Document KRqmRBqeBXJV6gwKO4BQZ4V40

214 CHAPTER 10 1954 Guide 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. 4 a ceiling 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 warm air from the dwelling, transporting much vapor to a danger area. More desirable ventilation of the house can be arranged. Crawl spaces under dwellings where the earth is damp and uncovered require a high rate of ventilation. At least four openings, one at each Table 3. Recommended Good -. P_e_a_c_ti_c_e_s-_L_o_ft__a_n_d'_A_t_t_ic__V_e_n_t_il_a_ti_o.n*. ;.' Flat Roop--Slope Less than 3 Inch^ in 12 Inches , -------------------------------------------- .---------- ;---------------------------- : ----.--. ------------- -- ------------- ------------ : ;----------- ;--------------------------:------------------- Condensation Zone Ic: Total net area of ventilation shouldbe Hoothb 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 Root--Slope over 3 Inches in 12 Inches Condensation Zone I: Total net area of at least 2 louvers on opposite sides located near ihe ridge to be J$othb - CopnludsenasavtaiopnorZobnaerrIieIr: iSnatmhee vteonptislatotiroyncaesilfinogr.Zone I. A vapor barrier i_s not considered necessary. Condensation Zone III: Same as for Zone II. Hip Roop Condensation Zone I: Total net area of ventilation should be J4oothb.with Jioothb distributed uniformly at the eaves arid Jiooth located at the ridge with all spaces interconnected. A vapor barrier should also be used in the top story ceiling. v Condensation Zone II: Same ventilation as for Zone I. A vapor barrier is not considered necessary. Condensation Zone III: Same as for Zone II. 15* Cable ob Hip Roop--With Oochpanct Contemplated :! Condensation Zone I:,Total net area of ventilation shoud be Hoothb with Hootbb distributed uniformly a( the eaves and Hoothb located at the ridge with all spaces interconnected. A vapor barrier should also be usee on the warm side of the top full story ceiling, the dwarf walls, the sloping part of the roof, and the attic j> story ceiling. -$ Condensation Zone II: Same as for Zone I. -'i' Condensation Zone 111: Same as for Zone 1 except that a vapor barrier is not considered necessary if insula* tioa is omitted. B It is reoognieed 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. c The zone numbers refer to Fig. 4. comer, as high as possible, should be provided.8 Their total net area ma be calculated by the formula: _ 2/, A_ = 100 + 300 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 ventilation is usually sufficient but cools the first floor so much th insulation is needed. A better treatment is a cover on the damp groun Water Vapor and Condensation in Building Construction 215 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 IQ 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. 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 a barrier 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 heat^i, 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, r "v.eVer' ^ lined with cement or other vapor permeant material, will allow snght amounts of vapor to pass and to that extent reduce the accumulation i 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