Introduction

The primary functions of all buildings is to adapt to the prevailing climate and provide an internal/external environment that is comfortable and conducive to the occupants, machines and materials.

Building environments need to contain certain traditional styles, materials and characteristics of the surrounding culture.
E.g. Buddhist turquoise internal wall wash, Shingsak decorative sunshade.

Building materials, structure and services(where necessary) need to be appropriately selected and designed.

Climate

• Macro climate vs. Site climate

Climatic data comes in the form of macro meteorological information, without fortune of physically measurement specific site climatic data will, more than likely, be unobtainable. However, attention should be given to vegetation, topography and other outstanding features which may dominate the local site.

"the designer's task..... to take advantage of the favourable and mitigate the adverse characteristics of the site and its climatic feature"

Manual of tropical housing - Koenigsberger, Ingersoll, Mayhew, Szokolay.

• Climate data requirements

The following basic information is required for the formulation of the building type, orientation and form given a desired function. It is also pertinent to the assessment of thermal comfort of the occupants.

1. Longitude and latitude
2. Air temperature
3. Humidity
4. Precipitation
5. Sky conditions
6. Air movement
7. Solar radiation

Basic climatic data which you need to obtain from CIBSE guide A, include Tables A2.22, 2.35 and Sunpath diagrams.

These tables and diagrams help to determine the position of the sun and optimise the built form, orientation and exposure of elements (windows, roof and walls) for maximum or minimum solar gain. Shading devices are designed from a knowledge of the suns azimuth and altitude along with the wall-solar azimuth.

Sunpath diagrams used in conjunction with solar irradiance data tables will provide the information needed to determine internal environmental temperature and the likely thermal comfort of the occupants.

CIBSE guide 'A' gives tables of solar azimuth and altitude for numerous latitudes, the same information in sunpath diagram for is provided in "Manual of tropical housing" - Koenigsberger, Ingersoll, Mayhew, Szokolay and 'Windows and Environment Overlays and Charts'

• Solar Irradiance

Hourly average solar intensity must be known for detailed design.

Example irradiance table from CIBSE Guide 'A' table A2.35.

Example - To estimate the maximum solar irradiance on a surface

Given that information given in tables is based on experimental and steady state conditions, weather factors attempt to account for cloudiness and sky clarity.

KD = direct radiation factor

kd = diffuse radiation factor

Humid climates: KD = 0.5, kd = 1.1

Arid climates: KD = 1.1 kd = 0.9

(1). Maximum solar irradiance on a horizontal south facing surface, latitude 0oN, sea level, at noon in June, cloudy sky.

Max Irradiance ITHd = (Direct irradiance IDH x kd) + (diffuse irradiance IdH x kD)

Max irradiance = (830 x 0.9) + (335 x 1.1)

ITHd = 1115.5 Watts/m^2

(2). Maximum solar irradiance on a vertical east facing surface, latitude 0oN, sea level, at 10am in June, clear sky.

Max Irradiance ITHd = (Direct irradiance IDH x kd) + (diffuse irradiance IdH x kD)

Max irradiance = (400 x 0.9) + (95 x 1.1)

ITHd = 464.5 Watts/m^2

• Heat Gain

In addition to solar heat gain through glazing (Qs=AI), heat gain by occupants and equipment, solar thermal conduction through opaque elements in hot climates is particularly importance. As with conductance heat loss calculations (Qc=AUT), solar heat gain due to conduction can be assessed by a knowledge of the inside and outside temperatures as well as the materials of construction. The Sol-air temperature is a method of determining the heating effect of the outside surface due to solar irradiance and is substituted for the outside temperature in calculations for opaque surfaces i.e T = Ts - Ti. The Sol-air temperature is calculated from:

Sol-air temperature Ts = To + ITHd x a/fo

Where:

To = outside temp

ITHd = solar irradiance (established above)

a = surface absorbance (see below)

fo =outside surface conductance (see below)

So, thermal conductance of solar energy through walls: Qc = AU(Ts-Ti)

A construction with a low U-value (air-to-air transmittance) will reduce all forms of conduction heat transfer through the building envelope. Conduction heat flow would be large if the temperature difference were large.

