Document 67mkZjLV97Vn82k20g69avXR

208 CHAPTER 9 .1952 Guide 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 view point. Sheet barriers,often contain asphalt as the vapor resisting ingredient; metal foils, so,placed that, they are not too cold, may also be used. : The interior wall board or finish material may itself be vapor resistant, or .a barrier coating-may be applied to its concealed side, when that-side will not be too cold. The interior finished surface may be coated with a suit able paint having the required -vapor resistance and also1 s2e*rv*in* g* *a*s*t1h0e decorative finish, or it may be covered by another coat. A paint coat on the interior finish, though of adequate resistance, is not likely to be so effective as a sheet barrier properly applied during the wall construction. This applies especially to houses of more than one story having cavities in ceilings which open into the outside walls. Such cavities allow vapor entering.Jhe ceiling to diffuse or be transported to the cold areas. Stoppage of this path is difficult, requiring normally the painting of the ceiling as well as the walls. Similar treatment niay be required on internal partitions, or at least the first stud space adjacent to the cold wall. The pecessary barrier resistance depends on a number of factors. When the vapior flow, occurs in annual cycles as in heated buildings, the require ment is not so exacting as it is for a cold storage room where there is no chance for drying out an accumulation of moisture.' in a heated structure covered-on-theoutsidewith-materialshighly-resistant-to_water_vapor_such as paint or roll roofing, the winter season is a time of moisture accumulation in the cold outer elements and their safe moisture holding capacity is an important factor in determining the barrier requirement. A house without sheathing requires a better barrier; and a prefabricated design with only a sheet of metal outside of insulation requires very high barrier resistance. The interior vapor pressure and the length and severity of the winter are also important. For typical frame dwellings with wood sheathing and siding in the north ern United States, a barrier permeance of one perm or less has been found satisfactory. There are cases, however, in residential construction where a one perm barrier would not be adequate and there are also many industrial applications in' which a very much higher vapor resistance is required. In any event, the choice of an adequate barrier implies that its permeance be definitely established. The usually accepted test procedure for this pur pose is a dry method at a temperature of 70 F to 80 F. If obtainable at reasonable cost (including good application), a barrier better than required should be chosen for any construction; Despite the theoretical possibility of safely discharging some vapor through a wall; a higher than minimum permeance is not preferred. An exact statement showing which buildings require a vapor barrier is not readily formulated. However, in view of the distressing results its omission may bring, it is tentatively recommended that the walls of every well constructed modern dwelling include a vapor barrier when the con struction includes any material-that would be damaged by moisture or its freezing. This applies to all condensation zones when the U value for the wall is lower than 0.25 Bto per (sq ft) (hr) (F deg), arid 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 which should be followed. First, the vapor barrier should be placed as near to the warm surface of the wall as practicable. Second, it Heat Transmission Coefficients of Building Materials 209 should be continuous with no direct openings through the barrier. Good workmanship 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 different temperatures, leaves a path for thermosyphon air rotation which will transport large amounts, of water vapor from the warmer space to the colder. If a membrane barrier is 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 two-inch'lap over a framing member will make a sufficiently tight joint when the interior finish is applied. Such a lap, however, without backing would not be adequate. Barriers attached to the warm side of insulation, should form a continuous unbroken membrane over the entire insulated area. Edges should be lapped'over framing members; ends of strips should be fastened by lapping over plates or headers. All openings Fig. 10. Water Vapor Balance in a Dwelling (Vapor Barrier, 1 perm; Wall and Ceiling Area 2000 sqft Insulated) for electrical fixtures and joints around window and door casings 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 is ventilation of the house. This meiasure is obviously necessary as an accompaniment to a vapor barrier since, if the barrier blocks entrance into the walls, the water vapor must be removed by other means. No great volume of air change is necessary, however, and normal infiltration alone is frequently all that is required in winter weather. The effectiveness of ventilation is shown in Fig. 10, which also shows the small amount of water vapor escaping into the barrier equipped, well, in sulated walls and ceilings (2000 sq ft) of a typical small dwelling, the floor being neglected. Evidently, ventilation of 2000 cu ft per hr will remove 21 lbs of vapor per day with the relative humidity at 40 percent, while at the same time 1.5 lbs escapes into the structure. The total vapor production (22.5 lb) is a typical amount. Double glass will be barely safe from visible condensation as shown in Fig. 6. By reference to Chapter 6, 2000 cu ft per hr appears to be near the minimum for odor control and ventilation would have to be higher when cooking is done. By reference to Chapter 10, it appears that usual infiltration will normally supply the necessary air change, but that supplementary ventilation may be necessary in kitchen