4.1.2 Structural considerations in environmental modification
Temperature and circulation can be modified directly through building structures. Relative humidity can be modified only indirectly through the effective use of natural ventilation or through technological control which will be discussed later.
East and west walls, which receive the brunt of the morning and afternoon sun should be protected and insulated so that the sun's heat is not transmitted to the interior of the building. The roof, which has a high level of exposure to the mid-day sun, should reflect heat and there should be an attic or ventilation space directly below the roof to provide insulation for the interior of the building.
Double wall construction is an excellent way of insulating buildings in the tropics. Air is an effective insulator, and prevents heat from passing through the outer wall to the inside of the building. In many places in the tropics, the hollow cement block is a staple of the building trade. It provides an economical, though not always aesthetic, building material, and provides adequate insulation for the interior of the building. A true double wall construction is more effective, but considerably more costly. This construction is also used effectively in temperate climates where extremes of cold and heat make long term operating costs for environmental control a primary consideration.
The brise soleil is a variation on the double wall. It may be designed as part of the building, or attached to the facade of existing buildings. Though not as effective as a double wall, it provides protection by absorbing the sun's primary radiation. It also reduces the light levels inside by shading the windows, and allows windows to remain open even in the rainy season. It may cover the entire wall, part of the wall, or in some cases, only windows though this latter construction is considerably less effective in reducing the transmission of solar radiation.
Shading of exposed walls can take a number of other forms, including exterior landscaping with trees and shrubs, the extension of over ranging roofs, and the installation of exterior canopies. The use of interior blinds, curtains or louvres can also reduce the transmission of heat through window glass. Kukreja5 provides a table evaluating the effectiveness of various shading devices based on their reduction of total heat gain, efficiency in ensuring cross ventilation, and the percentage of natural light resulting from that particular form of control.
Glass both transmits and intensifies heat. Large windows in tropical climates can significantly increase interior temperatures, yet are incorporated into many buildings for aesthetic reasons. Ultra violet and heat absorbing films are effective in reducing heat and UV light without obscuring views or lowering the light levels too greatly.
High ceilings are a common feature of older buildings in the tropics and are an effective means of diffusing interior heat. As the warm air rises, it can be pulled out of the building with ceiling or attic fans or through windows placed directly beneath the roof overhang.
In general, buildings in tropical climates should be oriented in such a way as to take advantage of any prevailing winds and designed so that cross ventilation is possible in all areas of the building.
Even buildings designed to utilize natural ventilation will require back-up systems of mechanical ventilation for those times when prevailing winds fail or shift.
Placement of windows is a principle means of assuring adequate air circulation, once the orientation of the building has been determined. Kukreja6 provides excellent diagrams of interior air movement for various window placements. These are useful not only in the design and modification of buildings, but in anticipating problems and determining stack arrangements. He notes that a single window serves no purpose as far as interior ventilation is concerned, and that if modifications are to be made, the best results are obtained by placing windows on opposite walls to insure cross ventilation. Enlarging the outlet window opening results in a definate increase in interior air movement, even when the inlet window remains unchanged. Circulation is also increased significantly by increasing the height of narrow inlet and outlet openings. He also compares the circulation figures for various window areas in relation to floor area, demonstrating that air movement peaks when window openings are equal to 25% of the area's floor space.
Louvred windows can provide excellent ventilation, and are common in the tropics, but are difficult to seal against rain and insects, and their use is best combined with other window treatments. All open windows should be screened with well fitted, fine mesh, fiberglass screens. Placement of the screens on the inside of the windows will facilitate their removal for cleaning, particularly if there is a brise soleil on the outer wall.
If the existing building has high ceilings, the installation of ceiling fans is an excellent investment. Used in conjunction with standing floor fans, or window fans adequate air circulation can be maintained at a relatively modest cost even in Af climates where there is very little natural air movement.