With small temperature differences between the inside and outside, the heat flow would be small anyway; an improvement in thermal insulation would not bring any significant reduction.

Remember, in a heat gain situation, with strong solar radiation, it is the sol-air temperature value which must be used to find the temperature difference, thus even if the air temperature difference is small, the actual temperature difference acting as a motive force for heat flow may be large, consequently insulation may be important i.e. low U value.

Insulation will be most effective under steady state conditions, or if at least the direction of the heat flow is constant for long periods of time- especially for heated or air conditioned buildings. Where the direction of heat flow is twice reversed in every 24-hour cycle, the significance of insulation will be diminished i.e. Hot arid climates.

Under these conditions (large diurnal temperature variations) the significance of thermal capacity will be much greater than of insulation. Some authors refer to the effect of thermal Capacity as Capacitive insulation, as opposed to resistive insulation provided by low conductivity materials and low transmittance constructions.

Periodic heat flow is the oscillation of temperature with time of day. The concepts of time-lag and decrement factor must be considered in selection of materials and construction of walls, roofs and floors. The time lag is the difference between the time of peak outside temperature and the time of the resulting indoor temperature.

Here we can set the question: 'However much thermal capacity, what length of time-lag, is desirable? A point often over-looked is that the thermal capacity can be too much, the time-lag can be too long. For example, a wall facing east receives its maximum heating at 10.00 hours. A time-lag of 10 hours would put the inside surface temperature maximum at 20.00 hours, when it is likely to be too hot anyway and the occupants may want to sleep but cannot.

The time at which the indoor temperature will be the warmest becomes apparent when a graph of the Sol-air temperature is draw for each wall.

Time lags for typical structures:

Cavity wall with two 100mm dense blocks and render = 10 hours

The choice of the surface colour is not of particular importance, the absorbance and surface conductance are. Surface conductance does not change by a large amount for different materials, however selection of a low absorbance material will reduce the solar heating effect.

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By far the greatest contribution to solar gain is from openings and particularly windows (greenhouse effect). Short-wave radiation entering heats the structure, is re-emitted as long wave radiation and is trapped inside the building.

There are four methods available to the designer for the reduction of solar heat gain through windows:

1. orientation and size.
2. internal blinds/curtains.
3. special glass.
4. external shading (mentioned above).

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Further reading on Structural Control:

1. CIBSE Guide 'A'.
2. "Manual of tropical housing" - Koenigsberger, Ingersoll, Mayhew, Szokolay and 'Windows and Environment Overlays and Charts'
3. 'Useful Solar Gain Through a South-Facing Window in the UK Climate' - MG Davies, University of Liverpool.

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Thermal Comfort

The all encompassing term, 'comfort', is very subjective and is a statement of not only the physical but also psychological satisfaction. The general acceptance of a set of measurable environmental conditions by the majority of the building occupants should be the objective of the designer. This is seen to be a very physical objective, there are however deeply psychological effects at work in the body's sensing of the thermal environment. For the majority, the happy/comfortable occupant will express positive attitudes of well-being, emotions and behaviour.

If the heat gain and heat loss factors are:

Gain: Met = metabolism (basal and muscular)

Cnd = conduction (contact with warm bodies)

Cnv = convection (if the air is warmer than the skin)

Rad = radiation (from the sun, the sky and hot bodies

Loss: Cnd = conduction (contact with cold bodies)

Cnv = convection (if the air is cooler than the skin)

Rad = radiation (to night sky and cold surfaces)

Evp = evaporation (of moisture and sweat)

then thermal balance exists when: Met - Evp Cnd Cnv Rad = O

Likewise in our examination of the relevant climatic data, the essential information for comfort is temperature (dry and wet bulb), humidity, radiation and air movement. The objective of the designer is to attain comfort by balancing the environmental effects of climate on the heat exchange of the body.

