Post 280 – by Gautam Shah
Windows served two main functions for interior spaces: Ventilation and Illumination. To this was added the view out with the advent of glass. The window became part of shop front and it served the purpose of view in.
For several centuries a window was a minor entity for Illumination of domestic interiors. The door provided enough day time illumination. Domestic finer activities, such as the needle craft were conducted just inside or outside the threshold of the door. In deeper spaces, such as inner rooms, roof holes provided basic illuminance. In early public buildings, illumination was provided through smaller openings covered with parchment or alabaster. The areas of window opening though small, was distributed over a larger surface made available through increased interior heights.
In a tropical house admission of light is usually accompanied by heat gain, but the breeze coming through a door balances the interior environment. In tropical climates interiors tend to be darker to reduce the heat gain compared to colder climates where greater illumination is perceived as warmth. Naturally illuminated lit spaces are perceived to be healthier.
Ventilation in extreme climates such as very warm and cold, occurs through the temperature gradient between outside and inside. In hot and humid climates, the temperature gradient is not acute enough to cause natural air movement of a sufficient quantum. The need for large volume air movement is significant for moisture control in hot humid areas.
Ventilation requirements of an interior space vary depending on the number of occupants, nature of indoor cooking activities, fuels used for indoor heating and cooking, duality of distinct entry and exit points, the structure of the dwelling and scope for micro passive ventilation. To a smaller extent the ventilation needs are governed by the siting of the dwelling, such as the densely populated urban colony. Ventilation also depends on the nature of opening (cracks, crevices, holes), size of opening, number, distribution, location, orientation, and external climatic conditions (snow, rain, windy).
Cracks and gaps being unintentional are usually insufficient for heavier needs of ventilation and cooling or heating of spaces such as for toilets, kitchens, production areas and public spaces. Planned openings like windows on external face provide for such needs at the location, elevation, depth and in required quantity (such as a rate of air change -dilution, and the rate of air flow). The effectiveness of windows in achieving desired ventilation depends also on which windows are opened, how far they open, and the nature of shutter fixing.
Ventilation requirements for a dwelling are regulated by the cooking activity. In hot arid climates cooking is done outside the house, in an attached facility or semi open lean-to shades. Kitchen areas are sited in isolated spaces or corners. Cooking with a centric hearth occurred where it also contributed heat for warming. Moisture dilution is an important factor of ventilation requirements. In hot humid climates water utilities like storage and usage (bathing and washing) are placed in the Chowk like interior courtyards, outside or away from the dwelling. According to cannons of Building design, the Vastu Shastra, place of water is in the North-East side. This orientation provides for exposure to south-west face, the warmest or Sun side in the Northern hemisphere.
Ventilation is required to dilute the odours, moisture, carbon dioxide, airborne pollutants such as dust, smoke, volatile organic compounds (VOCs), latent heat from air, objects, etc. and encourage evaporation of body moisture and thereby cause cooling.
Windows provide ventilation, more effectively in rooms with internal doors (that is a door not opening to an exterior face), and especially when the exterior face door is closed for security reasons, such as at night. Movement of air between indoor spaces, and not the outside, is called transfer-air. Transfer-air has very little role in diluting the polluted air.
Windows placed on opposite sides and on same axis are better ventilating devices. The position of window vis a vis the work plane or task is determined whether 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.
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.
Most building codes suggest minimum opening area (including doors, windows, etc.) @4 to 5% of the floor area. But actual ventilation requirements are higher such as during rainy days, moisture content is very high, or when during celebrations and social events lot of people gather in a room. Nominally openings (including doors) @20% of the floor area, are sufficient for the purpose of ventilation, provided some sections of the openings are located within the human height (1.75 mts). Even in unoccupied rooms some ventilation is required to remove fumes and moisture generated by materials, plants and condensation. Minimum volumetric requirements for ventilation are 23 to 25 CMt per person per hour, and 12 to 16 CMt per Kg of fuel burnt. Large sized openings create turbulent air movements, whereas cracks and crevices create a viscous or laminar flow.
Commonly ventilation is measured in terms of entire interior volume of air gets replaced per hour, it is called air changes per hour, ACH, but requirements for air for well being per person are also specified. Minimum 0.35 ACH, but the supplied air must be no less than 15 cfm/person or 7.5 l/s/person. Since 2003, the standards for ventilation have been changed on floor area basis which is from 3 CFM/100 sq. ft. or 15 l/s/100 sq. m. to the 7.5 CFM/person or 3.5 L/s/person. To find the total amount of outside air required, one needs to add 3 cfm/100 sq. ft. or 15 l/s/100 sq. m. to the 7.5 cfm/person or 3.5 l/s/person. Thus, the air change rate requirement will vary by the size of the house and the occupancy.
Ventilation is required for a fire emergency from areas like corridors, stairs, etc. Openings for ventilation are necessary for all climate conditions, but control requirements are very acute in warm and extremely cold climates, due to outward leakage of internal air.
For adequate ventilation the building must take full advantage of prevailing breezes on the site. This includes consideration of: seasonal and diurnal wind patterns, land contours and other topographical features, shape and form of the building, height of the openings, axial position of the openings, work or task plane, physical state and age of the occupants, etc. Other important conditions are position of the window, the form of the surrounds and projections and design of the window shutter.