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HEATING VENTILATING AIR CONDITIONING GUIDE 1941 more nearly equivalent to the boundary surfaces of an internal air space than they are to the external surfaces of a wall. It would be more nearly correct to use a value of 2.2 rather than the usual value of 1.65 as coef ficients for these surfaces. In most cases this would make only a minor change in U. It should be noted that the over-all coefficient should be multiplied by the ceiling and not the roof area. If the unheated attic space between the roof and ceiling has no dormers, windows or vertical wall spaces the combined coefficients may be used for determining the heat loss through the roof construction between the attic and top floor ceiling. If the unheated attic contains windows and vertical wall spaces these must be taken into consideration in calculating the roof area and also its coefficient Z7r. In this case an approximate value of Ut may be obtained as the summation of the coefficient of each individual section such as the roof, vertical walls or windows times its percentage of total area. This coefficient may be used with reasonable accuracy in the above formulae. If, however, there are roof ventilators such that the attic air is substantially at outside temperature, then the roof should be neglected and only the coefficient for the top floor ceiling construction used. Basements and Unheated Rooms The heat loss through floors into basements and into unheated rooms kept closed may be computed by assuming a temperature for these rooms of 32 F. The coefficients of transmission for concrete floors on ground (Table 10) are based on the assumption that the heat-resisting value of the floor extends downward and stops at the under side of the concrete. It is probable, however, that the dirt underneath has some heat-resistance value extending to a considerable depth, which would result in substan tially lower heat transmission coefficients than given in Table 10. This problem is now the subject of research. Additional information on the inside and outside temperatures to be used in heat loss calculations is given in Chapter 5. CONDENSATION IN BUILDINGS The water vapor or moisture mixed with the air in buildings will be transmitted through many types of building construction if there is a difference in the vapor pressures on the two sides of the structure. Such water vapor will also condense whenever it comes in contact with surfaces or objects at or below the dew-point temperature. Thus two types of condensation problems are encountered in building practice, namely (1) Surface condensation or condensation on the interior building surfaces including the walls, ceiling (or roof) and glass, and (2) Interstitial con densation or the transmittance of the vapor through the building materials and condensation of the moisture on surfaces or voids within the materials of construction. Condensation within the construction as well as condensation on the interior surfaces does not necessarily occur in all buildings but only, in isolated cases when conditions conducive to such condensation exist. The probability of condensation increases with the relative humidity or vapor pressure and with the temperature difference and, in the case of inter- 98 CHAPTER 3. HEAT TRANSMISSION COEFFICIENTS AND TABLES stitial condensation, decreases with the vapor resistance on the warm side of the wall. Condensation on interior building surfaces5 (surface condensation) may be eliminated by either reducing the relative humidity or by maintaining the interior surfaces at or above the dew-point temperature. Permissible relative humidities for various wall, roof or glass coefficients and tempera ture differences may be determined from Fig. 2. The permissible relative humidity for any specific type of construction may be determined by first ascertaining the coefficient of transmission (U) of the construction and then locating this coefficient on the horizontal scale of Fig. 2. A vertical line drawn to the proper outside temperature curve and then to the left hand scale will indicate the permissible relative humidity for the --involved. The dotted line shown in Fig. 2 indicates the per- Fig. 2. Permissible Relative Humidities for Various Transmission Coefficients missible relative humidity (64. per cent) if surface condensation is to be avoided, for a frame wall having a coefficient of 0.26 and for an outside temperature of --10 F. Condensation within the construction may likewise be prevented by eliminating the moisture at the source or by providing a barrier on the warm side of the insulation construction. A good vapor barrier con struction may be obtained with a vapor-proof paper properly applied under the plaster or a vapor-proof finish on the interior surface of the wall6. In the case of attics, the greater the heat resistance in the top floor ceiling, the lower the attic temperature and consequently the `Permissible Relative Humidities in Humidified Buildings, by Paul D. Close (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, December, 1939, p. 766). Condensation within Walls, by F. B. Rowley, A. B. Algren and C. E. Lund (A.S.H.V.E. Transactions, Vol. 44. 1938. p. 95). 99