Document 3QKmoRQN8rOE4xBZkD43Ej9b6

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. 2 3 It is notable that high relative humidity on either side of a speci men increases its permeance and the average permeability of the piece: <2f; 'M\ PERCENT RELATIVE HUMIDITY ON THE HIGH PRESSURE SIDE Fig. 2. Permeability of Wood (Sugar Pine) 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. 2 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 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 Water Vapor and Condensation ih Building Construction 207 the cup contains water, the wet-cup method. Usually the outside at mosphere 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 specimen are likely to be much different, the wet method producing the higher values. It 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 Table 1. Conversion Factors fob Vapor Transfer Units Multiply Number op- WVT Units to Obtain ^ grams % (24 hrs) (sq m) grains (hr) (sq It) grams (24 hra) (sq m) grains (hr) (sq ft) Perms by same method* by same method* by same method* method A & B, 73.4 F method E, 1UU ' F 1 0.0697 0.144 0.0840 0.0344 16.7 1 2.41 1.41 0.676 Nominal Test Conditions Sides op Specimen: Method Tern perature-F In cup Outside cup A 73.4 0 50 B c 73.4 90 100 0 60 50 D .E 90 100 60 100 0 90 Data obtained by one method cannot be reliably converted to another method. 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 can 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 (5) 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 1 presents the conversion. factors applicable to the commonly used units and test methods. Table 2 presents some data on typical