Document mBDEaQLeo4zgp09rYGgqnwk2Z

412 CHAPTER 23 Fig. 5.... Relative Humidity in Dwellings The resulting balance may be in terms of relative humidity if the inside temperature is 70 F. The relative humidity in boated 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 humidification 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 GO percent, the latter fig ure applying to a very crowded and unventilated dwell ing. A 40 percent level is considered representative of a sub stantial number of modern tightly constructed small bouses Although the average house relative humidity is probably below 25 percent. Surveys in residences show that the rela tive humidity increases as would be expected in warmer weather. Fig. 5 represents the results of one such Burvey.8 When water vapor is allowed to enter a wall and condensa tion occurs on its outer cold elements, it appears as frost or 1965 Guide And Data-Book liquid. If the weather temperature rises frequently, frost melts and becoming liquid, is likely to penetrate capillary materials like wood, or run down when the surface is non absorbing or is already saturated with water. In weather that is continuously cold for a long period, the frost may build back into a cavity or fibrous insulation and, when it reaches a warmer plane, will run to lower, cooler levels where it forms a miw of ice. Water seepage to the weather side may occur harmlessly in masonry walls when the weather is above freesing but water seepage into the building must obviously be avoided. In typical frame construction with wood sheathing which has large water absorbing capacity, seepage is rare and occurs only after a long period of steady cold weather. More generally, moisture accumulates in wood sheathing and siding through the colder months and reaches a peak in late winter, after which the drying of spring and summer completes the anmiftl cycle.17 The average winter temperature and its dura tion are factors in the.condensation problem. In fig. 6 the map of the United States is divided into condensation zones based on winter weather conditions. The solid lines separating the zones follow state lines, and are those recommended by the Hmising and Home finance Agency for the guidance of own ers, builders, and architects.18 The dashed lines are the --20 F, 0 F, and +20 F isotherms of winter design temperature taken from The Guide, 1958, fig. 1, Chapter 12. It will be noted that there is reasonably good correlation between the zones determined by the two sets of lines. Zone I roughly includes those areas where the design temperature is --20 F or colder; Zone II those for which the design temperature is zero to --20 F, and Zone III those at zero and wanner. Within each zone, similar degrees of condensation trouble are to be ex pected, and similar corrective measures apply. In roofs, the condensation problem is much the 6ame as in walls. The roof covering may be even more resistant to the escape of vapor than wall coverings such as paint; and while i i Moisture in Building Construction 413 paint is likely to be ruptured by excessive, moisture, no such relief occurs in roofs. Thus roofs furnish conspicuous examples of rapid decay in lumber. In crawl spaces over uncovered damp ground, a large water evaporation sometimes occurs and causes condensation on the outer ends of floor joists and other members that are below the floor line and near the outdoors. Water vapor from the crow) space may also enter walls, be transported by rising air in a stack effect, and even reach the attic by this route when the wall structure permits. Ventilation, as discussed later in this chapter, is an important correction factor in these cases. Insulation in a wall or roof reduces heat loss and lowers the temperature of the outer elements of the structure, thus in creasing the possibility of condensation if the vapor path to the cold surface is not blocked. Since low vapor resistance is a characteristic of fibrous insulation, the needed vapor resist* fnc* must be provided by other means. It is to be noted that in typical residential conditions, condensation does not occur in fibrous insulation itself, except when frost has formed on sfroftthing and gradually built backward among the fibers. Wet insulation may result from this condition or from liquid condensation seeping down from a higher level. CONTROL OF CONCEALED CONDENSATION An excessive accumulation of moisture in walls (or roofs) can be prevented by one or more of the following measures: (1) providing a vapor and air barrier to limit vapor entrance into the wall, (2) ventilating the building to reduce vapor pressure therein, or (3) ventilating the wall cavity to remove vapor that has entered. 