sum of irradiance reflected and diffused by clouds, diffused by the at- mosphere (water and dust particles) but, foremost, reflected from the ground, neighbouring landscape elements and adjacent buildings. The only way to reduce incidence of indirect radiation is to minimise reflectance of the ground and building surfaces, espe- cially outside windows facing the sun. This can be achieved, for instance, with the help of low growing shrubs planted in front of the building’s walls.
Compact building forms are subject to less heat- ing when outdoor temperatures are high than ones that are spread out. This is because compact forms minimise roof and wall areas – limiting solar heat gains by their surfaces. In these terms, cube-like multi-storey buildings of medium height seem to have advantages over the single-storey ones. Presum- ably, compact buildings are advantageous in hot and dry climates, if they can be sealed for the day and the requirement of sufficient night-time ventilation can be met. In hot and humid climates, however, this strategy is not nearly as effective. It seems that mini- mising heat gains in the wet tropics should be achieved through appropriate shading and heat dis- sipation, for instance through ventilation, rather than through minimising the area of a building’s pe- rimeter, which also hinders heat dissipation. The roof is usually difficult to shade and instead it should be well insulated.
Any shading devices should be formed and placed so that they do not impede airflow through the building. For instance, vines growing on a trellis
should be avoided as a means of shading verandas and patios if they interfere with airflows – especially in and out of the building. It appears that trees and bushes can be very effective in shading east and west walls. Sunshading of entire east and west walls (as opposed to shading openings only) in the tropics seems to be a necessity. A somewhat more expensive alternative would be a good thermal insulation. At the same time, roof overhangs are the most practical and feasible means of shading north and south walls from direct radiation from the high midday sun. Al- though they are not very effective at shading east and west walls, at a depth of 1.2 m, in Cooktown (15280S) they provide complete shading of south walls 2.4 m high during summer months from around 8:00am to 4:30pm in December and from around 7:20am to 4:40pm in November and January. During the remaining 2–2.5 hours after sunrise and before sun- set, the walls are partially shaded or out of the sun (Figure 2.13).
The contribution of solar heat gain by windows of a typical timber framed building is the major heat load, even if the windows constitute on average less than 8 per cent of the total envelope surface area.
Openings, which are glazed with ordinary glass, are largely transparent to solar radiation. The solar ener- gy that passes through such openings is absorbed by internal surfaces, which in turn become heat radia- tors. Because the re-emitted heat is a long-wave radi- ation, it cannot pass back through the glass – it is
‘trapped’ indoors. This phenomenon is known as the ‘greenhouse effect’. To prevent rising mean radi- ant temperature (MRT) of the interior through this mechanism, it is necessary to carefully choose the Figure 2.11 Components of solar irradiation.
location of openings and/or shade the openings ex- posed to sunshine. Generally, it is recommended that there are no openings in western and eastern walls and that plants do not obstruct the free flow of air over the building envelope (including air movement parallel to the wall surfaces). When windows are pro- tected with shading devices, these are best placed on the outside of the glazed openings, so that they can lose heat absorbed from the sun to the ambient air rather than to the interior (Figure 2.14).
The effectiveness of shading is expressed by the shading coefficient: a ratio of the solar energy passing through a shaded opening to the energy that would pass through the opening if it were unprotected. Usu- ally, a simple window (3 mm float glass) is taken as a
reference. Any shading coefficient above 0.2 must be considered too high in the tropics.
In both movable and fixed categories, there are three types of shading devices:
* horizontal, for shading from overhead sun radia- tion;
* vertical, recommended for shading from radiation falling sideways;
* a combination of both horizontal and vertical (sometimes called ‘egg-crate’ shades).
The most popular are shades of the first type. They can take the shape of large surface devices, such as awnings, or be designed as divisible screens, for ex- ample louvres. Vertical shades can be incorporated into the building structure as ‘wing walls’ or be used as blade screens, similarly to horizontal shades. Shad- ing devices are often designed so that they can be operated to allow for seasonal or current desirable adjustments. In such cases, vertical louvres, when used at east or west sides of a building, have an ad- vantage over horizontal ones in that they need ad- justment less frequently.
