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414 CHAPTER 23 1965 Guide And Data Book . (Vapor tarrimr, I pmratf Wall and Ceiling Area 2000 iq ft Insulaledl Fig. 7 .... Wafer Vapor Balance in a Dwelling house. This measure is obviously necessary as an accompani ment 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 with outdoor air is shown in Fig. 7, which also shows the small amount of water vapor es caping into the barrier-equipped, well-insulated walls and ceilings (2000 sq ft) of a typical small dwelling, the floor being neglected. Evidently, ventilation of 2000 cfh will remove 21 lb of vapor per day with the relative humidity at 40 percent, while at the same time 1.5 lb 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 be seen in Fig. 4, while triple glass will prevent visible conden sation. By reference to Chapter 7,2000 cfh appears to be near the minimum for odor control, and ventilation would have to be higher when cooking is done. By reference to Chapter 25 it appears that usual infiltration will normally supply the necessary air change, but that supplementary ventilation may be necessary in kitchen and laundry for proper vapor control and for the reduction of peaks in relative humidity that would otherwise occur in those areas. 3. Ventilation of Structure. The third measure listed for the control of concealed condensation is ventilation of the struc ture itself. It is effective in certain cases especially as a supple ment to warm-side vapor resistance which is considerable but not of itself fully adequate. Air from outdoors is used. The vents must be shielded from the entrance of run and insects. Attics and crawl spaces may be considered as parts of the structure, and for these portions ventilation is practically a ' necessity. Attic ventilation has long hap.n an established prac tice but its effectiveness is likely to be diminished by the newer practice of adding insulation to ceilings. Insulation re quires added ventilation which in turn necessitates adequate insulation. The recommended ventilation shown in Table 3 for dwellings1* is based on such insulation. The net area refers to the total of all openings free from obstructions. The use of louvers and 8-mesh screen (usually recommended) requires a gross area 2.25 times that listed. In Zone I of Fig. 6 a coiling vapor barrier is recommended for all constructions. It is also necessary that stray openings from walls into the attic, - or around a loose fitting attic door be avoided. The stack effect ' allows a large inflow of w&rm'air from the dwelling, transport ing much vapor to a danger area. More desirable,ventilation of the house can be arranged. Table 3 ,.. Recommended Good Practice1* for Loft and Attic Ventilation4 Flat Roof--Slope 3 ladies in 12 laches or less Condensation Zone I*: Total net area of ventilation should be Vmth* 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 os for Zone I. Cable Beef--Stop* ont 3 Inches fa 12 lnefai Condensation Zone I: Total net area of at least 2 louvers on opposite sides located near the ridge to be Vnothb 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 Boof Condensation Zone I: Total net area of ventilatioo should be Vnsthb with Ycuthb distributed uniformly at the eaves and Ycethh located at the ridge with all spaces interconnected. . A vapor barrier should also be used in the top story wiling Condensation Zone II: Same ventilation as for Zone. I. A vapor barrier is not considered necessary. Condensation Zone III: Same as for Zone II. . Gabfa or Hip Hoof--With Ot Condensation Zone I: Total net area of ventilation should be Yicothb with Ywathh distributed uniformly at the eaves and Ythb located at the ridge with all spaces interconnected. A vapor barrier should also be used on the warm side of the top full story ceiling, the dwarf walls, the lnping part of the roof, and the attic story ceiling. Condensation Zone II: Same as for Zone I. Condensation Zone III: Same as for Zone I except that a vapor barrier is not considered necessary if insulation is omitted. * It b racoenbed that fa many Area* increased veotOatfao may be riririraTilr for summer comfort. For winter comfort, fa recommcoded between e tiring space and a toft or attic rentOated at these ratec. b Refera to area enrlnaert within bcilding line* at cave teveL * The aoae cumbers refer to Ffa. 6. Crawl spaces under dwellings where the earth is damp and uncovered required a high rate of ventilation. At least four openings, one at each comer, as high as possible, should be provided.1* Their total net area may be formula: by the 2L A 100 + 300 (10) tohere L = the perimeter of the crawl space, linear feet. A - the area of the crawl space, square feet. ` a = the total net area of all vents, (or the gross area if a 4-mesh screen ts used) square feet. This ventilation is usually sufficient but cools the first floor so much that insulation is needed. A better treatment is a cover on the damp ground. This cover may be a concrete slab, or merely heavy roll roofing or 0.004-0.006 in. thick polyethylene plastic'film laid on a graded surface with its edges lapped 2 in. (but not necessarily, cemented). With this barrier, the vent area may be reduced to 10 percent of that calculated by Equation 10. In building walls, cavity ventilation can be applied'in a' moderate climate as the sole vapor control system. However, air passages in walls designed to remove an unrestricted vapor supply are unduly Large and may waste considerable heat. On the other hand, a barrier as the only control measure would, . in some cases, require so High a resistance as to be impractical. Ventilation of the structure in conjunction with a vapor. Moisture in Building Construction 415 barrier, is a procedure with important applications, but its general utility has not been fully investigated. Ventilation is most effective when each structural space has a clearly de fined air passage with an inlet and outlet. In walls a small convective effect may be utilized by locating one vent at the bottom and one at the top of each space. The best to vapor-proof a building is during its con struction. After a building is completed, ventilation of the occupied space is the most easily applied of the three basic control measures. Paint that is chosen for its low vapor per meance can be applied as a barrier on the interior with good reults, care being taken that all areas, including parts of partitions and ceilings which offer an indirect vapor path to the cold wall, are covered. Ventilation of the wall cavity is effective in certain cases especially to supplement the fore going measures. When