Document XO9n9jNmGjL99wVM2jJMeNy

136 CHAPTER 9 I960 Guide T. D. Phillips: Efleet of Ceiling Insulation upon Summer Comfort (National Bureau of Standards Report BMS52, July 1, 1940). L. V. Teesdale: Thermal Insulation Mode of Wood-Base Materials, Its Application and Use in Souses (U. S. Forest Products Laboratory Report No. R1740, October 1949). F. B. Rowley and A. B. Algreo: Heat Transmission through Building Materials (University of Minnesota, Engineering Ex periment Station Bulletin No. 8). Paul D. Close: Building Insulation (American Technical Society, Chicago, 1951, 4th ed.). CHAPTER 10 MOISTURE IN BUILDING CONSTRUCTION Properties of Water m Air; Water in Building Materials; Vapor Transmission; Permeance and Testing; Visible Condensations Concealed Condensation in Heated Buildings; Control of Concealed Condensation; Condensation in Cooled Structures. HE behavior of moisture is too often overlooked or given humidity. The vapor pressure of the water present in .the air, Tscent attention in the design and construction of build* although not shown on the chart, can be calculated readily inga It is present as a vapor in all air and as adsorbed mois from the vapor pressure at saturation and the relative hu ture in most building materials. It may be present also at midify, since relative humidify is very nearly equal to the tirpoa in the free liquid state or as ice, in the solid state, within ratio of the actual vapor pressure to the saturation pressure the range of temperatures encountered in many buildings. at the existing temperature. Problems involving moisture may arise from changes in mois The increasing relative humidify accompanying cooling ture content, from the presence of excessive moisture, or from from the condition represented by A on the chart to point B effects associated with its changes in state. can readily be followed. At B, however, at 44.6 F, the relative Of particular interest is the change from the vapor to the humidify becomes 100 percent, and.the air-vapor mixture is liquid or solid state, known as condensation. This may be said to be saturated. The temperature at which this particu associated with a reduction of temperature with time, or lar air-vapor mixture, upon cooling, becomes saturated is its may occur as a result of migration of water vapor to regions dew-point temperature. Upon further cooling, to 3S F, the of lower temperature. Moisture problems involving condensa original amount of water vapor can no longer be retained and tion are therefore most likely to occur in buildings in any is reduced, in this case, to the condition represented by C, climate in which there is a source of water vapor at tempera from 0.0633 lb per lb dry air to 0.0427 lb per lb dry air. The tures above normal, or in cooled structures, and in buildings process ABC is typical of that which an air-vapor mixture in cold climates. experiences when it comes in contact with a cool window Moisture problems in residences occur in winter and be surface. Cooling from B to C results in visible condensation come increasingly important as homes are built smaller and on the gl>wH surface. If the point C were below 32 F, the con tighter. Water vapor originates from such necessary living densation would be in the form of frost. - requirements as cooking, laundering, bathing, and the breath- - Once the temperature drops below the dew point, or frost mg and perspiration of people. In a typical family of four, point if below 32 F, the vapor pressure at the condensing the average daily production of water vapor from these sources surface is also reduced, thereby establishing a gradient of may be as much as 25 lb, and may be much greater where such vapor pressure from the room air to the window surface. This appliances as humidifiers, automatic washers, and dryers gradient will operate, in conjunction with the convective ac are used.1 Another large source of water vapor is sometimes tion within tiie room, to move water vapor continuously to the bare earth in a crawl space or basement. AU this water . the window surface to be condensed, so long as the concentra vapor must escape from the dwelling. tion of water vapor in the room is maintained. PROPERTIES OF WATER VAPOR IN AIR A common winter process is that, shown by DE, .showing air at 20 F, saturated, being heated to 70 F with a resulting Water vapor in air is a gas which occupies all the space, large decrease in relative humidify. This explains, in part, along with the air present. In many ways, the water vapor can act independently of the air, since in general its proper ties do not depend on the presence of the air. It exerts its own vapor pressure, and can move about through air in a space, or move through materials under differences in its own vapor pressure, independently of the air. However, when the air is moved suddenly or is heated or cooled, the water vapor pres ent is similarly affected, so that it is usually necessary to con sider it as a part of an air-vapor mixture. The properties of mixtures of air and water vapor are rela tively well known and are discussed in Chapter 3, Thermo dynamics. Changes in these properties with heating and cool ing can be followed readily with the aid of a psychrometric chart, shown in outline in Fig. 1. The saturation line repre sents the limiting concentrations of water vapor which can exist as vapor at various temperatures. A common condition inside buildings, 70 F and 40 percent relative humidity, is represented by point A. This is a condi tion of partial saturation; i.e., less than 100 percent relative Fig. 1.... Two Typical Heating and Cooling Processes m Air within Buildings Shown on ASHAE Psychrometric Chart 137