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228 CHAPTER 10 1958 Guide'1-.'! Class A. For uses where a high degree of water vapor resistance is required. ,:r 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, this V specified WVT Test being a dry method at 73 F: 't Class A paper shall have a minimum tensile strength in each direction of either 35 . Table 2. Conversion Factors for Vapor Transfer Units Multiply Number op WVT Units to Obtain ^ grams (24 brs) (sq m) grains (hr) (sq ft) * b grama (24 hra) (eq m) graina (hr) (sq ft) Perms .by same method* by same method* by same method* method A & B, 73.4 F method C & D, 90 F method E, 100 F 1 0.0597 0.144 0.0840 0:0344 1 2.41 1.41 0.575 - it; ' Nominal Test Conditions % Relative Humidity on the Two Sides op Specimen: *>) A Method TemperatureF In cup Outside cup A 73.4 0 50 B 73.4 100 50 C 90 0 50 D 90 100 50 A 100 0 90 * Data obtained by one method cannot be reliably converted to another method. (24 hr) (sq--m7) -(;-m---m--- tHogp)erms multiply by 1.62. lb per inch width, or 20 lb per inch width, as specified in the invitation for bidsr, Paper of both strengths shall have a minimum water resistance of 24 hr, and a masiSJj mum water vapor permeability (WVT) of 4 grams per (square meter) (24 hr) ?' 0.576 perm. ' -'-A Class B paper shall have a minimum tensile strength in each direction of either 35g. lb per inch width, or 20 lb per inch width, as specified in the invitation for bid?' 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) - 0.864 perm. Al'-t Class C paper shall have a minimum tensile strength in each direction of either o ^ , lb per inch width, or 20 lb per inch width, as specified in the invitation for bidgy Paper of both strengths shall have a minimum water resistance of 8 hr. . -, Class D paper shall have a minimum tensile strength in each direction of 20 lb p| inch width. The paper shall have a minimum water resistance of 10 min., a mi)S! 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 forms 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, ased on a value of 1.65 for the inside surface conductance, Fig. 4 shows e relative humidity in a room at 70 F when visible condensation will ppear at various (7-values. The curves for single and double glass at comm181 ^'va^ues are included. It should be noted that (/-values as SDOt. n ^i Usec^ are an average for a large area within which there may be fughe' the s^uds in an insulated wall, where the. transmittance is at tb F K i* 6 uls*(ie surface temperature of a wall will, in general, be lower fects of.u-om due to such things as stratification of inside air and the ef- densat' aif e, age and of convection in walls with air spaces. Since con- with ca?rt'See a he ci(iest sPot> values from Fig. 4 can be applied only geneous ii a result the limit of relative humidity for a non-homowall is lower than might be inferred from its average (/-value. condensat^' rflost likely spot in a heated room for appearance of furniture11011' k an uninsulated wall surface shielded from radiation by avoided, th^ 7? dually cold. In some cases where condensation is barely ' ne sunace, being but little warmer than the dew-point tempera-