such venting is required, each cavity epa^ isolated by framing should be separately vented with an inlet and outlet judiciously placed, to accomplish proper ' bit change. CONDENSATION IN COOLED STRUCTURES Water vapor is sometimes an even greater problem in cooled than in heated structures, but the basic facts of its migration and condensation on cold surfaces are the same. Refrigerators, cold pipes, and cold vessels all require insula tion and should be provided with a vapor barrier. The barrier, as always, should be placed on the warm tide of the insulation and as close as possible to the warm surface of any enclosure. It resists the movement of water vapor toward the colder parts of the structure, and it3 job is even more exacting than that required in residential construction. In the case of an insulated cold pipe line, the process is likely to be uninter rupted for years and there is no chance for vapor that entersthe insulation to dry out periodically. Also, no vapor can escape horn the cold tide, if this is a metal (vapor tight) pipe. For such an application vapor control requires insulat ing material that is itself very highly resistant to water ' vapor or a jacket or coating whose permeance is the minimum obtainable, not over 0.1 perm. Metal coverings are desirable but difficult to apply. Similar - considerations apply in the case of cold rooms whether constructed inside a heated building or as a separate building. In the latter case, cold rooms operating above freeting provide some periods of vapor reversal in winter but such drying can be of little help. Refrigerators, if lined with cement or other vapor permeable material, will allow slight amounts of vapor to pass and to that extent reduce the accumulation of moisture that may have penetrated the barrier. However, emphasis must be placed on an adequate warm-tide barrier not greater than 0.1 perm. The infiltration of air around a vapor barrier can carry considerable quantities of water vapor into the insulation and create a condensation problem. The air find* passages through gaps in joints, tears in the barrier, or throughcracks where pipes or similar items penetrate the wall. The mass flow of air and water vapor moves under the influence-of unbalanced pressures created by temperature differences and wind. These forces are active continuously even though they may be variable. The opening and closing of cold room doors causes a pumping of air which also includes water vapor. These actions require that holes and cracks be kept to a mini mum and that extreme care be exercised in installing the vapor barrier and in sealing it around pipes and lines which penetrate the cold room wall. Summer air cooling for comfort does not normally create senous vapor problems in exterior walls and ceilings. Normally the cooled air is little, if any, colder than the dew point of the outdoor atmosphere and there arc so areas cf condensa tion. Outdoor dew-point temperatures, especially peak values, may exceed the design wet-bulb temperatures in common use, but these will seldom be greatly in excess of 75 F for any pro longed period of time. Condensation within exterior walls which are exposed to indoor at 75 F will seldom be as serious as for the winter condition. A vapor barrier to prevent condensation within exterior walls under summer cooling conditions would normally be located on the outside of the insulating, whereas the location for the winter conditions is on the inner side. The use of vapor barriers at both locations is not without difficulty because they not only restrict the entry of moisture into the insula tion but also restrict the escape of any moisture. It is generally considered best in dwelling construction to locate the vapor barrier for the more serious case for condensation in winter, and to disregard the summer case even when some condensa tion may be expected. The corresponding situation in coldstorage buildings in which there may be a much more serious reversal of vapor flow conditions from winter to summer poses a wall design problem not yet fully resolved. Some observations made by the National-Research Council of Canada, but not yet publicly reported, would indicate that serious wetting within walls can occur in summer under special conditions. The walls of test huts of brick masonry, but finished inside with furring, insulation, vapor barrier, and plasterboard interior finish, were opened during a sunny period that followed a rain. Extensive wetting was observed in the insulation, aQd particularly on the' back of the vapor barrier. The bride wall which was unusually absorptive bad taken on substantial quantities of water during the period of rainfall. Subsequent heating by the sun had driven the moisture as vapor into the wall where it condensed and caused serious wetting. The construction in question did not have protection for the strapping and insulation in the form of parging or paper on the inside of the brick. Walls having absorptive exterior coverings capable of absorbing and storing considerable quantities of water during a rain' and providing little resistance to vapor flow into the insulation from the outdoors, may experience serious interior wetting by condensation under such unusual conditions. No wetting occurred in a construction similar to that described above when a saturated sheathing paper was used between the insulation and the brick. It would appear that the provision of moderate resistance to vapor flow such as that provided by parging or a good exterior grade sheathing paper on the outside of the insulation can be effective in such cases. Vapor which diffuses inwards under cooling conditions adds to the .cooling load. Vapor barriers in walls installed for winter conditions are a help in reducing this. However' if condensation does occurs on the back of a vapor, barrier installed on the inside of the insulation, the latent heat which is then released on the inside of the insulation may add to the ^ heat gain through the wall. REFERENCES 1 S. C. Hite and J. L. Dray: Research in Home Humidity Control (Purdue University, Engineering Experiment Station Research Series No. 106, November 1948). *8. C. Chang and N. B. Hutcbeon: Dependence of water vapor permeability on temperature and humidity (ASHAE Transactions, Vol. 62, 1956, p. 437). 1F. A. Joy and E. R. Queer: Permeance measurement im proved by special cell (ASHVE Transactions, VoL 56, 1949, p. 377). * F. A. Joy, E. R. Queer, and R. E. Schreiner: Water Vapor Transfer Through Building Materials (Pennsylvania State College, Engineering Experiment Station. Bulletin No. 61, December 1948).