Document Eq1NZdEB047DyoG5qGZKqN4E0

. j: i, \ ' t} \ : I'; '',f `M ; * .3i ; !r II !l ', *ij; ! iH> 208 CHAPTER IQ,. 1954 .Guide HI Table 2. Permeance and P^eability of Materials to Water Vapor Air (still) Insulation Cellular glass Corkboard CSotrrukcbtouarardl Insulating Board (vegetable, uncoated) Mineral Wool (unprotected) Wood Sugar Pine (see Fig. 2) Plywood (Exterior type 3 ply D.F.), }i in. Plywood (Interior type 3 ply D.F.), fi in. Masonry CCoonnccrreettee ((1S:'2c:4oreMdixb)lock wall, limestone agrgt.) Brick wall--with mortar--4 in. Tile wall--with mortar--4 in. Interior Finish Plaster on wood lath Plaster on metal lath-- Plaster on plain gypsum lath (with studs) Gypaum wall board--plain--H in. Insulating wall board (uncoated)--H in. * Paint--2 coats Asphaltio paint on plywood Aluminum in varnish on wood Enamels, brushed on smooth plaster Primere or Sealers on insulating wall board Various Primers 4-1 coat flat paint on plaster Flat paint (alone) on insulating wall board Water Emulsions on insulating wail board Paint--Exterior, 3 coats White lead & oil prepared paint on wood siding White lead-sine oxide & linseed oil on wood * Building Papers and Felts Duplex sheet, asphalt laminae, aluminum foil one Sastiduerated and coaled felt heavy roll roofing Kraft and asphalt laminae. Reinforced 30-120-30 Insulation back up, asphalVeat., one side glossy Asphalt-saturated and coated sheathing paper Asphalt-saturated sheathing paper 15-pound asphalt felt 15-pound tar felt Single sheet Kraft, double infused t* MDeetshcordipst:iodn--isdarygcuuidpe; own--lyw, aent dcudpo;est--ntowt oinsteumreppeerarmtueraensc; eb.--special cell; v--air velocity both sid es;;.^' 4--$avReerfaegrenocfefso. uNrom. e7thaolsdos.includes Bulletins 22 and 25 of the Engineering Experiment Station, Univertil?? of Minnesota. No. 13 includes data to be published by the Engineering Experiment Station, The PennsylvonpYxI State College. building materials showing, in each case, the source and method and, where - appWliacatebrl-ep,rothoefedthibcuknildeisnsgtpeastpeedr.s are listed in Federal Specifications ITU-P-; 147, May 24,1948, according to water vapor resistance required as: 'g; 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 and of water resistance is required. ' CCllaassss DC.. FFoorr uusseess wwhheerree laomworedseirsattaencdeegtroeewoaftewravtaeprorer sisistraenqcueiriesdr.equired. . it}- Water Vapor and Condensation in Building Construction 209. It may be noted that a paper may be .walerrproofyi.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 73 F: 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 invitation for bids. Paper of both strengths shall have a minimum water resistance of 24 hr, and a maxi mum water vapor permeability (WVT) of 4 grams per sqiiare meter per 24 fir, (i.e.1 0.576 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 hi-, (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 teusile 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 ter 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 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 residencies show that the relative humidity increases as would be expected in warmer weather. Fig. 3 represents the results of one such survey.4 When water vapor is allowed to enter a wall and condensation occurs on its outer cold elements, it appears as frost or liquid. If the weather temperature rises frequently, frost melts and becoming liquid, is likely to penetrate capillary materials like wood, or run down when the surface is non-absorbing or is already saturated with water. In weather that is con tinuously cold for a long period, the frost may build back into a cavity or nbTous;insulation and, when it reaches a warmer plane, will run to lower, cooler levels where it forms a mass of ice. Water seepage to the weather side may occur harmlessly in masonry walls when the weather is above breezing but water seepage into the building must obviously be avoided; ? typical frame construction with wood sheathing which has large water absorbing capacity, seepage is rare and occurs only after a long period of steady cold weather. More generally, moisture accumulates in wood deathing and siding through the colder months and reaches a peak in late liter, after which the drying of spring and summer completes the annual