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228 CHAPTER 10 1957 Guide- 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. 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 water-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 73 F: Class A paper shall have a minimum tensile strength in each direction of either 35 Table 2. Conversion Factors for Vapor Transfer Units Multiply Numbeb or WVT Units ^to Obtain grams grains ... (24 hrs) (sq .m) (hr) (sq ft) grama (24 his) (sq m) by same method* 1 16.7 groins (hi) (sq ft) Perms by same method* by same method* method A & B, 73.4 F method C A D, 90 F method E, 100 F 0.0597 0.144 0.084l 0.0344 1 3.41 1.41 . 0.575 . Nominal Test Conditions % Relative Humidity on the Two Sides op Specimen! Method Temperature-F In cup A B C D E 73.4 73.4 90 90 100 0 100 0 100 0 Data obtained by one method cannot be reliably converted to another method. Outside cup 50 / 50 50 60 90 Tloo ccoonnvveerrtt (2g4rahmr)-(-s--Q---m---)---(--m---m---- Hg-) to perms multiply by 1 52. lb per inch width, or 20 lb per inch width, as specified in the invitation for bidsPaper of both strengths shall have a minimum water resistance of 24 hr, and a manmum water vapor permeability (WVT) of 4 grams per (square meter) (24 hr) *-, 0.57C6lapse3rmB. 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 man" mum water vapor permeability (WVT) of 6 grams per (square meter) (24 hr) *- 0.86C4lapsesrmC .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 bidfi. Paper of both strengths shall have a minimum water resistance of 8 hr. :.d Class D paper shall have a minimum tensile strength in each direction of 20 lb J\' inch width. The paper shall have a minimum water resistance of 10 min., a miw-;.- mum water vapor permeability (WVT) of 35 grams per (square meter) (24 hr) **> 5.04 perms. ' <=% Moisture in Building Construction 229 VISIBLE CONDENSATION Just as moisture collects on the outer surface of a glass of cold water, so does it also condense on other cold materials. In winter, visible con densation may collect on cold closet walls and attic roofs and is commonly observed on frosted window panes. Although condensation, if liquid, may enter an unpainted surface as fast as it fonns and thus be unseen, any condensation on a visible surface will for convenience here be called visible condensation to distinguish it from concealed condensation. Within residences and public buildings, visible condensation occurs in winter and may damage decorative finishes and window sash. Interior visible condensation occurs when any surface is colder than the dew point of the near-by air. The temperature of any such surface-- wall, roof, or glass--is dependent upon the air temperature inside and out- Fig. 4. Relative Humidity at Which Visible Condensation Will Appear on Inside Surface side the building and the heat transfer coefficient U of the surface structure. Based on a value of 1.65 for the inside surface conductance, Fig. 4 shows he relative humidity in a room at 70 F when visible condensation will appear at various (/-values. The curves for single and double, glass at heir usual (/-values are included. It should be noted that (/-values as commonly used are an average for a large area within which there may be Pots, such as the studs in an insulated wall, where the transmittance is at tlfrV 'ns^e surface temperature of a wall will, in general, be lower fe i .horforn due to such things as stratification of inside air and the ef- cts of air leakage and of convection in walls with air spaces. Since con- with 10n- ^ee^ts coldest spot, values from Fig. 4 can be applied only Een Cau^on- ^ a result, the limit of relative humidity for a non-homo- ^ j.f0US wad is lower than might be inferred from its average (/-value. conH0rmal!y'gkss *s most likely spot in a heated room for appearance of fu TMn, but an uninsulated wall surface shielded from radiation by avoidpJTB' e9uall.y cold. In some cases where condensation is barely ecl, surface, , being but little warmer than the dew-point tempera-