VENTILATION in TROPICAL BUILDINGS
Post 370 ⇒ by Gautam Shah →
Ventilation requirements and modalities for Buildings in tropical climates are very different from buildings of cooler climates. For buildings’ indoor climate four important issues are heat, moisture, air movement and contaminants. Of these four, the Heat needs to be removed, Moisture conserved or scrubbed off the air depending on dry or a humid tropical zone, Air movement encouraged, as it positively impacts the rest of the three issues, and Contaminants including CO2, CO, odours, and airborne particulates diluted.
The removal of heat from an interior space is a contradictory concept from heat conservation, being practised in buildings in colder climates. Moisture management in a tropical climate is mainly with the concept of comfort, rather than environmental damaging factor. Effective air movement take care of heat from body, evaporative cooling of perspiration and free moisture, and removal of odours and other contaminants from space pockets.
Ventilation in a building in a tropical climate zone occurs in many different ways. Openings like doors and windows remain open for substantial part of day-night period (except at nights and in few winter months), providing air-change. When the openings are shut, the cracks, crevices and structural micro gaps (such as thatched roofs, terracotta tiles, etc.), provide ventilation needs. Cracks and gaps, are usually insufficient for heavier needs of ventilation for toilets, kitchens, production areas and public spaces. Cooking, bathing, washing, require intensive ventilation, but in many tropical dwellings these occur in outdoor areas or distinctly adjunct spaces. Where climates are benign, intensive outdoor home or public life trims the indoor ventilation requirements.
Historically Government offices and administrative posts in India and Asian countries have relied on two architectural provisions for climatic comfort: High volume (tall spaces) rooms and Verandahs for conducting their businesses. Concept of cross openings (opposite ends) in buildings has been a natural practice. In Iran, stack-effect through towers and water bodies, both have helped ventilation (air change) and moisture addition. Spaces like Livan has been a feature of Mideast dwellings.
In western countries, such as Europe and US, ventilation requirements are specified in terms of air changes per hour, ACH where entire interior volume of air must get replaced per hour. But this is presuming a room to an enclosed (sealed) space in colder environments. This concept is universally applicable (in all types of climates), to an extent, for air-conditioned urban buildings.
Outdoor air at any location is relatively less polluted because it has greater scope of dilution compared to any restricted interior air mass. However, in dense urban localities and at locations near known pollution activities such as power plants, industry, open mines, etc. outdoor air can be a source of pollution. High level of humidity in certain coastal locations, and during very heavy rainy seasons can be considered a contaminant of air. Where such air is used to replace the interior air, some control or processing by passive or active filtration devices, integrated within the opening system are required.
NATURAL VENTILATION: Natural ventilation advantageously uses wind movement speeds, pressure-driven flows, and temperature differences (stack effect) to supply and remove air through enclosed space. Windows placed on opposite sides and on same axis are better ventilating device. The position of window vis a vis the work plane or task is determined weather one wants a draught-breeze over the body and the task or avoid it. The nature of shutter opening also determines the direction of the internal air movement. Air moves because of differences in temperature and pressure across the supply and exhaust points. In single storey houses, the movement of air due to pressure differences is main factor then the differences in temperature across the supply and exhaust points. A high-pressure area occurs air strikes a building and low-pressure areas are created as the air moves over and around the building. To encourage movement of air within a room, the openings, through which the air leave should be larger, than those through which it enters. The direction and strength of air movement are altered due to draperies, furniture items, people, local heat sources (heating, cooking, machines) and air movement devices (fans).
TRANSFER VENTILATION: Movement of air between indoor spaces, and not the outside, is called transfer-air. It has, though limited a role in ventilation, but is a great tool for thermal comfort. This concept has been in use in traditional tropical homes and settlements. Interior space modules are mutually connected through openings, exposed to space ingresses such as cutouts, Chowks or courtyards or are exposed to an exterior face. At a settlement level several buildings share community spaces for transfer ventilation. The openings nearly remain open or are latticed. The buildings are closely spaced, so few sides are exposed to sun.
Transfer air ventilation for interior spaces work best when spaces are used at different times of the day, for different tasks and by varied occupation (number of space users). Transfer air ventilation combines horizontal flows and vertical stack effects.
VENTILATION and OPENING DESIGN: Windows with shutters opening outward often obstruct the wind path, but double hung sash windows and sliding shutters which open within the frame are better as receptors. Casement window shutters with offset hinges or friction stay which create a small gap on the jamb side help in catching the breeze. Hoppers, awning and jalousie windows direct the breeze due to the angle of opening. The depth of a window and its surround also affects the nature of ventilation. Splayed sides create funnel effect to catch the breeze.