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• Example climates and balance:

Six categories of tropical climates:

1. Warm-humid - 15oN and South of the equator, e.g. Lagos, Mombasa, Colombo, Jakata etc.
2. Warm-humid island - equatorial and trade wind zones, e.g. Carribean, Philippines and Pacific Islands etc.
3. Hot-dry desert - 15o to 30o North and South, e.g. Baghdad, Alice Springs, Phoenix etc.
4. Hot-dry maritime desert - latitudes as (3), coastal large land mass, Kuwait, Karachi etc.
5. Composite Monsoon - Tropic Cancer/Capricorn, Lahore, Mandalay, New Delhi etc.
6. Tropical uplands - Tropic Cancer/Capricorn, 900 to 1200 metres above sea level (plateaux and mountains), Addis Ababa, Mexico City, Nairobi etc.

Threr Climates and their Balance with Comfort

Calm, Warm air moderate humidity In a temperate climate, indoors, when the air temperature is around 18°C, when the air is hot is calm, air i.e. air velocity does not exceed 0-25 m/s, and humidity is between 40 and 60%, a person engaged in sedentary work will dissipate the surplus heat without any difficulty, in the following ways:
by radiation 45%
by convection 30%
by evaporation 25%

if the temperature of bounding surfaces is approximately the same as the air temperature.

Hot air and high radiation As the air temperature, approaches skin temperature (31 to 34 oC), convective heat loss gradually decreases. Vasomotor regulation will increase the skin temperature to the higher limit (34oC), but when the air temperature reaches this point, there will be no more convective heat loss.

As long as the average temperature of opposing surfaces remains below skin temperature, there will be some radiation heat loss.

Hot air, radiation, humidity and appreciable air movement When the air is hot (equal to or above skin temperature) so that the convection element is positive, when surface temperatures are warm or there is a substantial radiant heat source, so that the radiant element is also positive, and when the air is humid (but less than 100% RH) the movement of air will accelerate evaporation thus increase heat dissipation: even if Its temperature is higher than that of the skin. This has limited value, the layer of air in immediate contact with the skin soon become saturated which prevents any further evaporation from the skin. Faster moving air will remove this saturated air envelope and the evaporation process can continue, discomfort may occur if air velocities exceed around 1.5m/s.

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There are numerous scales of comfort perception assessment:

1. Effective Temperature
2. Corrected Effective Temperature
3. Equivalent Warmth
4. Operative temperature
5. Equatorial comfort index
6. Resultant temperature
7. Predicted four hour sweat rate
8. Heat stress index
9. The bioclimatic chart
10. Index of thermal stress
11. Predicted Mean Vote
12. Tropical Stress Index

.................and more.

An assessment of comfort conditions in a variety of tropical climates is shown below, measurements were taken and a personal comfort perception made. The interrelationship of temperature, humidity and air movement is clearly shown. Methods (11) and (12) were used to see how they correlate:

Note: Method 11 - Fangers Predicted Mean Vote (PMV) report is available on request.

Method 12 - Tropical Stress Index (TS I) Results Table

BUILDING	tw (C)	te(C)	V(m/s)	tsi(C)
1) CBRI (Roorkee)	28	31	1	30.5
2) Yogi Lodge (Varanasi)	28.5	30	1	30
3) Taj Mahal (Agra)	30	31	0.5	32
4) House Boat (Srinagar)	23	28.5	0.1	28.5
5) Palace View (Leh)	17	27	0.5	24.5
5a) above at night	12	20	0	19
6) Ecology Centre (Leh)	10	20	0.5	17
6a) above at night	12	22	0	20.5
7) Amarpurkashi (Uttar Pradesh)	30	35	0.1	35.6

The PMV scale was developed by europeans a rating of zero is neutral (Comfort), +3 = Hot and -3 = Cold.

The TSI method is widely used in Asia and the tropics, comfort TSI = 25oC to 30oC, Cool TSI = 190C to 25oC and Warm TSI = 30oC to 34oC.

Clearly there is a difference in acceptable comfort requirements.

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