Document NbvZyO83VLr23p1GRQXZkk4p

234 CHAPTER 10 1958 Guide ' " 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 O' ~P. Fig. 7. Water Vapor Balance in a Dwelling (Vapor Barrier, 1 perm; Wall and Ceiling Area 2000 sq ft Insulated) omission may bring, it is tentatively recommended that the walls of every ` iV well constructed modem dwelling include a vapor barrier when the con- ] structiqn includes any material that would be damaged by moisture or its freezing. This applies to all condensation, zones in Fig. 6 when the U value for the wall is lower than 0.25 Btu per (sq ft) (hr) (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 which should be followed. First, the vapor barrier should bci_ placed as near to the warm surface of the wall as practicable. Second, it^fi; should be continuous with no direct openings through the barrier. Goody ,'j workmanship and application are very important. Workmanship thatU y leaves two openings through the barrier, or around its margin, at differentia' levels, connecting air spaces at only slightly different temperatures, 1eaves.i- y a path for thermosyphon air rotation which will transport large amounts of.y*: water vapor from the warmer space to the colder. If a membrane barner.) j.. is used back of the plaster or interior finish, its joints should be made overly some solid framing member, and not between the 3tuds or in similar places,yv- : Usually a two-inch lap over a framing member will make a sufficiently^ by tight joint when the interior finish is applied. Such a lap, however, without?^- backing would not be adequate. Barriers attached to the warm side or, Moisture in Building Construction 235 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 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 the barrier blocks entrance into the walls, the water vapor must be removed Table 3. Recommended Good Practice "`-Loft and Attic Ventilation* Flat Roof--Slope Less than 3 Inches in 12 Inches Condensation Zone Ic: Total net area of ventilation should be %oothb distributed uniformly at the eaves plus a vapor barrier in the top story ceiling. Free circulation must be provided through all spaces. Condensation Zone II and III: Same as for Zone.I. Gable Roof--Slops oveb 3 Inches in 12 Inches Condensation Zone I: Total net area of at least 2 louvera on opposite sides located near the ridge to be }ioothb plus a vapor barrier in the top story ceiling. Condensation Zone II: Same ventilation as for Zone I. A vapor barrier is not considered necessary. Condensation Zone III: Same as for Zone II. , Hip Roof Condensation Zone I: Total net area of ventilation should be )ioothb with Woothb distributed uniformly at the eaves ond )6oothb located at the ridge with all spaces interconnected. A vapor barrier should also be used in the top story ceiling. Condensation Zone II: Same ventilation as for Zone I. A vapor barrier is not considered necessary. Condensation Zone III: Same as for Zone II. Gable ob Hip Roof--With Occupancy Contemplated Condensation Zone I: Total net area of ventilation shoud be Hoothb with )tooth b distributed uniformly a tne eaves and >tooth located at the ridge with all spaces interconnected. A vapor barrier should also be used on tne warm side of the top full story ceiling, the dwarf walls, the sloping part of the roof, and the attic story ceding. Condensation Zone II: Same as for Zone I. Condensation Zone III: Same as for Zone 1 except.that a vapor barrier is not considered necessary if insula* tion a omitted. recognised that in many areas increased ventilation may be desirable for summer comfort. For b comiort, insulation is recommended between a living space and a loft or attic ventilated at these rates. e ^e^ers area enclosed within building lines at eave level. The tone numbers refer to Fig. 6. 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. 7, which also shows the small amount of water vapor escaping into the barrier-equipped, well-in- ated walls and ceilings (2000 sq ft) of a typical small dwelling, the floor emg neglected. Evidently, ventilation of 2000 cu ft per hr will remove 21 sol vapor per day with the relative humidity at 40 percent, while at the (22 5 iM'6 **bs scapes into the structure. The total vapor production crmri a Epical amount. Double glass will be barely safe from visible 2000enSftn 85 w'^ he seen 'n 4. By reference to Chapter 6, ventil01*' ^er aPPears to be near the minimum for odor control, and to Ch& tD Wou.i<i have to be higher when cooking is done. By reference necessR61^*' ^ aPPears that usual infiltration will normally supply the ty 411 change, but that supplementary ventilation may be neces-