Man's first wall was built as a stockade around his village. Its function was to protect from intruders and to contain livestock. In the evolution of house forms, herders de-emphasized the importance of the hunter's roof-tent in favor of wall embellishments based on the village stockade. First, he drove posts into the ground and wove wattle in between. Later, he pressed stiff mud into the wattle — a direct predecessor of our ever-popular suburban "half-timber" style.' Finally, a few thousand years before Christ, the wall builders invented the brick, which essentially brings us to the modern scene.
As contemporary man continues to place brick on brick to form the wall — stockade around his "living" space, he armors against life in much the same manner as his pastoral ancestors. The function of walls for protection persists even though the early pastoral significance has been lost in our present non-pastoral communities. Some self-conscious design reformers later succeeded in disturbing these notions when they started the bring-the-outdoors-in campaign. Result; the picture-window became a stock item and indoor planters defied the "outside" feeling as much as the concrete patio did the "inside" feeling.
It's about time we re-examine the wall in terms of purpose and function. A house can be thought of as having many purposes; but primarily it is a contrivance for regulating (controlling) a segment of our environment. This conditioned environment is enclosed with roof, floor and walls so that weather factors such as air movement, humidity, precipitation, temperature, and light may be regulated. Walls are structural membranes separating the indoor environment from the outdoor environment. On the other hand, the greater the difference between the indoors and the outdoors, the more elaborate must be the wall's inherent properties. The more important characteristics which a wall must have include strength and durability, flow-control (heat, moisture and air), and good design at low cost. These are the more significant properties that will concern us in this chapter.
Generally speaking, about one-half of the cost of a house is the cost of the materials which go into it, the remainder being labor costs. With this fact in mind one would naturally assume that builders would have a thorough knowledge of the nature and properties of all building material possibilities. The functional performance of a wall material is number one consideration, but at the same time the most economic solution requires a selection of components having the lowest combination of initial cost and maintenance. This long-range cost is too often overlooked by builders in their concern with how much money must be raised in the beginning.
Among the many considerations for selection and evaluation of wall materials, listed below in order of importance. I would certainly include a seemingly far fetched salvage value. Cheaper and easier demolition becomes significant in this era when the average useful life of a building is comparatively short. Plywood paneling and heavy timber framing have high salvage values; brick and concrete have low salvage values.
Check List for the Selection of Wall Materials
1. initial cost and subsequent maintenance cost.
2. compressive and traverse strength.
3. resistance to natural weathering, chemical attack and atmospheric pollution.
5. ease of handling (size, weight, shape) and erection.
6. resistance to scratching and impact.
7. dimensional changes with temperature and moisture content changes.
8. susceptibility to insect attack.
9. appearance in color and texture.
10. moisture penetration resistance.
11. sound insulation and absorption.
12. adaptability to future changes of layout and salvage value.
Wall materials must have the necessary strength in compression, bending, shear and tension to carry applied loads and to resist such external pressures as wind loads. Accurate strength properties of the material must be known before one can economically design a wall. In the design analysis it is important to remember that the loads in question apply only to the net width of the wall, excluding door and window openings. From a design standpoint this would suggest greater consolidation of openings and adequately spaced, solid wall panels to provide bracing effect. Types of bracing and methods of fastening are the main factors which influence the strength and rigidity of a wall.
The durability of a wall is more dependent on heat flow (temperature change) and moisture than any other factors. Moisture deterioration can take place in all organic building materials, as such materials are hygroscopic-absorbing moisture from the air in proportion to relative humidity. Dimensional change in a wall primarily causes weakness and failure of fastening members. Practically all wall materials change their dimensions according to whether they are wet or dry. When a wet material is dried, shrinkage occurs; when the material becomes wet again, expansion takes place. But the subsequent expansion is often less than the initial contraction, leading to irreversible expansion. Swelling and shrinking due to moisture-changes also causes a breakdown of surface finishes. The pore structure of organic wall materials easily permits capillary moisture attraction from the atmosphere. As much as 40% of the dry weight of a material can be taken up and held with water. This is to say nothing of direct moisture contact from rain or melting snow.
