Document e5JR3OnVo1zYRmy0b9xgKVreq

202 CHAPTER 9 1952`Gvude series is the sum of the resistances of its component parts provided con densation does hot take place within' the assembly. Expressed in the more usual terms, the permeances (Afi, iUj, M3, etc.) of the individual pieces may be combined by use of the'formula'" - ilf-----J-_--------J--_----,----1----+-------------J--L-_- (10) Equation 10 holds for materials that are reasonably homogeneous and in a condition of steady state where the transmission at all points is a vapor diffusion process as, for example, in a vapor transmission test. Actually, the conditions of moisture movement through a building wall are generally PERCENT RELATIVE HUM0ITY ON THE HIGH PRESSURE SIDE Fio. 7. Permeability of Wood (Sugar Pine) different. A steady state, where the entering and leaving moisture are equal, rarely exists, and frequently, the moisture in some portion of the path is liquid, in which case forces of capillarity and gravity are usually more important. It is therefore evident that the formula can be used only for certain portions of a building structure. Another caution is that the permeances of the several pieces must apply at the existing conditions. The permeability of a material has been defined as one of its properties but it is not a fixed property for. all conditions of exposure. Some materials like wood, because of their structure and hygroscopicity,. are much more permeable to water vapor when, the relative humidity is high. Since the equilibrium moisture content of permeable materials is increased to a greater or less degree by exposure to high relative humidity, it is likely that this sorbed moisture contributes to the mechanism of transfer. The variations in the permeability of sugar pine wood are shown in ' Fig. 7.18 It isnotable that high relative humidity on either side of a speci men increases its permeance and the average permeability of the piece. Heat Transmission Coefficients of Building Materials 203 The spot permeability is shown, but-the average, is more readily, used in practical calculation. Temperature also affects permeability,; but for.most materials is considered a minor factor, although data are .few, .These variations are to be expected in most materials and therefore, due care is required in choosing for each the proper value at its exposure conditions. Exact calculation by any of the preceding formulas, therefore, requires, a knowledge of such variations as shown in Fig. 7 for each material, but approximate calculations are readily made and are adequate for most requirements. Permeance Data and Testing The simplest method of finding the vapor permeance of a specimen is to seal it over the top of a cup containing desiccant, or water, placing it in a Table 21. Conversion Factors for Vapor Transfer Units Multiply Numbeb 07 WVT Units to Obtain J, grams (24 his) (sq m) grains (hr) (sq ft) grams (24 hre) (eq m) grains (hr) (sq ft) Perms by same method* by same method* by same method* method A A B, 73.4 F ' method C A D, 90 F method E. 100 F . . 1 0.0597 0.144 0.0840 0.0344 16.7 1 2.41 1.41 0.575 . % Relative Humidity on the Two Sides 07.Specimen: Method Temperature-F In cup Outside cup A. B C: D 73.4 73.4 90 90 100 0 50 100 50 0 50 100 . . 50 0 90 . Data obtained by one method cannot be reliably converted to another method.' controlled 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 at? mosphere is heid 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 specimen are likely to be much different, the wet method producing the higher values. ft is obvious that any statement of permeance of a specimen should include the conditions of test. The permeance of a piece of material in a given service is best known if. tested under conditions duplicating the serv ice. Its permeance may be adequately judged, However, if it is tested by both dry and wet methods thus providing relative humidity conditions that usually include those to be encountered in service. Unfortunately, the conditions of testing have not been standardized, and test data have frequently been presented in terms of -weight transmitted