THE ZAMBIA INSTITUTE OF ARCHITECTS: Function of walls in constructing houses
THERE are two recent developments in the construction of houses in Zambia that seem to have received wide acceptance by most developers.
Firstly most houses are now being constructed out of 150mm thick concrete block work external walls and secondly there has been a tendency to construct all walls up to wall plate level only
F with no walls being taken up to the underside of the roofing sheets.
The justification advanced by most builders and developers has been that this method of construction has resulted in some “cost savings”.
In construction building economy is one of the many other considerations taken into account in the design process and therefore cost alone ought not be the sole criteria for evaluating the architectural quality of a building.
This article therefore examines the implications of this trend in the construction industry.
In one of the articles under this column “ARCHITECTURE DERIVED FROM THE VILLAGE CONCEPT” it was shown the important role a family plays in national development and the importance of having acceptable homes from which to raise families.
This article focuses on the functions of walls in house construction in view of the above stated recent trends in order to help our readers have a clearer and broader appreciation of the implications of these trends.
A wall plays several functions in the performance of a house and these functions need to be fully understood in order to create suitable and comfortable homes for our families.
Walls are used as boundary markers to define the various functional spaces within a house plan and there are two types of walls in a house; one type is the internal walls which define the interior spaces while the external wall separates the interior spaces from the external surrounding space.
As space dividers, walls also act as acoustic barriers ensuring acoustic privacy between the various rooms which tends to be compromised when dividing walls are only built up to wall-plate level.
Sound or noise will be transmitted from one room to another mainly through airborne transmission as the effective separation between rooms in this kind of construction is only the thickness of the non-continuous ceiling board with the weakest entry points being the joints between the ceiling boards.
In such type of construction privacy will be compromised as discussion meant to be private and restricted will easily be audible in next adjoining rooms.
In order to slow down the rate of flame spread in case of fire within a building, architects apply various techniques including the creation of fire compartments which would contain the fire for some time before spreading to other parts of the building.
One method is to take the block work walls between rooms up to the underside of the roof; for instance and depending on the quality of the aggregates used a 100mm thick solid brick work wall may have a Fire Resistance Rating of up to one and half (1.5) hours while a 200mm thick hollow concrete block work wall may have a Fire Resistance Rating of up to four (4) hours.
This may make a significant difference between what may or may not be lost during a fire depending on how a house is constructed and the fire insurance you take may also be affected by the method of construction used in your house.
Depending on the method of construction adopted walls may be used as structural elements or load bearing walls used to provide an anchor and support to the roof structural system.
The walls used in this manner must have the required capacity to carry all the loads imposed on it including the walls’ own weight failure to which a building may develop structural cracks which may lead to the ultimate and eventual building failure and shortened life span.
This possibility is even more pronounced especially now that concrete blocks are usually made to anyone’s specifications which are usually inferior to the minimum preferred crushing strength of not less than 3.5N/mm2
Partition walls within a house can be used to resist lateral movement in a building and provide cross-bracing in the roof structural system. All these structural benefits may be lost when all internal walls are only built up to wall plate level and out of 150mm thick walls.
Geographically Zambia lies between latitudes 8 and 18 degrees south of the equator and between longitudes 22 and 34 degrees east, climatically Zambia is classified as a Tropical Upland.
The generally high altitudes tend to lower the air temperatures but the clear skies, however result in strong solar radiation being the dominant climatic design consideration.
A building, among other functions, is also a climatic filter and among the various building elements the roof surface receives the highest solar radiation intensity followed by wall surfaces facing east and west.
Walls facing south receive the least, if any, solar radiation intensity.
There are two major contributors to indoor temperatures which may result in the uncomfortable overheating of indoor spaces.
The first is what is commonly known as the “Green House” effect; window glass panes are practically transparent to short-wave infra-red radiation emitted by the sun entering the house through windows but glass panes are almost opaque to long wave radiation emitted by objects within a room.
The consequence of this is that the solar radiation, once it has entered the window, gets trapped inside the building and the temperature just builds up.
The second contributor has to do with the Periodic Heat Flow where in the hot period heat flows from the environment through the built fabric into the inside of a building.
Now depending on the thermal characteristics of the wall material the out-door temperature may have reached its peak and started decreasing before the inner surface temperature has reached the same temperature level.
The retention of night-time low wall surface temperatures is desirable in the hot season and the construction of walls with thermal capacities such as of solid masonry or concrete walls with nine (09) to twelve (12) hours thermal time-lag in heat transmission may be used to lower indoor temperatures.
However the thinner the thickness of an external wall the more transparent it will be to heat flow and the less will be the thermal storage capacity, in other words the indoor temperatures may reach their peak at the same time as the out-door air and solar temperatures.
This means a 150mm thick external concrete block work wall may not be as effective in helping lower internal temperatures as may be the case for more massive masonry or concrete walls unless, of course thermal insulation is used.
A building, therefore constructed from 150mm thick external concrete block walls is more likely to overheat during the hot season.
An external wall also acts as a moisture barrier separating the outer and wet environment from the inner environment and the effectiveness of the wall may depend on the absorptive capacity of the external fabric used.
Normally an architect may need to know whether intense rains are associated with strong winds or the likelihood of driving rain or the Driving Rain Index which characterises a given location and expresses the degree of exposure.
Although this index only broadly classifies the given location, the actual rain penetration will depend on the instantaneous rain intensity and the simultaneous wind velocity.
A thin external wall which is likely to easily allow passage of moisture into the inside is unlikely to offer protection against this element even where generous roof overhangs are allowed this may not completely offer protection to such walls and may lead to damp internal wall surfaces during the rainy season.
It is advisable therefore that when developers are making decisions on what materials to use on their project they consult because cost of materials is not the only important consideration to take into account, the various characteristics of building materials are equally important for the satisfactory performance of the building functions.
(The author is the current Chairperson of the Zambia Institute of Architects- Copperbelt Chapter) / Times of Zambia