Document gbq5wRJoXEzg5mOQZKq6zeLvJ

230 CHAPTER 10; : 1958 Guide r ture, is exposed to moisture damager^-such as.swelling, mold or discolora- tion. Visible condensation may occur also in summer. It is often seen, on basement concrete walls and floors which are cooled by the earth and which being massive, tend,to hold a constant temperature from day to day while, tiie weather dewTpoint temperature rises. When no water vapor is re leased in the space, the dew point tends to equal that of the outside (though' it is likely to lag when there is slight-ventilation). At times the dew:' point temperature rises above the temperature of walls and floors and . condensation results. If the basement is decorated the trouble may be serious. As an operating problem, the solution may be to reduce ventila-' tion at times of high weather dew point, to warm the walls, or to dehumidify Fig. 5. Relative Humidity in Dwellings the space. Warming the walls, a slow process at best, is generally accom^ plished in favorable climates by excess ventilation. In a climate having,> low temperature at night, it may be feasible to ventilate only at night anor-i thus reduce the moisture content of hygroscopic materials which will thet^ act .as a desiccant to retard the dew-point rise during the day. In basey; ment walls and floors insulation should be applied in the concrete <?r. its o itside. Insulation placed on the inside of such walls eliminates visible,-j- condensation'but fosters concealed condensation. The control of the latter* will be discussed. A practical arid fully effective technique for such control^ has, however, not been developed. Usually, sub-surface dwellings shoujdg. be designed for occupancy and should be dehumidified. -.Sip A similar situation is seen in dwellings with coricrete floor slabs laid .|^| the ground where such slabs are not a part of the heating system. northern climate where high dew-point temperatures occur in summer, densatidn or very high relative humidity may damage rugs which are them;y selves contributors to the trouble since they reduce the floor slab tempera-.,-, ture. Slab floors above grade are not so serious a problem as floor slabsogjy grade but their response to air temperature change is slow. Faster warW-y irig is accomplished by the removal of rugs and abundant ventilatiori'*.^ proper times. In their design, floor slabs should have as low specific beAg^ and as high thermal resistance as is consistent with other requirement^ Suitable insulation below the slab, especially well drained gravel, will help Moisture in Building Construction 231 somewhat. A top surface cover of insulating value that is unaffected by water on its lower side would be desirable in the less favorable northern climates. The avoidance of interior visible condensation is partly a construction' and partly an operating problem. It is accomplished by reducing the in terior dew-point temperature or by raising the surface temperatures that are below the dew-point, or both. The dew-point temperature may be lowered by giving attention to the sources of the moisture, and in winter, may be controlled by ventilation, or possibly by some moisture absorption process. The temperatures of the inside room surfaces in winter may be increased by adding insulation to outside walls, by double glazing of win dows, by circulating warm air over the surface, or perhaps by direct heating of the surface. The most expedient method of overcoming a surface con densation difficulty will depend upon special conditions surrounding the problem. CONCEALED CONDENSATION IN HEATED BUILDINGS Water vapor produced in a building necessarily raises the vapor pressure above that outside thus providing the force that causes its diffusion into exterior walls. The amount of vapor pressure rise in the building depends on the amount of vapor produced and inversely on its chance to escape. The resulting balance may be expressed in terms of relative humidity if the inside temperature is 70 F. The relative humidity in heated buildings covers nearly all of the possible range. In zero weather it may be only 10 percent in an office, and 85 percent in an industrial plant where humidifi cation is required for a process, or where vapor release is incidental to a process. In residences the relative humidity in cold winter weather ranges from 10 percent to 60 percent, the latter figure applying to a very small, crowded and unventilated dwelling. A 40 percent level is considered representative of a substantial number of modem tightly constructed small houses although the average house relative humidity is probably below 25 percent. Surveys in residences show that the relative humidity increases as would be expected in warmer weather. Fig. 5 represents the results of one such survey.6 , water vapor is allowed to enter a wall and condensation occurs on tpmU * C e*ements, it appears as frost or liquid. If the weather ,, Perfure frequently, frost melts and becoming liquid, is likely to non6 rate capillary materials like wood, or run down when the surface is timm i F i ?T ^^dy saturated with water. In weather that is con- fihrm!f y i or a lnS Period, the frost may build back into a cavity or cooler llns, 'ation and, when it reaches a warmer plane, will run to lower, side m 6VelS wae/'e it forms a mass of ice. Water seepage to the weather freezing. *CUr aarmlessly in masonry walls when the weather is above In tvnir it wa^er seepage into the building must obviously be avoided. absorb-8 tram construction with wood sheathing which has large water steady colrf15801^' seepaSe's rare and occurs only after a long period of sheathimr * ,wa. r- More generally, moisture accumulates in wood winter aftf; l- , g trough the colder months and reaches a peak in late cycle. ' Th F WtUTM drying of spring and summer completes the annual condensaHe,,aVeri!?e ^^ter temperature and its duration are factors in the ' to condense*; pro`em' In Fig. 6 the map of the United States is divided in- -separating *i,0n zones bnsed on winter weather conditions. The solid lines & e zones follow state lines, and are those recommended by the