Document Lo4v8DNEKMXXaXBdJg3yD70G7

`224 CHAPTER 10 1958 Guide .1 - { 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 iof temperatures and relative humidities indicated, and 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 of 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 meansof 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 tobe 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) fore, material such as wood. The vapor permeability is shown to vary onlfe moderately at low humidities, but to increase at an increasing rate J? higher humidities are reached. The dry-cup test of this material carried > out with 0 percent relative humidity one side and 50 percent on the others will experience throughout its thickness, because of the variation in relative humidity, a variation in spot permeability. The average permeability fr` I pdp ' r> .iff'. p, is by definition (Equation 3), given by _ , and since at a temperature there is a linear relationship between vapor pressure a^.;> relative humidity, this expression can be seen to correspond to the m*?,,:. height of the area under the spot permeability curve, between the appfO" 1 Moisture in Building Construction 225: Table 1. Permeance and iPermeability of Materials' .' \ to Wateb 'YApob ! '. M-ATEBIAL . -1 Aib (still) InCseolllualtaior nglass Corkboard . Cork board Structural InsulatingBoard (vegetable, uncoated) Mineral Wool (unprotected). WSouogdar Pine * Plywood (Exterior type 3 ply D.F.), )4 in. Plywood (Interior type 3 ply D.F.), fi in. Masonbt Concrete (1:2:4 Mix) CBorincckrewteal(l8--*wcoitrhedmbolrotackr--w4ailnl,.limestone agrgt.) Tile wall--with mortar--4 in. Intbriob Finish Plaster on wood lath Plaster on metal lath--$4* Plaster on plain gypsum lath (with studs) Gypsum wall board--plain--% in. Insulating wall board (uncoated)--% in. Paint--2 coats Asphaltic paint on plywood Aluminum in varnish on wood Enamels, brushed on smooth plaster Primers or Sealerson insulating wall board 1 Vanous Primers + 1 coat fiat paint on plaster Mat paint (alone) on insulating wall board' Water Emulsions on insulating wall board * --Exterior, 3 coats 5W?h'ittee lead-zinocilopxirdeepa<rfceldinpseaeindt ooinl ownoowdoosidd"ing i:/ Per: " Id BANCS 'Perm' ' Permea-! BILITT , Perm-inch RHi-RHi wV jlHrQ ^ HO' s Rbe.J 120. ; ' 92-73 ! ' ubv'`; ;; 3 o:o . 2.1-2.6 9.5 20^50 .116. . .>-:75-0:> .100-46 ,s' ; 40-x . .100-30. . wj d> : t- ,w 4-: ' ` .. : 97 . -6 0.72 1.86 6.4-5(4 various 5050- tv 4 4 3 11 11 2.4 0.8 0.12 11. 15. 20. 50. 50-90 0.4 0.3-0.5 0.5-1.5 0.9-2.1 1.64.-3. 30.-85. 3.2 f ijj r i r 100-45 79-68 50-x 50-x w t t t 100-30 40-x 40-85 50-20 40-x w t t V t 100-30 95-0 92-0 40-x 40-x 40-x 40-x w d b t 't t t 9 3 8 8 67 3 13 7 6 10 3 7 7 7 7 0.3-1.0 0.9 . 50-0 d 13 95-0 d | 10 Boilding Papers and Felts Duplex sheet, asphalt laminae, aluminum foU one and,'=<ffelt heavy roll roofing Id-pound asphalt felt ^ 15-pound tar felt Single eheet Kraft, double infused per 500 sq ft Permeance-Perms dry cup wet cup .43 326 34 31 43 22 70 70 . . 16 . 0.002. ' 0.05 0.3 0.4 0.3 3.3 1.0 4.0 . ' 30.8 ' . -0.176 0.24 1.8 0.6-4.2 0.6 20.2 ` >6.6 . 18.2 , ' 41.9 12 12 12 12 12 12 12 - 12 12 --------------- o guae t Methods: d--dry cud- 4-average 0f f,VoUuIr muieeintohaSs j, quu uug uuii insure permeance. we^ CUP *--*wo temperatures; b--special cell; v--air velocity both sides; ^Afinnraota^of^'indlfdM1)?^? SnUetins 22 and 25 of the Engineering Experiment Station, University state College. '"niuaea data to bo published by the Engineering Experiment Station, Tie Pennsylvania the humidity limits. The average permeability as found for for theywPtPo,,C"ns should therefore have the value mi Similarly the value shnMt between 50 percent and 100 percent relative humidity, wood-fiber Tnt - i **2' ^ *s n0*1 uncommon for these values for wood and The mafcena s to he in the ratio of 1 to 3, or higher. (See Table 1.) ticular temruP Perm?ahility p for any other relative humidities at a parcurve of snotrat,ure 1.s. given by the mean height of the area under.'.the p permeability for the material, at that temperature., between