The main functions of walls are:
• exclusion of heat or cold, rain, wind, dust, noise, and other undesirable climatic and environmental elements;
• regulation of indoor climate (temperature, moisture, air movement);
• security against human and animal intrusion;
• support of ceiling and roof structure (though not the case in frame constructions with infill walls).
There are principally two ways of building a wall:
• massive or loadbearing wall construction;
• skeleton or frame construction with non-loadbearing walls.
Massive wall constructions usually comprise materials of high compressive strength (eg stone, earth, brick, concrete), by virtue of which they support their own weight and that of the ceiling or roof.
A skeleton structure consists of vertical, horizontal and angular members (eg timber, bamboo, reinforced concrete), which are joined together to form the loadbearing framework of the building. The space between them may remain open or can be filled in with in-situ wall construction materials (eg masonry wall, straw-clay) or prefabricated panels (eg timber and composite boards, concrete, ferrocement and brick panels). These help to strengthen the frame to prevent distortion. Well-braced frames can also carry a lightweight cladding (eg plywood and bamboo boards, fibre concrete, slate).
• In warm humid regions, diurnal and annual temperatures remain fairly constant, so that walls of low thermal capacity are required, together with large openings for cross-ventilation.
• In hot arid zones, in which diurnal and annual temperature variations are large, it is desirable for walls to absorb heat during the 9 - 12 hours of solar radiation and then to emit the heat to the interior until the cold pre-dawn hours, thus maintaining thermal comfort inside the building at all times (time-lag design theory). Small openings, located at higher levels should permit hot air to escape, and exclude solar radiation and glare.
• In all warm climates, the long axes of buildings should be orientated in east-west direction, with openings in the walls facing east and west being avoided or kept small, as it is difficult to shade them from the low morning and evening sun. Openings in walls facing south and north are easy to shade from the high noon sun by means of wide roof overhangs.
• While the east-west orientation of buildings is important, in warm humid regions priority must be given to orientation for air movement; in hot arid zones, importance must be given to exclusion of hot air, sand and dust.
• The absorption of solar heat can be greatly reduced by reflective wall surfaces. The ground adjacent to the building should be shaded or have some vegetation to avoid reflection onto walls, but heat emission at night should not be hindered.
• Solid walls with high thermal capacities are common in hot arid climates, as they transfer the absorbed heat to the interior with a time lag, thus restraining the heat when external temperatures are high, and releasing it when temperatures are low.
• Typical solid walls are made of stone, earth, burnt clay bricks and concrete.
• Insulation on the outside of a solid wall gives a four times greater time lag than if it were placed on the inside, but it also hinders heat dissipation during the night.
• Double walled construction has many advantages, both in warm humid and hot arid regions:
• the outer layer protects the inner layer from direct solar radiation, which first heats up the outer layer. With a reflective outer surface, this heat absorption is greatly reduced;
• only a part of the heat that passes through the outer layers reaches the inner layer by radiation or convection, and if provided with a reflective surface, it will not absorb all the heat;
• if the cavity is not ventilated (as in hollow or perforated bricks), it will act as an insulator, which can be advantageous, but can also hinder the passage of heat from the inside to the outer skin;
• openings at the top and bottom of the cavities allow the hot air, which will have accumulated within, to escape at the top, while fresh air is drawn in at the lower side (however, this ventilation of the air space does not affect the radiation from the outer to the inner layer); curing the day, when the fresh air is also hot, air circulation will have no cooling effect, so that it would be ideal (but not practical) to be able to close the openings during the day and open them at night;
• sound transmission is reduced by the air space.
• In warm humid climates, double walled constructions have the additional advantage of protecting the inner layer from rain and moisture penetration. Any moisture that passes through the outer layer is removed be ventilation, and condensation water can trickle down and out through the opening below.
• The materials used for cavity walls can be of various types, depending on several factors, such as temperature range, intensity and duration of solar radiation, humidity, rainfall, building usage, nature of immediate surroundings.
• In warm humid conditions, the inner skin should not be impermeable, as moisture movement is required, while the outer skin (usually thin panels or tiles on lathing) can be either impermeable or not, but care must be taken to avoid moisture bridging from the outer to the inner skin.
• In hot arid environments, materials of lower thermal capacity can be used in cavity walls, for instance, if the outer skin has good reflectivity and thermal insulation. However, the inner layer is generally a soil, brick or concrete construction, but of less thickness than for solid walls, as the heat accumulation over a 9 to 12 hour time-lag period is greatly reduced. The outer skin is typically of thin brickwork, concrete elements or a cladding of flat or corrugated sheets or tiles (eg metal, clay tiles, slate, fibre concrete).
• A disadvantage of cavity walls is that insects and vermin may nest in them. To avoid this problem, the interior surfaces of the cavity should be smooth and hard, and occasional washing will remove any accumulated dirt or insects.
• These are usually thin panels, matting, sheets or tiles of low thermal capacity, fixed to a framework. In some rare cases they can be thermally insulating.
• Such walls are only of use in warm humid regions, where heat storage is not needed. The main functions of lightweight walls are to provide shade and privacy, as well as protection from wind, rain and intruders.
• Sufficient openings facing the main wind direction are required to facilitate cross-ventilation for the improvement of indoor comfort.
• Lightweight walls are advantageous in earthquake zones, because their failure cannot cause as much devastation as heavy walls. However, in hurricane zones, lightweight walls can be susceptible to serious damage under strong wind pressure, hence strong connections, and avoidance of small elements and projecting parts are essential requirements.
• Depending on the type of material and construction system, wall surfaces can be left untreated or be treated to increase their durability by protecting them against rain, abrasion and vermin, to improve the thermal and moisture performance of the wall, or to improve its appearance by covering unsightly surfaces, or applying decorative effects and colours.
• Cement or lime mortars and a variety of stabilized mud plasters are the most common types of surface treatment on concrete, brick and earth structures, whereby special knowledge and experience is required in using the correct rendering for each type of wall material.
• Other types of surface treatment are lime and cement washes, varnishes (on timber) and several types of paints (principally oil-based or emulsion paints). Wall paper is less common in tropical regions, but decorative woven fabric and mats are fairly widespread.
Common Wall Construction Materials
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