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CBD 48-Requirements for Exterior Walls by Hutcheon, N.B. 1963
Introduction to Microeconomics (ECON 201)
Concordia University
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Requirements for exterior walls
Hutcheon, N. B.
Canadian Building Digest
Division of Building Research, National Research Council Canada
CBD 48
Requirements for Exterior Walls
Or iginally published December 1963
N. Hutcheon
Please note
This publication is a part of a discontinued series andis archived here as an historical reference. Readers should consult design and regulatory experts for guidance on the applicability of the information to current construction practice.
Designs for exterior walls for buildings have seldom been developed in a systematic, rational way. They have evolved slowly, keeping pace with gradual changes in social and economic patterns and environmental requirements. Today, with a dynamic architecture and many new materials, components and construction techniques ava ilable, a large number of new designs are possible. Unfortunately, some are being adopted without adequate consideration, and evaluation by the slow trial by use methods of the past is no longer adequate.
Although trial by use must al ways be the ul timate test, the cost of full -scale experiments is usually so great that considerable study and effort is justified in order to ensure that the design will perform satisfactorily. It must be recognized al so that the conditions for such full -scale, long -time trials in use may not be those for which the information is later required. This is particularly true in Canada today where important changes such as humidification in winter are being introduced. It is now possible with the aid of building science to recognize the pertinent factors affecting the performance of walls and to analyse wall designs systematically as to probable performance in respect of their varied requirements. With this capability one may begin to discriminate between various designs for particular uses, and, even more important, to provide a basis for the development of improved designs.
Functions of a Wall
It is important, at the outset, to recognize that the over -all function of an exterior wall, in conjunction with fl oors and roofs, is to provide a barrier between indoor and outdoor environments, so that the indoor environment can be adjusted and maintained within acceptable limits. The requirements for a wall must, therefore, relate to its ability to remain in place and to be durable for a required length of time, while providing the necessary barrier or filter to wind, rain, solar radiation, heat, noise, fi re, particulate matter, insects, animals and even humans. It may be required to transmit light (windows), while imposing a barrier to other factors; must not itself be a hazard to life or property; must contribute suitably to the form and aesthetics of the building generally; and fi nally must satisfy a number of lesser requirements such as colour, texture and porosity. All of this must be achieved as far as possible at an acceptable cost, including both initial and maintenance costs.
in most designs. The designer may often be required, as part of the design problem, to vary the proportion of anticipated initial to operating costs to suit the economies of a particular venture. Initial costs may be reduced by selecting lower quality components at the expense of increased maintenance or reduced service life, or compromises may be adopted in other directions. Insulation may be reduced at the expe nse of heating cost or a simpler wall or window arrangement, with some sacrifice in function, may be adopted.
Aesthetic quality can al so become a major factor in cost, although this is not always so. Form, colour, texture and pattern can usually be varied over a wide range without affecting other requirements. But insistence upon a particular aesthetic feature or quality may make it difficult to satisfy the service requirements or may force unsatisfactory solutions.
Aesthetic considerations frequently becom e in volved in other ways. For example, inadequacies in design may allow cracks to develop so that water-staining occurs, surfaces spall or finishes degrade. Thus the aesthetic qualities of a building will be more readily maintained when the basic requirements have been satisfied.
The last of the requirements to be discussed, strength and rigidity , is a functional requirement that has important inter-relationships with other requirements. There is a further complication with walls because they may interact with the structural frame to contribute to its strength and rigidity. This inter -relationship may vary widely at the designer's choice, from curtain walls supported on the frame to designs in which walls contribute racking strength and rigidity or assist incarrying vertical loads. The extent of this inter-dependence must be considered in the structural design of both frame and wall.
There are, in a ddition, loads arising from wind that must be borne by the exterior wall and its parts; these must be transferred, together with the dead load of the wall, through suitable structural connections with the structural frame. This kind of structural analysis and design has been well established for many years and need not be discussed further. There is, however, one kind of loading arising out of dimensional changes in materials that deserves far more attention, particularly in the design of exterior walls and building frames, than it has yet received.
Dimensional Changes and Induced Loading
The strains and deformations in building elements from stresses produced by loading are well recognized and are regularly dealt with in s tructural design. But other small dimensional changes take place in materials due to causes other than loading. These may give ri se to deformations, loads and stresses that are not always adequately taken into account. Creep, a deformation with time under load, and thermal expansion and contraction are usually considered in the analysis of the structural frame, but they may be neglected in the desig n of the wall itself. Dimensional changes may al so result from changes in moisture content in certain materials and from aging and degrading effects produced by environment.