Movable shades are better suited to conditions changing in a broad range. They can be most effective in providing the required shading although they can also pose problems of stability (and, as a conse- quence, safety during the cyclone season) and main- tenance. There are few examples of fixed shades that are efficient at controlling the direct component of solar radiation and which, at the same time, permit a view. In the design of fixed shades a design procedure can be used in which the desired (‘free’) geometric form is compared with calculated (manually or by a computer) vertical and horizontal shadow angles for a given design period. The redundant parts can be then removed, which may result in original and at- tractive shapes for the shades (Figure 2.15).
Shading can also be provided by vegetation and topography of the site. Neighbouring landforms, structures and vegetation can all be used for this pur- pose. In the tropics, where overheating is likely throughout most of the year, it is sensible to take advantage of land features and construct the building on a part of the block which is best shaded during the year. To do this, the sun path as well as orientation and tilt of the land must be considered together with exact location and type of vegetation used.
Design aids for sunshading both manual, such as shading protractors, and various computer programs, have been available for many years, however there is no evidence of their widespread use. Likewise, many architectural designs exhibit total disregard for sun- shading principles and a purely formal treatment of Figure 2.12 Self-shading of the wall.
Figure 2.13 Rule of thumb: an overhang’s size is effective in shading most of the wall area from high altitude sun.
this problem – so important in this climate (Figure 2.16). Sunshades should not be mere orna- ments but offer effective protection against solar ra- diation at all periods when required.
The use of parasol roofs or ventilated roof spaces (attics) is somewhat controversial. Some would argue that the thermal benefit of such a solution does not justify the likely higher cost. As a heat gain preven- tion strategy, ventilated attics can only prevent con- vective heat flow from the roof, which is a minor part of a total heat transfer at less than 10 per cent of the total heat flow. Nevertheless, the role of the roof act- ing as a parasol for the ceiling – especially if the un- derside has a low-emittance surface (such as
‘coolclad’), thus effectively shading a ceiling struc- ture from the impact of solar radiation – cannot be
discarded. A carefully designed parasol solution sup- ported with appropriate materials can ensure that such a roof/ceiling assembly will provide internal ceiling temperature at not-higher-than ambient air level. This, in turn, would mean that radiation from the ceiling would not need to be taken into consider- ation. It would then be sufficient to remove hot air from under the ceiling to appreciably lower the tem- perature of the top layers of air within the building’s volume.
The final aspect of the heat gain minimisation strategy is a consideration given to the material solu- tion. Direct solar radiation increases the temperature of sunlit surfaces. This accelerates the rate at which heat flows into the body of the material. The increase very much depends on the character of the sunlit Figure 2.15 Shading should be sought from both vegetation and landforms.
Figure 2.16 Ventilated attic.
surface. There are several material solutions to mini- mise solar gains occurring this way. Building materi- als used in construction of a particular element can either resist heat gain from solar radiation or re-emit it as soon as the sun moves into a position from which further irradiation does not take place. The former are basically materials insulating due to their surface qualities (absorptance/emittance), i.e. providing re- flective insulation (see below) of the surfaces exposed to solar radiation. In the latter materials heat storage effects can be utilised: the majority of lightweight materials have very little storing capacity and easily give up any heat stored.
In naturally ventilated tropical buildings, where air temperature differences between outside and in- side are low, the heat flow through the fabric is too small to consider thermal insulation as a means of reducing the heat flow. However, even in these con- ditions, insulating envelope elements can be worth- while. Such a need must be established using a different criterion. In a heat gain situation, with strong solar radiation, it is the sol–air temperature value that must be used to find the temperature dif- ference. In the sol–air temperature (SAT) concept, the SAT comprises the ambient air temperature value and a value which creates the same thermal effect as the incident radiation in question.
SAT¼Tþ ðGa=foÞ ð2:4Þ where T is air temperature; G is global irradiance;ais absorptance; fois (outside) surface conductance
Thermal insulation, as a material solution, can take the form of either reflective, resistive or capaci- tive insulation. The difference between them is that the first two resist heat flow instantly (that is, they insulate) while the third one operates on a time-lag principle, slowing the heat flow down. Thermal insu- lation is most effective under conditions of steady heat flow, i.e. when the direction of the flow is con- stant (occurs in one direction) for long periods of time. Thus, in the tropics, it has a bigger importance in the warmer half of the year.