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212 CHAPTER 10 . 1956 Guide ure curve, thereby confirming that the critical plane for condensation was correctly assumed. '. , With the vapor pressures thus established, the relative humidities may be found, by reference to the saturation vapor pressures. The permeances originally assigned to the various elements may then be re-examined in the light of the service conditions of temperatures and relative humidities indicated, ana the analysis repeated, if necessary, using more appropriate permeance values. In a more detailed analysis, individual values might be assigned to the elements forming the outer portion of the wall, which is here dealt with as a composite, homo geneous element. The transmission of water vapor as outlined is based on the assumption of a diffusion process. The possibility of vapor being transferred as part of a moving air stream has . thus far been ignored, except in Example 1 ,in which it is implied that the air circulation on either side of the wall will be sufficient to eliminate surface film resistances to vapor flow. Differences .in total pressure of the air may result in a transfer of vapor with air, aug menting and at times over-riding the effects of the flow produced by vapor pressure gradients alone. This can be particularly important in the trans fer of vapor through cracks and pinholes or through air-permeable building constructions. This means of vapor transfer is similar to that of transfer off heat by air leakage in and through building constructions, requiring for purposes of calculation information on the nature and amount of the air leakage. It will seldom be important in constructions without air spaces and having parged or plastered surfaces. It may, however, be an important means of vapor transfer through constructions lacking in air tightness, and may contribute to condensation difficulties, since the mechanism of condensa. tion is not dependent upon the way in which the vapor is transferred. PERMEANCE AND TESTING The simplest method of finding the permeance of a specimen is to seal it over the top. of a cup containing desiccant or water, placing it in a con trolled atmosphere, and weighing it periodically. The steady rate of weight gain or loss is normally the water vapor transfer. When the cup contains a desiccant the procedure is called the dry-cup method and when the cup contains water, the wet-cup method. ' Usually the outside atmos phere is held at 50 percent relative humidity, thus providing, in either method, substantially the same difference of vapor pressure, but the results obtained by the two methods on the same specimens are likely to be much different, the wet cup method producing the higher values. The relationship between these values can best be understood by reference to Fig. 3, which shows a typical variation of spot permeability with relative humidity at one particular temperature (isothermal conditions) for a material such as wood. The vapor permeability is shown to vary only moderately at low humidities, but to increase at an increasing rate as higher humidities are reached. The dry-cup test of this material earned out with 0 percent relative humidity one side and 50 percent on the other, will experience throughout its thickness, because of the variation in relative humidity, a variation in spot permeability. The average permeability [\ip p, is by definition (Equation 3), given by , and since at a fi*^ temperature there is a linear relationship between vapor pressure and relative humidity, this expression can be seen to correspond to the mean . height of the area under the spot permeability curve, between the appr' Moisture in Building Construction ; Table 1. Permeance and Permeability op Materials to Water Vapor Material Azs (still) INSULATION Cellular glass Corkboard Corkboard Structural Insulating Board (vegetable, uncoated) Mineral Woo) (unprotected) . Wood Sugar Fine Plywood (Exterior type 3 ply D.F.), M in. Plywood (Interior type 3 ply D.F.), H in. Masonry Concrete (1:2:4 Mix) Concrete (3* cored block wall; limestone egret.) Brick wall--with mortar--4 in. Tile wall--with mortar--4 in. Intsrior Finish Plaster on wood latb Plaster on metal lath--H* Plaster on plain gypsum lath (with studs) Gypsum wall board--plainin. Insulating wall board (unedsted)--H in. * Paint--2 coats Asphaltic paint on plywood Aluminum in vanish on wood - Enamels, brushed on smooth plaster Primers or Seeders on insulating wall board Various Primere 4-1 coat flat paint on plaster ' Flat paint (alone) on insulating wall beard Water Emulsions on insulating wall board * --Exterior, 3 coats White lead <ft od prepared paint on wood siding White lead-sine oxide & linseed oil on wood 213 Ref.J BuitDiya Fatehs and Frltb Duplex sheet, asphalt frwifa**, aluminum fou one side Saturated and coaled felt heavy roll roofing Kraft apd urpAolt laminae. Reinforced_30-120~30 Insulation back up, asphalt-sat,, one side glossy Asphalt-saturated and coated sheathing paper AsphalVsaturated sheathing paper 15-pound asphalt felt 15-pound tar felt Single sheet Kraft, double infused _D_e_s_cr_ipt_io_n_is_a_g_ui_de_o_nly_,- _an_d_d_oe_s_n_ot_in_s_urei 16permeance. . Methods: d--dry cup; w--wet cup; t--two temperatures; b--special cell: v--air velocity both side#; ^average of lour methods. of re.ac^r' ^*.T ak includes Bulletins 22 and 25 of the Engineering Experiment Station, University Stqjj Ccttc^t ^ includes data to be published by the Engineering Experiment Station, The Pennsylvania th13^ relative humidity limits. The average permeability as found for ,!*e "ry-eup conditions should therefore have the yalue pi . -'.'Similarly r the wet-cup test between 50 percent and 100 percent relative humidity, w<wiUe skuld be pi. It is not uncommon for these values for wood and 0a-fiber materials to be in the ratio of 1 to 3, or higher. (See Table 1.) average permeability p for any other relative humidities at a parcurv JemPerai'ure is given by the .mean height of the' area under the rve f spot permeability for the material at that temperature, between