The Construction of Buildings 2 [antikvár]

METRIC EDITIONS: NOTE For linear measure all measurements are shown in either metres or millimetres. A decimal point is used to distinguish metres and millimetres, the figures to the left of the decimal point being metres and those to the right millimetres. To save needless repetition,the abbreviations 'm' and 'mm' are not used, with one exception. The exception to this system is where there are at present only metric equivalents in decimal fractions of a millimetre. Here the decimal point is used to distinguish millimetres from fractions of a millimetre, the figures to the left of the decimal point being millimetres and those to the right being fractions of a millimetre. In such cases the abbreviations 'mm' will follow the figures e.g. 302.2 mm. R. BARRY CHAPTER ONE WINDOWS A window is an opening formed in a wall or roof to admit daylight through some transparent or translucent mateiial fixed in the opening. This primary function of a window is served by a sheet of glass fixed in a frame in the window opening. This simple type of window is termed a fixed light or dead light because no part of the window can be opened. As the window is part of the wall or roof envelope to the building, it should serve to exclude wind and rain, act as a barrier to excessive transfer of heat and sound and spread of fire in much the same way as the surrounding wall or roof does. The functional material of a window, glass, is efficient in admitting daylight and excluding wind and rain but is a poor barrier to the transfer of heat, sound and the spread of fire. The traditional window is usually designed to ventilate rooms through one or more parts that open to encourage an exchange of air between inside and outside. Ventilation is not a necessary function of a window. Ventilation can as well be provided through openings in walls and roofs that are either separate from windows or linked to them to perform the separate function of ventilation. The advantage of separating the functions of daylighting and ventilation is that windows may be made more effectively wind and weathertight and ventilation can be more accurately controlled. FUNCTIONAL REQUIREMENTS The primary function of a window is: Admission of daylight and the secondary functions are: A view out, and Ventilation. Daylighting is a necessary function and a view out is generally a desirable function. Ventilation is neither a necessary nor a desirable function yet it is traditionally expected of a window. The functional requirements of a window as a component part of a wall or roof are: Strength and stiffness Exclusion of wind and rain Thermal insulation Sound insulation Fire resistance. DAYLIGHT The prime function of a window is to admit daylight for daytime activities in sufficient quantity for efficient performance. Good sense dictates taking the maximum advantage of this free source of illumination when the modem alternative, electric light, is so extravagantly wasteful of natural fuel sources and grossly expensive. The accepted minimum level of daylight for the performance of various activities varies from for bedrooms, 1 for living rooms to 6 for drawing offices, the figures given being the daylight factor which is the percentage of daylight admitted through a window from the hemisphere of an unobstructed sky. Accepting that daylight is to be the main source of light during the day then the level or amount of daylight at a particular point in a room used, for example for drawing, is critical whereas in a room for living it is desirable but not critical. A calculation of the quantity of daylight is therefore important for rooms such as those used for drawing and less so for those used for general purposes such as living. Table 1 sets out typical recommended daylight factors. The quantity of light admitted depends in general terms on the size of the window or windows in relation to the area of the room lit, and the depth inside the room to which useful light will penetrate depends on the area of the window and the height of the head of the window above floor level. Common sense and observation suggest that the quantity of daylight in rooms is proportional to the area of glass in windows relative to floor area and this is confirmed by measurement. The average daylight factor in side-lit rooms is roughly equal to one fifth of the percentage ratio of glass to floor area. In a room with windows on one long side with no external obstructions and a room surface reflectance of 40%, where the glass area is one fifth or 20% of the floor area, the average daylight factor will be 4 and the minimum about half that figure. Conversely, to obtain an average daylight factor of, say 6, in a room with a floor area of 12 m^, a glass area of about 6x12x5/100 = 3.6 m^ will be required. This broad average calculation is generally sufficient when used for general activity purposes such as in living rooms and it is an adequate base for preliminary assumptions of window to floor area which can be adjusted later by a more accurate calculation of the light required for activities in which the lighting is critical. For many purposes a daylight factor of 2 is adequate (see table 1). A broad measure of the penetration of useful daylight into rooms is, taking an average figure of 2 as the daylight