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214 CHAPTER 10 1953 Guide . Table 3. Recommended Good Pbactice6-Loft and- Attic Ventilation* Flat Roof1--Slope Less than 3 Incheb in 12 Inches Condensation Zone I: Total net area of ventilation should be Hoothb distributed uniformly at the eav 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--Slope over 3 Inches in 12 Inches Condensation Zone I: Total net area of at least 2 louvers on opposite sides located near the ridge to be $4oothb 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 Hoothb with *6oothb distributed uniformly at the eaves and ^6ooth 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 or Hip Roof--With Occupancy Contemplated Condensation Zone I: Total net area of ventilation shoud be Hoothb with ^6oothb distributed uniformly at the eaves and Hooth located at the ridge with all spaces interconnected. A vapor barrier should be used on the warm side of the top full story ceiling, the dwarf walls, the sloping 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 insula* tion is omitted. * It is recognised that in many areas increased ventilation may be desirable for summer comfort. For winter comfort, insulation is recommended between a living space and a loft or attic ventilated at these rates. b Refers to area enclosed within building lines at eave level. comer, as high as possible, should be provided.5 Their total net area may be calculated by the formula: 2L A_ 100 + 300 (6) where: L -- the perimeter of the crawl space, linear ft. 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 is used), square feet. This ventilatiqn 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 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 6. In building walls, cavity ventilation can be applied in a moderate climate as the sole vapor control system. In general, 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 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 defined air passage with an inlet and outlet. Water Vapor and Condensation in Building Construction In walls a small thermosyphon effect may be utilized by locating one vent at the bottom and one at the top of each space. The best time to vapor-proof a building is during its construction. 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 permeance can be applied as albarrier on the interior with good results, 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 sup plement the foregoing measures. When!; such venting is required, each cavity space isolated by framing should be separately vented with an inlet and outlet judiciously placed, to accomplish proper air change. CONDENSATION IN COOLED STRUCTURES Water vapor is sometimes an even greater problem in cooled structures than in those which are heated, but the basic facts of its migration and condensation on cold surfaces are the same. Refrigerators, cold pipes and cold vessels all require insulation and should be provided, with a vapor barrier. The barrier, as always, should be placed on the warm side 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 its 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 uninterrupted for years and there is no chance for vapor that enters the insulation to dry out periodically. Also, no vapor can escape from the cold side, if this is a metal (vapor tight) pipe. For such an appli cation vapor control requires insulating material that is itself very highly resistant to water-vapor or a 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 con structed inside a heated building or as a separate building. In the latter case, cold rooms operating above freezing provide some periods of vapor reversal in winter but such drying can be of little help. Refrigerators, however, if lined with cement or other vapor permeant material, will allow slight amounts of vapor to pass and to that extent reduce the accumulation of moisture that may have penetrated the barrier. While this is helpful, emphasis must be placed on an adequate warm side barrier not greater than 0.1 perm. Summer air cooling for comfort does not involve serious vapor problems and vapor control measures are not essential. Normally the cooled air is little, if any, colder than the dew-point temperature of the outside at mosphere and there are no areas of condensation. However, the interior vapor pressure is often below that outside and therefore, vapor, which diffuses inward, adds to the cooling load. Vapor barriers in walls installed for winter needs, are a help in reducing this element of the cooling load. REFERENCES 1 Research in Homs Humidity Control, by S. C. Hite and J. L. Dray (Purdue University, Engineering Experiment Station, Research series No. 106, November 1946). * Permeance Measurement Improved by Special Cell, by F. A. Joy and E. R. Queer (A.S.H.V.E. Trans actions, Vol. 55, 1949, p. 377). * Water Vapor Transfer Thrpugh Building Materials, by F. A. Joy, E. R. Queer and R. E. Schreiner (Penneylsania State College, Engineering Experiment Station Bulletin No. 61, December 1948).