HUMIDITY MANAGEMENT in BUILDINGS

Post 556  by Gautam Shah

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There are few obvious factors that affect the humidity in buildings. The most important ones are, location of the building and season of the year in consideration. Other factors include, ventilation system, heat input in the interior spaces, location of humidity generating sections within a built space, inter-space air exchanges, and the nature-duration-zoning of activities.

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wikipedia image by NepGrower-commonswiki

The humidity management in buildings has two extremes of high and low humid conditions of external environment. The external humidity affects the interior climate of the building by way of air exchange. The air exchange, adds to the humidity content and also dilutes the interior humidity. Adding humidity to space is comparatively easier than subtracting or scrubbing the excess moisture. Low moisture conditions can occur in hot arid areas and occasionally in cold climate zones. High humid conditions are associated with rains, sea coast areas, forested and tropical locations. Within a built space, toilets, kitchens and washing areas are prolific generators of humidity. Internal humidity is high in areas with high occupation such as meeting rooms, gymnasiums, cinema halls, industrial process areas such as quenching, etc. and in spaces with insufficient air-change or ventilation. External humidity is highest during a rain-shower and a little while after that. The same condition begins to occur, soon after, in interior spaces. But the external environment has greater scope of natural dilution of humidity compared to internal spaces.

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Smaller openings in hot arid zone for viscous air flow at Gurna mosque Luxor Egypt by Hasan Fathy

Level of humidity is the amount of vapour held by air at a particular temperature. If there is a rise in temperature, the air expands, and as a result more vapour can be accommodated in the same volume of air. Inversely when temperature drops, the air density increases, and its capacity to hold the vapour decreases. Many people cannot sense the fluctuations of relative humidity in the range of 25% to 60%, except through the side effects on the body over a longer period.

Human body cannot cope up with rapid rates of moisture removal, as it has limited amounts of water available within it. Low level of humidity can remove even the moisture that helps skin to remain soft and supple. In cold arid climates the body has, neither excessive amounts of heat nor moisture and so even minute perspiration will evaporate readily. In contrast to this in hot and humid climates the perspiration does not evaporate readily, causing a heat buildup within the body. This is coped up in several manners such as shifting out to low humidity zones, being in the dominant air movement sections, reducing the metabolic activity of the body by resting and quality of food and adopting appropriate clothing. Air with high percentage of humidity is comparatively deficient in oxygen and may cause problems to people with TB or asthma.

Low and High levels of humidity, directly affect health and buildings, and ‘sick buildings’ further affect the well being. For both humidity level extremities, the common solution lies in air exchange management between sections within the building and with the outside. Condensation and its consequence effects can be prevented by improving thermal resistance of buildings shell. Humidity generation can be managed by adding or isolating high humidity sections of the dwelling or processing plants, by including or avoiding water bodies in the surroundings.

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Desert cooler or evaporative cooler Wikipedia image by Billy Hathorn

Low relative humidity can lead to discomfort, shrinkage of building components mainly of wood and static electricity discharges. Dry conditions discourage growth of mould, bacteria, and viruses, but dehydrate the protective mucus linings of the respiratory system.  In hot arid climates, low level of humidity and movement of air cause rapid evaporation. The rate of evaporation governs the perspiration and sweating, the prime mechanisms to dissipate the body heat.

High level of humidity does not allow adequate evaporation of the perspiration, resulting in the rise of body temperatures, and one has to resort to other methods of heat dissipation. High level humidity allows condensation of water over colder surfaces. Inner faces of exterior walls and window glasses are vulnerable to condensation. Such surfaces, if porous or textured, allow mould growth, encourage corrosion dust mite infestation and mildew in fabrics and furnishings.

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A laminar flow through low level openings for Hot humid climate Wikipedia image by Arayilpdas

Air movements within and around a building shell are very effective for humidity management, in all types of climates. In colder climates inter-room air exchange helps in dilution of humidity without major change in a heat profile of the body. Conversely direct ventilation of toilets, kitchens and such other zones removes humidity from acute areas. In warmer climates high speed air movement -the ‘winds’, help in heat and humidity exchange. Winds have turbulent or laminar air flow properties. A laminar air is more comfortable than turbulent air. The later one creates greater displacement of air mass, but the former one achieves a better mix of air. This is the reason why in hot arid climates small size opening is used to create turbulence or a viscous flow, and in hot humid climates a laminar flow is generated through body level openings to displace the humidity.

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