Document p2pwNN4V0Yr7Xkp6Z2Qjd81nj

204 CHAPTER 9 1952 Guide per- (unit area) (unit time). Such, data may be called water vapor trans mission data or WVT data and values are either high or low depending on the difference of vapor pressure chosen for the test: When this difference is known, WVT data ban be converted to permeance, care being taken if conversion of'the basic units (weight, area-and time) is also required. -The following formula applies: /^ WVT rating Permeance = ---------- Ap (11) where, WVT rating = weight of vapor transmitted, grains per (sq ft) (hour). . Ap = vapor pressure difference in the test, in inches of mercury. Permeance is expressed in perms.' Table 21 presents tlie conversion factors applicable to the commonly used units and test methods. Table 22 presents some data on typical building materials showing in each case the source and method and, where applicable, the thickness tested. Water-proofed building papers are listed in Federal Specifications UU-P147, May 24, 1948, according to water vapor resistance required as: Class A. For uses where a high degree of water vapor resistance is required. Class B. For uses where a lower degree of water vapor resistance ana of water resistance is required. Class C. For uses where a moderate degree of water resistance is required. Class D. For uses where low resistance to water vapor is required. . It may be noted that a paper may be tvoter-proof i.e. possess water re sistance, and still have low water vapor resistance. Detail requirements in these specifications are given as follows, the specified WVT Test being a dry method at 73F: Class A paper shall have a minimum tensile strength in each direction of either 35 lb per inch width, or 20 lb per inch width, as specified in the inyitation for bids. Paper of both strengths shalf have a minimum water resistance of 24 hr; and a maxi mum water vapor permeability (WVT) of 4 grams per square meter per 24 hr, (i.e. 0.676 perm). Class B paper shall have a minimum tensile strength in each direction of either 35 lb per inch width, or 20 lb per inch width, as specified in the invitation for bids. Paper of both strengths shall have a minimum .water' resistance of 16 'hr, and a maxi mum water,vapor permeability (WVT) of 6.grams per square meter per 24 hr, (i.e. 0.864 perm). Class C paper shall have a minimum tensile strength in each direction of either 35 lb per inch width, or 20 lb per inch width, as specified in the invitation for bids. Paper of both strengths shall have a minimum water resistance of 8 'hr.' ' Class D paper shall have a minimum tensile strength in each direction of 20 lb per inch width.- The paper shall have a minimum water resistance of 10 min., a mini mum water vapor permeability (WVT) of 35 grams per square meter per 24 hr, (i.e. 5.04 perms). Concealed Condensation in Heated Buildings Water vapor produced in a building necessarily raises the Vapor pressure above that outside thus providing' the force that causes its diffusion into exterior walls. The amount of vapor pressure rise in the. building depends on the amount of vapor produced and'inversely on its chance to escape. The resulting balance may be expressed in terms of relative humidity if the inside temperature is 70 F. The relative humidity in heated buildings Heat Transmission Coefficients of Building Materials 205 Table 22. Permeance and Permeability op Materials to Water Vapor Material Air (still) Insulation Cellular glim* Corkboard `- Corkboard ' Structural Insulating Board (vegetable, uncoated) Mineral Wool (unprotected) Wood 'i; Sugar Pine (see Fig. 7) . Plywood (Exterior type 3 ply D.F.), \i in. Plywood (Interior type 3 ply D.F.)* H in. Masonrt Concrete (1:2:4 Mix) ' Concrete (8* cored block wall,' limestone agrgt.) Brick wall--with mortar-r4 in. . . Tile wall--with mortar--4 in. Interior Finish Plaster on wood lath Plaster on metal lath--H* Plaster on plain gypsum lath (with studs) Gypsum wall board--plain--H in. Insulating wall board (uncoated)--H in. . Paint--2 coats Asphaltic paint on plywood Aluminum in varnish on wood Enamels, brushed on smooth plaster Primers or Sealers on insulating wall board . Various Primers + 1 coat flat paint on plaster Flat paint (alone) on insulating wall beard Water Emulsions on insulating wall board Paint--Exterior, 3 coats White lead <fc oil prepared paint on wood siding White lead-sine oxide A linseed oil on wood Per PERMEA meance BILITY* . Perm .; Perm-inch 120. o;o` 2.1-2.6 9.6 20-60. 116.: . 75-0 .100-46 40-x 100-80 0.72 . 1.86 0.4-6.4 various. 50- i 50*. - 2.4 0.8 . 0.12 11. 2105.. 50. 50-90 3.2 100-45 79-68 50-x 50-x 100-30 40-x 40-85 60-20 40-x 0.4, 0.3t0.5 0.6-1.6 0.9-2.1 1.6-3. 4. 30.-85. 100-30 95-0 ' 62r0 40-x 40-x 40-x 40-x 0.3-1.0 0.9 60-0 96-0 b d. -wd..: 1f tv 44 Re7.{ 21 24 : 22 19. 18 26' 26 24 18 23 19 22 18 28 22 19 26 18 22 22 22 22 per 500 sq ft Permeance-Perms dry cup wet cup * Building Papers and Felts Duplex sheet, asphalt laminae, aluminum foil one side Saturated and eooterf felt heavy roll roofingKraft and asphalt laminae. Reinforced 30-120-30 Insulation back up, asphait-sat., one side glossy Asphalt-saturated and coated sheathing paper . Asphalt-saturated sheathing paper 15-pound asphalt felt 16-pound tar felt Single sheet Kraft, double infused' 43 326 34 . 21 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 0.64.2 . 0.6 .. 20.2 ' 5.6' 18.2 41:9 27 27 27 27 .27 27 ;'27 27 27 * Description is a guide only, and does not insure, permeance.. ( .t Methods: d--dry cup; w--wet cup; t--two temperatures; b--special cell; v--air velocity both sides; 4--average of four methods. ` t References. No. 22 also indudes Bulletins 22 and 25 of the Engineering Experiment Station, University of Minnesota. No. 28 includes data to be published by the Engineering Experiment Station, The Pennsylvania State College. covers nearly all of the possible range. In zero weather it may be Only 10 percent in an office, and 85 percent in an industrial plant where humidifi cation is required for a' process, or where vapor release is incidental: to a process. In residences the relative humidity in Cold winter weather ranges from 10 percent to 60 percent, the latter figure applying to a very small, crowded and unventilated dwelling. A 40 percent level is considered representative of a substantial number of modem tightly constructed small' houses although the average house relative humidity is probably below 25 percent. Surveys in residences show that the relative humidity increases