Post 588 –by Gautam Shah
Micro ventilation is a very important passive method of adjusting heat and moisture for Hot-arid and Hot-humid climates. It is based on three factors: Air movements due to the differing pressures outside the building, like windward and wind-off sides. Buoyancy forces that results from temperature across the buildings interior and exteriors, and Pattern of circumstantial and designed apertures in the building shell. Micro ventilation relies more on external or macro conditions of the locality, but needs appropriate interior design.
Micro ventilation ensures that AIR enters or leaves a space through cracks, crevices, gaps or apertures, diluting the interior contaminants and adjusting the temperature and moisture. It is one of the easiest and consistent ways of managing comfort in enclosed and semi-open spaces. Micro ventilation systems are of vernacular design and time-tested solutions that have come down from one generation to another.
Entry and exit of air occur due to Differential Pressures along a point to point paths of air movement. The operative pressure is governed by both, the size and shape of the buildings layout scheme and individual characteristics of the building such as shape, size, height etc. Air movement occurs as a Buoyant Force due to the temperature difference across buildings’ interior and exteriors in general, and across the openings in particular. The air pressure is also affected by temperature of surfaces and surroundings, near the windward and wind-off sides. In a building micro openings are, circumstantial and designed. The micro openings are very much smaller in size in comparison to buildings’ formal openings like doors, windows, gaps etc. Micro openings yet have a characteristic size, shape, passage section, adjunct elements on internal and external faces, and their closeness to the location of need (for ventilation). The circulation or movement of air is affected, by the space profile (section), the task intensive volume and its datum, levels of ventilation (import-export) nodes, the hindrance by elements such as size and shape of external overhangs, the sill depth and its profile shape.
Micro ventilation operates as outside air has lesser moisture (except during actual raining conditions) then indoor air, so any level of ventilation, dilutes the interior humidity level and adds to the comfort.
Micro openings are circumstantial and designed, as casual parts of the buildings’ structure, components and systems (including openings), or formal or passive elements. But broadly these can be classified by their locations. Movement of air also encourages evaporation and increases cooling in the interior space. In dry arid climates smaller and deep-set openings create a strongly directional air movement allowing dwellers to locate their activities suitably. Deep-set openings also increase absorption of heat in the mass of the built-form or add simple cooling devices such as water wetted grass mats or fountains.
● ROOF RELATED GAPS are such as in the thatched and country tile roofs, loosely laid roofs of slates, stone sheets. Formal devices include vents such as lattices, chutes, hoppers, etc.
● GAPS IN UPPER SECTION OF WALLS are such as the unpacked ends of corrugated roofing sheets or terracotta tiles, ends of purlins and trusses when not sealed, and eyelets or oculi like holes in gables.
● OTHER GAPS are circumstantial that exist in and around openings such as doors and windows, as loose joinery, leaky fitments, ajar shutters, door bottom space, peepholes, openings without shutters (gaps), latticed constructions such as of woven mats, fabrics, or louvered openings, crack or fissures in building elements, expansion joints, unsealed joints, etc.
Micro ventilation requirements inside a building vary depending on the climate, season of the year, use of the space, tasks, work-schedules, crowding in the space and presence of heat evolving means (hearths, machines, etc.) The ventilation requirements also depend on the amenities used for conducting the tasks.
In many old buildings the micro ventilation system is almost sufficient for dilution of fouled air. Micro ventilation of the building is further exploited by life style settings attuned to the air flow movement paths, pressure gradients and qualitative variations at different locations and time schedules. Task activities are continually shifted around to match the seasonal and diurnal variations.