When these expansions or contractions are not restrained, the element merely chang es dimension. This in itself may often be a problem. For example, a temperature differential of 100 deg. in s teel or concrete gives rise to dimensional changes of about 0 per cent. When these are restrained by adjacent material or by an adjoining elemen t, strains and corresponding stresses can develop. A strain of only 0 per cent is required to fail normal plain concrete in tension. A dimensional change of this magnitude can be produced by a temperature change of about 20 deg. Normal concrete blocks will shrink as much as 0 per cent when dried from a saturated state. Cracking, crazing or buckling of exposed surfaces or of surface coatings often occur as a result of temperature or moisture changes that promote either shrinkage or expansion.
Once cracks have developed, severe wetting of the wall will usually follow. This results not only from direct penetration of rain but al so from the condensation of water vapour carried in ai r leaking outward through the cracks.
Obviously this whole subject of dimen sional changes in materials and the induced loads and failures that can result merits more extended treatment. There is some discussion of the subject in CBD 30 and this will be extended in future Digests. It has been introduced here because of its obvious relationship with other requirements. Dimensional stability, although a highly desirable characteristic of all materials, elements and components, and having particular implications in exterior wall design, bas not been listed as a requirement. It is implicit in the requirements for strength and durability and in the barrier requirements, because these influence temperature, moisture and other conditions that ma y produce dimensional changes. Discussion of it has served to emphasize further the inter -relationships and the inter - dependence of the various requirements that have been set out. It has also demonstrated the importance of a knowledge of the dimensional stability characteristics of elements and components as well as of materials in any rational approach to wall design.
Environmental Considerations
Determination of the outdoor environment and establishment of that desired indoors is an essential fi rst step in exterior wall design. Only when these factors are known is it possible to assess the over -all performance requirements of the wall acting as a separator. Inevitably, at this stage, some aspects of the building services become involved. Through them cert ai n features of the indoor environment are adjusted to the desired levels. The daylighting characteristics of the transparent portions of the wall must be considered in relation to lighting; and heating and cooling requirements are related to the nature of the wall as a barrier to solar radiation, heat, moisture and ai r during both winter and summer. Other barrier requirements such as those related to dust and smoke may have implications also for the services required. One consideration, that of the relativ e humidity to be carried indoors, is of such potential importance as a factor in indoor environment and the design of exterior walls for cold weather conditions that it merits special attention.
The nature of relative humidity, its effects upon building oc cupancy, and some of its implications in the operation of buildings have been discussed in CBD 1. Further developments, particularly the added precautions that must be taken to avoid difficulties, are outlined in CBD 42. Relative humidity, either indoors or outdoors, poses a problem mainly when it represents, at the particular temperature at which it is measured, a dew -point above the temperature on the cold side. During the summer, ai r- conditioned buildings and cold storage buildings may have special problems. Fortunately, the maximum outside dew-point in Canada is only likely to exceed the indoor temperature for short periods during the summer in buildings cooled for normal occupancy. Usually, the most perplexing situations occur in winter when the dew -point temperature indoors greatly exceeds that outdoors. In these cases special provisio ns may have to be made in the barrier characteristics of the wall or window for the control of heat, moisture, air and water vapour. Specifically, substantial relative humidities indoors with low temperatures outdoors introduce the possibility of wetting on the surface as well as within the wall (see CBD 30 ).
Once the indoor and outdoor environmental conditions and their various requirements have been established, the plan ni ng of a suitable wall may proceed. Estimates should be made of temperature and moisture conditions throughout the wall and any adjoining parts of the building frame for both winter and summer conditions. These thermal and moisture gradients should then be examined to determine the effect of the environment at any plane in the wall on the materials selected for its construction.
Conclusion
The differences in the properties of inside and outside atmospheres to be separated by a wall dictate the requirementsof that wall. In combination with the properties of the materials to be used, these differences determine the environments within which each element must perform. The service life of a material is determined by the properties of the material and the conditions of the environment to which it is subjected.
CBD 48-Requirements for Exterior Walls by Hutcheon, N.B. 1963
Course: Introduction to Microeconomics (ECON 201)
University: Concordia University
Recommended for you
Students also viewed
- Straube Smegal-High-R Walls Case Stydy Analysis-BA-0903-Rev
- Shirtliffe, C.J.-Thermal Resistance of Building Insulation-CBD 149 1972
- BSD-018 The Building Enclosure Building Science Corporation
- Quirouette, R.L. BPN 54-Diff bw Air Barrier Vapour Barrier 1985
- BSD-014 Air Flow Control in Buildings Building Science Corp
- Kenzel,H-Smart Vapour Retarder 1998