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Weather exclusion is concerned with methods of ensuring that wind and water (rain and snow) don’t adversely affect the fabric of a building or its internal environment. Wind and water penetration Wind can cause direct physical damage by collapse or removal of parts of a building. It can cause dampness by driving moisture into or through a building fabric, and also excessive heat losses from the interior of a building by uncontrolled air changes. Water penetration can produce rapid deterioration, and cause the fabric of a building to become moist enough to support life, including bacteria, moulds, mildew, other fungi, plants and insects. Saturated materials also permit the quick transfer of heat (water is a good conductor) and this, together with the other factors mentioned, will cause an uncomfortable, unhealthy and uneconomical building. The sources of water likely to penetrate a building include not only those from rain and snow, but also those from moisture contained in soil or other material in immediate contact with the building fabric. For water to penetrate there must be openings or passages in the building fabric through which it can pass, and a force to move it through these openings or passages. Without these two factors, the building fabric would remain in a water tight condition. Most buildings, however, have window and door openings, are made from lapped or jointed parts, or from porous materials ready to absorb moisture; wind currents and eddies are also normally present. Condensation may also create moisture problems. Damage by flooding is beyond the scope of this guide. Exposure zones Construction methods of earlier periods were generally capable of permitting a certain amount of wind and water to penetrate through to the interior of a building. Shapes for buildings were devised for particular climatic exposures which best provided an initial defense, and the constructional detailing endeavored to provide a final barrier. By way of progress, modern construction methods are expected to give almost total exclusion against wind and water penetration. With ever-changing fashions for building shapes - sometimes borrowed from areas vastly different in climatic influences - care must be taken to ensure that constructional methods are suitable for the exposure conditions dictated by the specific location and disposition of a building. That is to say, before considering form and construction method research must be carried out to reveal the degree to which a proposed building will be exposed to driving rain. In the United States, initial assessment can be obtained by reference to the driving rain index (DRI) for the particular location as published on maps by the Local Building Research Agency. Values are obtained by taking the mean annual wind speed in meters per second (m/s) and multiplying by the mean annual rainfall in millimeters. The product is divided by 1000 and the result is used to produce contour lines linking areas of similar annual driving rain index in m^2/s throughout the country:
The value for a particular location within 8 km of the sea, or a large estuary, must be modified to the next zone above (sheltered to moderate, and moderate to severe) to take account of unusual exposure conditions. Furthermore, modifications may also be necessary to allow for local topography, special features which shelter the site or make it more exposed, roughness of terrain, height of proposed building and altitudes of the site above sea level. The pro portion of driving rain from various directions within one particular location can be obtained by reference to driving rain rose diagrams. For greater accuracy, reference should be made Code of practice for assessing exposure of walls to wind-driven rain. This provides an alternative rose analysis for the extent of driving rain in various locations, to permit an exposure expectation for each face of a building. It’s expressed in liters/m^2 per spell, where spell is the period that wind-driven rain occurs on a vertical face of a building. Figures for a wall annual index expressed in liters/m^2 per year can be derived from this data, as can local spell and local annual indices. This does not mean that a designer need adjust construction detailing for different elevations, but it will reveal which aspect of a building is most vulnerable to water penetration. This guidance is also helpful in assessing areas most compatible to lichens, mosses and other growths that can have a deteriorating effect on structure. The FEDERAL BUILDING code provides a series of maps of the United States with superimposed rose diagrams allocating numerical values to specific areas. One of the most important lessons to be learnt from driving rain indexes and roses is that design and construction details, of necessity, may vary from one exposure zone to another; details suitable in a sheltered exposure zone would probably leak if simply transferred to a severe exposure zone without modification. For instance, in the severest of exposures structures should not have walls constructed with full cavity fill insulation. A minimum 50 mm air gap is necessary to prevent dampness bridging the insulation. Macro- and microclimates ---Alteration of general climate conditions as a result of building disposition and shape. However, there may be danger in using only meteorological climatic data of this nature for the final selection of appropriate material and constructional detailing. This data reveals the general climate, or macroclimate, liable to affect a building in a particular location. There is also a microclimate surrounding the immediate outer surface of a building, i.e. not more than 1 m from the surface of a building. This is created by specific environmental conditions arising from the precise form, location, juxtapositions and surface geometry of a building. A designer is expected to know when design ideas are liable to cause the microclimate to vary significantly from the macroclimate and make adjustments in materials and/or constructional detailing accordingly. Where there is no past experience, it may be necessary to make models of a building and its surroundings, test them under simulated environmental conditions and record the precise effects. For example, detailed analysis has revealed that tall buildings can receive more rainwater on their walls than on their roofs, especially on elevations facing the wind. Under these circumstances, rain is often driven vertically up the face of the building, making it necessary to use constructional details different from those considered suitable for lower buildings in the same exposure zone. --- Movement of water in walls of different construction. --- Movement of water in roofs of different construction. --- Types of jointing to resist the penetration of water. Movement of water Besides the numerous construction methods, there is a very wide range of materials and combinations of materials from which to choose when designing a building. Providing they are carefully matched to exposure conditions, all options can function with equal efficiency in controlling the movement of water from the exterior to the interior of a building. The precise way this is achieved will vary; it will depend upon the properties of the materials employed and the manner in which they are arranged to form the weather barrier. --- illustrates the three basic arrangements for the external fabric of a building where:
When rain falls or is driven on porous building materials, such as most brick types, some stones, or blocks, it’s absorbed then subsequently removed by natural evaporation. This process occurs when water adheres to the pores of the material. and if the adhesive force between the water molecules and the wall material is greater than the cohesive force between the molecules themselves, the water is drawn in by capillary action. A strong wind increases the rapidity of absorption, and only evaporation resulting from changes in the climatic conditions (rain ceases, temperature rises, air currents become warmer) will prevent the moisture penetrating through the thickness of the material. Earlier construction forms ensured that this thickness was sufficient to prevent water penetration by capillary action. Current economic trends and the need for energy conservation (saturated material loses about 10 times more heat through it than when dry) have now firmly established construction methods using materials of thinner cross sections. These are employed to provide an external initial weather check which is combined with materials used to provide internal thermal insulation. The external and internal components are separated by a water barrier (or air cavity) to interrupt the continuous flow of water from outside to inside. In this way, potentially wet areas are isolated from those which must remain permanently dry. Constructional detailing must attempt to reduce the movement of water as much as possible. It’s particularly important to ensure that the water barrier is incorporated in such a way that moisture is not trapped and kept in a position for a period of time liable to cause damage. For similar reasons, when rain falls or is driven on building materials theoretically assumed to be entirely impervious, e.g. dense concrete, glass, metal, bituminous products or plastics, precautions must be taken to ensure quick and efficient run-off. The quantity of water must never be underestimated; on a glass wall of a building it can be as much as 5 liters per 10 m^2 of façade. A typical flat roof construction to provide a weather resisting barrier would consist of sealed lengths of multi layer bituminous felt or a continuous homogeneous layer of asphalt. A comparable pitch roof construction to resist the penetration of water under the influence of gravity would incorporate an outer surface finish of lapped tiles or slates backed by an impervious water barrier. Further comments about this form of construction are included. --- Types of jointing to resist the penetration of water. --- Construction method used to resist water and wind penetration through a window opening in a fully insulated brick/block masonry wall. Note: this diagram is representative of many existing constructions, but the lintel and reveal details no longer satisfy new-build thermal insulation requirements for housing in the US. Joints The need for joints arises because of the necessity to link, lap or bond materials together when providing the continuous and efficient weather enclosure for a building. Unless the many interrelated factors which influence their position and type are very carefully considered, they can form the weak link in the enclosure. The first rule is to ensure that as much water as possible is kept away from this vulnerable point where two or more materials are brought together, each perhaps having different characteristics and connected in some way with yet other types of materials. The designs for roofs and walls can provide shelter to their joints, or channel free-flowing water in predetermined directions to permit either collection or discharge to less damaging areas. The effects of wind-driven rainwater must always be taken into account in this respect, and boldly profiled upstands and overhangs at joint positions are desirable when it’s difficult to form a continuous 'membrane-type' seal between two building components. The actual method of forming a joint will initially depend upon the physical and chemical properties of the materials involved. The comments made regarding movements and appropriate sizes are particularly relevant. The joint can be expected to behave in a similar manner to the surrounding surfaces by stopping water penetration at the outermost places, or by allowing water to be collected from its recesses and returned to the outside. Within these two extremes, there is a vast range of jointing possibilities. Some of the main joints necessary to provide a weather resisting enclosure for a simple design of a timber-framed window in a brick/block cavity wall. These include the use of mortar for the brick work outer leaf of the wall, an impervious water barrier façade. The surface texture created by the need to channel water away from widely spaced joints of preformed wall units also helps in creating the particular character and expression of a building, membrane (DPC) between brickwork and window frame, a seal between window frame and glass, and draught proofing between window sash and frame. The profiles between the fixed and opening parts of the window frame are also specially designed to reduce the movement of wind-borne water to the interior of the building. Draught excluding devices can also be fitted in this gap to eliminate the flow of air and the possibility of heat loss. The relationship between the type of brick and the type of mortar used in the outer leaf of brickwork is also important, as indicated: How water can penetrate brickwork. One of the most important aspects of joints in a building involves their effect on appearance. The particular type of brick bond (stretcher, Flemish, Dutch, Quetta, etc.) and the width, profile and color of mortar joint can have as much impact on the appearance of brickwork as the color and shape of the bricks themselves. Similarly, the precise location and profile of the joints in preformed panel and in situ reinforced concrete walls will assist in determining not only the overall scale and proportion of a building, but also the pattern and rhythm of features on the façade. The surface texture created by the need to channel water away from widely spaced joints of preformed wall units also helps in creating the particular character and expression of a building. --- How water can penetrate brickwork. Previous: Building
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