Wall moisture originating from within the building can be an even more damaging factor than outside penetration. As the inside temperature increases, inside water vapor is transmitted into the wall and there condenses. This condensation results in wetting of structural materials and consequent loss of insulating qualities. It also gives rise to such serious problems as chemical, physical or biological deterioration of the materials (corrosion of metal, spalling of brick, rotting of timber).
In wood-frame walls a vapor barrier will control inside condensation. But in masonry walls the vapor barrier prevents moisture from entering the room, and consequently, moisture condenses behind the moisture barrier as the hot summer sun follows a rain, driving moisture inside the wall. Waterproofing the outside of a masonry wall will prevent moisture migrations from the outside, but this same waterproofing causes a build-up and damming of water by condensation from the inside. In short, a wall must be designed to limit this entry of water from the outside via capillarity and at the same time permit the flow of water vapor to the outside in winter. But the outside wall should have a partial capacity for water storage, separated capillarity-wise from the inside wall. Furthermore, the transfer of vapor to the inside in summer should be controlled by venting. Obviously, the best choice for a masonry wall is one of the vented-cavity type. Where insulation is used as well as a vapor barrier, a higher indoor humidity is possible without the occurrence of surface condensation. However, incorrect use of thermal insulation can increase the danger of condensation. It can increase temperatures on the warm side of the wall thereby decreasing temperatures on the cold side and causing condensation towards the cold side of the wall.
In the wall-material literature one finds ample reference and speculation on the "wonder" material which satisfies all the requirements of an inexpensive, durable, strong, simply constructed wall. Realistically, this ideal material should also exhibit no thermal expansion, no moisture expansion, be impermeable to moisture, and resistant to heat flow. Also there are summer moisture-heat-gain and winter moisture-heat-loss factors to balance out, so it is quite unlikely that such a miracle product exists or could ever be inexpensively manufactured.
The more practical approach to wall building is for the owner-builder first to plan his walls in relation to areas for winter functions and summer functions, daytime use and nighttime use, inside partitions and outside exposure, bearing walls and curtain walls, storm sides and sunny sides. Our practice of building all four walls of a house of equal thickness and insulation is absolutely absurd. The nighttime, sleeping areas certainly have opposite requirements from daytime, living areas.
The thermal performance of a wall is determined by its (a) degree of direct solar heat penetration through windows, (b) absorptiveness of the exposed surface to solar radiation, (c) heat-storing capacity, (d) insulation characteristics, (e) ventilation rate. Heavyweight wall materials like tamped earth are cool during the day and warm during the night in regions where the daily variation in outdoor air temperature and solar radiation are great. But in regions where the daily outdoor temperature variations are small but solar radiation intensities high, such heavyweight wall materials do not permit sufficient cooling off during the night for comfortable sleeping. In such regions, lightweight materials should be used.
Another gross extravagance in the home building industry is the use of heavyweight materials in non-load-bearing partitions. This is an example of a builder's failure to select a wall material from the point of view of purpose and requirements as well as availability and other economic considerations. An interior, non-load-bearing wall certainly does not require even similar properties as does an outside supporting wall.
Wall materials in a house should be as varied as window sizes or roof coverings. The success or failure of a wall design—and the completed house as well—is determined by the degree to which the builder relates the wall material to inside and outside environment. A wall is a sort of transition between these two environments; it can express this function with insensitivity or by a statement of simple logic. When peppered with openings and conflicting materials, a wall lacks clarity and composition. Of all that man has created, simplicity of structure is a constant characteristic of the finest. You can get a unified design by reducing the number of materials used, by consolidating the areas and spaces within. You can simplify linear design by extending lines both horizontally and vertically—resulting in a neat, uncluttered silhouette. Simplicity means balance in form, scale, texture and color. Simple structure also means economy. All this is to tell the owner-builder that structure is effective as it satisfies your human need. Set your reasoning to get the simplest, most effective means possible. Immerse yourself in this economy-of-means. After a while you will not have to think effectiveness and balance. It will become part of your nature, with your actions taking place on a more intuitive, non-verbal, non-intellectual level.