Document QkJxd3grKqJOkMaEDkyY1aK7k

;212 CHAPTER 10 1954 Guide . 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 haying cavities in 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 this path is difficult, requiring normally the painting of the ceiling as well as the walls. Similar treatment may be required on internal partitions, or.at least the first stud space adjacent to the 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 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 the outside with materials highly 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 he 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. i 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 i freezing. This applies to all condensation zones in Fig. 4 when the U vahMjv for the wall is lower than 0.25 Btii per (sq ft) (hr) (F deg), and it appliesT, 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 b; 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; 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 barrieris used back of the plaster or interior finish, its joints should be made ov#t 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 Oft; insulation should form a continuous unbroken membrane over the entire - insulated area. Edges should be lapped over framing members; ends P*/ Water Vapor and Condensation in Building Construction 213i strips should be fastened, by lapping over plates or headers. All openings, 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 measure is; obviously necessary as an accompaniment to a vapor barrier since, if thebarrier 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. 5, 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 will he seen in Fig. 1. By reference to Chapter 6, Fig. 5. Water Vapor Balance in a Dwelling (Vapor Barrier, 1 perm; Wall and Ceiling Area 2000 sq ft Insulated) 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 11, it Appears that usual infiltration will normally, supply the necessary air change, but that supplementary ventilation may be neces sary in kitchen and laundry for proper vapor control and for the reduc tion of peaks in relative humidity which would otherwise occur in those areas. 3. Ventilation of Structure. The third measure listed for the control of concealed condensation, ventilation of the structure itself, is effective in ^tain cases especially as a supplement to warm side vapor resistance w"ich is considerable but not of itself fully adequate. Air from outside is used. The vent--s--m--u--s--t--be Nsh.UieV.lUdUeUd fruoimu tmhiep oennptriaainivc^ec Lo/if irtauinii aannad ninusseeccttss.. frwv?08 anc* craw* spaces may be considered as parts of the structure, and , r these portions ventilation is practically a necessity. Attic ventilation kslong been an established practice but .its effectiveness is likely to be furnished by the newer practice of adding insulation to ceilings. In- insi 1 011 re<iu*res added ventilation which in turn necessitates adequate. Elation. The recommended ventilation shown in Table 3 for dwellings5