1. Vapor Barrier. A vapor barrier is the principal and most obvious correction, but each measure b more effective if aided by the effect of another. In habitations, some ventila tion of tiie living space, either incidental or planned, b neces sary. Also, a small amount of cavity ventilation b-essential in cases where the vapor inflow b not completely stopped and the moisture storing capacity of the outer wall elements b slight. This applies to some prefabricated designs nring metal siding. Vapor-barrier sheets are often built into the wall n^r the warm surface. In wood frame walls they may be applied to the inside surface of the studs. They are sometimes attached to the warm side of the insulating materials, or they may be applied on the cold side of plaster base materials. Special designs may be attached like wall paper to the inside of the wall, when satisfactory from the decorative viewpoint. Sheet barriers, such a3 polyethylene films and laminates containing asphalt, paper arid metal foil, so placed that they are not too cold, may also be nd The interior wall board or finish material may itself be vapor resistant, or a barrier cn&ting may be applied to its concealed side if that side will not be too cold. The interior finished surface may be coated with a suitable paint having the required vapor resistance and also serving as the decorative fiffish, or it may be covered by another coat. _ A paint coat on the interior finish, thnngh of adequate resstance, b not likely to be as effective as a sheet barrier properly applied during the wall construction. This applies especially to houses of more than one story having cavities m ceilings which open into the outside walls. Such cavities allow vapor entering the ceiling to diffuse or be transported to the cold' areas. Stoppage of thb path b difficult and requires normally the painting of the filing as well as the walla. Similar treatment may be required on internal partitions, or at least the first stud space adjacent to a cold wall. The necessary barrier resistance depends on a number of factors. When the vapor flow occurs in annual cycles as in heated buildings, the requirement b not as exacting as it b for a cold storage room where there b no chance for drying out an accumulation of moisture. In a heated structure covered on the outside with materials highly resistant to water vapor such as paint or roll roofing, the winter season b a time of moisture accumulation in the cold outer elements and their safe moisture holding capacity b an important factor in determining the barrier requirement.17 A house without sheathing requires a better barrier. A prefabricated design with only a sheet of metal outside of the insulation, requires a very high resistance barrier on the warm ride. Materials such as plastic faced ridings, aluminum riding and decorative metal panels may present problems of condensation directly behind the cold side finish. The interior vapor pressure and the length and severity of the winter are also important. For typical frame dwellings with wood sheathing and riding in the northern United States, a barrier permeance of one perm or les3 has been found satisfactory. There are cases, however, in residential ,construction where a one perm bar rier would not be adequate and there are also many industrial applications in which a very much highwr vapor resistance b required. In any event, the choice of an adequate barrier imT plies that its permeance be definitely established. The usually accepted test procedure for thb purpose b a dry method at a temperature of 70 F to 80 F. If obtainable at reasonable cost (including good application), a barrier better thun required should be chosen for any construction. Despite the theoretical possibility of safely discharging some vapor through a wall, a higher-than-minimum permeance b not preferred. An exact statement showing which buildings require a vapor barrier b not readily formulated. However, in view of the distressing results its omission may bring, it b tentatively recommended that the walls of every well constructed modem dwelling include a vapor barrier when the construction in cludes any material that would be damaged by. moisture or its freezing. Thb applies to all condensation zones in fig. 6 when the V value for the wall b lower than 0.25 Btu per (hr) (sq ft) (F deg), and it applies in Zone I and Zone II to walls of higher transmittance. In applying vapor resistance to a wall, there are certain fundamental principles that should be followed. First, the vapor barrier should be placed as near to the warm surface of the wall as practicable. Second, it should be continuous with no direct openings through the barrier. Good workman ship and application are very important. Workmanship that leaves two openings through the barrier, or around its margin, at different levels, connecting air spaces at only slightly dif' ferent temperatures, leaves a path for convective air ro tation which will transport large amounts of water vapor from the warmer space to the colder. If a membrane barrier b used back of the plaster or interior finish, its joints should be made over some solid framing member, and not between the studs or in similar places. Usually a 2-in. lap over a framing member will make a sufficiently tight joint when thtf interior finish b applied. Such a lap, however, without back ing would not be adequate. Barriers attached to the warm side of insulation should form a continuous unbroken mem brane over the entire insulated area. Edges should be lapped over framing members; ends of strips should be fastened by lapping over plates or headers. Avoid stapling insulation flanges to the sides of the framing. If thb-must be done, a separate vapor barrier, such as polyethylene plastic sheeting should be installed over the entire wall for best results. All openings for electrical fixtures and joints around window, and door carings should be carefully sealed. Holes accidentally made in the barrier should be sealed.. 2. Ventilation of Living Space.. The second, measure, listed for the control of concealed condensation b ventilation of the