EEM-00259 Permeability of Common Building Material to ...

1 Permeability of Common BuildingMaterial to Water VaporWHAT IS A PERM RATING?If a Material has a perm rating of , we know thatin 1 hour, when the vapor pressure difference be-tween the cold side and the warm side of the materialis equal to 1 inch of mercury (1" Hg), 1 grain of watervapor will pass through 1 square foot of the grain of water is equal to 1/7000 of a pressures depend on the temperature andrelative humidity (RH) of the air as temperatureand RH go up, vapor pressure gets higher. Thegreater the vapor pressure differential across orthrough a Material , the greater the tendency for watervapor to migrate from the high pressure side to thelow pressure OF Material THICKNESSThe perm ratings given are for stated thicknesses ofmaterials.

2 Generally, doubling Material thickness halveswater vapor transmission: if 1 inch of a Material hasa perm rating of , then for 2 inches, the perm ratingwould be With paints, however, adding a secondcoat more than halves the water vapor VAPOR BARRIERSB ecause of Alaska s wintertime vapor pressure differ-entials, and the lengths of the cold spells, the idealvapor barrier has a perm rating approaching Themost widely used vapor barrier is 6 mil polyethylene,which has a perm rating of Given the combina-tion of high RH indoors, and very cold weatheroutside.

3 Measurable amounts of water vapor will passthrough 6 mil polyethylene. For high moisture build-ings, such as those housing swimming pools orJacuzzis, 10 mil polyethylene is often specified. Inpractice, however, it is not usually the perm rating ofthe water vapor barrier which determines how muchwater will pass into the insulation, but the quality ofthe vapor barrier installation. A carefully installed,well sealed 4 mil polyethylene vapor barrier is muchto be preferred to a 6 mil VB with unsealed seams,gaps, tears at electrical boxes and unsealed atticscuttle OF DATAPerm ratings are established by testing, but severaldifferent test methods are used, and different tests onthe same materials yield different results.

4 For thisreason, published perm ratings are not to be regardedas the truth, but as a design guide. When the vaporbarrier performance of a Material is critical to aninstallation, it is best to rely on manufacturer s speci-fications, or conduct an independent test. For anenlightening discussion of this issue, see The AmericanSociety of Heating, Refrigerating, and Air Condition-ing Engineers (ASHRAE s) 1985 Handbook of SAMPLE CALCULATIONFor demonstration purposes, we shall now assume aset of conditions and make a calculation concerningthe amount of water vapor which will migrate througha vapor barrier.

5 The relevant formula is:WVT = A x T x delta P x permsWVT stands for "water vapor transmission."Unit of measurement: grains. One pound equals7000 means "area." Unit of measurement: square feet(ft2). We will assume that this is a two story house, 24'wide by 40' long by 17' high. Area of the vaporbarrier, then, is about 3900 ft2, allowing 10% offloor area for doors and IN ALASKAEEM-00259T means "time." Unit of measurement: hours. Wewill perform this calculation for the month of Janu-ary, which has 744 P means "difference in vapor pressurebetween inside and outside.

6 " Unit of measure-ment: inches of mercury (" Hg). In order to derive thisnumber, we must assume a temperature and arelative humidity (RH) for both inside and out. Let theinside temperature be 70 , and the outside -10 ,which is about average for January in Fairbanks. Letthe inside RH be 40%, and the outside be 70%. TheTable of Vapor Pressures gives figures for saturatedair (100% RH); to get the vapor pressure at an RHless than 100%, you multiply by the percent RH. In thiscase, (.7392 x 40%) - (.022 x 70%) =.

7 2803 " means "perm rating."Unit of measure-ment: grains of water vapor per hour per square footper inch of mercury vapor pressure differential (gr/ft2/hr/" Hg). We will assume that the average permrating of the installation (including unpatched tears,nailholes, etc.) is these assumptions, then:WVT = A x T x Delta P x Perms3900 x 744 x .2803 x = 81332grains of water. This is pounds, or about Fin Hg Fin Hg Fin Hg Fin HgTABLE 1 VAPOR PRESSURES FOR SATURATED 2 PROPERTIES OF WEATHER BARRIERS AND Building PAPERSP roductTyvekAirtight-Rufco-WrapTu-TufBarr icadeConventionalNamehousewrap,WrapRaven Air Seal,Building15-lb asphalt-DupontPresec, feltTypeSingle bondedMicro-perforated cross-laminatedSpun-BondedProductPolyeth ylene high-density polyethylene filmpolyester(1)(2,3,4)(5)(6)(1)(2)(3)(4 )(5)(6)Permeance (perms)ASTM E-96 Method A9 4{ }{ } [ ]Method B103{ }{ } (mils)

8 (lb/1,000 ft2) rollsdimensions (ft) (3x100)8x195 Tensile strength, (lbs)ASTM resistance (lbs)ASTM [ ][ ][ ][ ][ ] [ ][ ][ ][ ][ ]Air porosity (seconds)Gurley [ ][ ][ ] [ ]Water resistance (cm)AATCC Method [ ][ ]NOTES:1. Extrapolated from, "Wrap Wars," Alex Wilson, New England Builder, August Values in brackets, [ ], obtained from DuPont literature on Values in brackets, { }, low because film installed backwards (worst-case situation).4. All other values obtained from company literature or personal communication with Perm: Vapor transmission rate of 1 grain of water vapor/ft2/hr/in.

9 Of mercury pressure The data are provided to permit comparison of Material and selection of air-vapor barrier or weather barrier Exact values should be obtained from the manufacturer of A continuous air-vapor barrier is usually placed on the warm side of the shell before the gypsum board (drywall) is The air-vapor barrier should have a permeance of less than 1 The weather barrier should have a high permeance, a low water resistance, a high air porosity, high tensile strength, and hightear The compilation is from a number of sources; values from dry-cup and wet-cup methods were usually obtained from investigationsusing ASTM E96 and C355.

10 Other values were obtained using techniques such as two-temperature, special cell, and air : Axel R. Carlson, Professor Emeritus, University of Alaska Fairbanks, Fairbanks, Alaska, 9/3 3 WATER VAPOR PERMEANCE OF CONSTRUCTION MATERIALSM aterialThicknessPermeancePermeability(in .)(perm)(perm/in.)MATERIALS USED IN CONSTRUCTIONC oncrete, 1:2:4 block, cored limestone masonry, cement 4-8 with oil base on metal on wood on plain gypsum lath, with wall board, sheathing, asphalt 0 Structural insulating board, sheathing insulating board, interior , , , built up, hot mopped0 Wood, sugar , Douglas Fir, exterior , Douglas Fir, interior , glass fiber reinforced , glass fiber reinforced INSULATIONSAir still120 Cellular glass0 Mineral wool, unprotected116 Expanded polyurethane, R-ll, board polystyrene, polystyrene, foam.