Document 2JoQ7jGRZBELvwEq6gx7X9JRR

208 CHAPTER 9 1951 Guide obtained by the different methods, nor in regard to a single standard to be used. Notwithstanding the uncertainties as to the theory and the lack of complete agreement as to test methods for measuring vapor permeability or permeance, the requirements are well understood and the relative values of certain types of materials have been established with sufficient precision to indicate their vapor permeance. Some values which may be used as a guide are given in Table 21. Water-proofed building papers are listed in Federal Specifications UUP-147, May 24, 1948, according to water vapor resistance required as: Class A. For uses where a high degree of water-vapor resistance is required. Table 21. Permeability of Various Materials to Water Vapor Group Material Permeability Grains per (Sq Ft) (Hr) (Inch Hg) Plaster base and plaster. H in------- -------------------- -------------------------------------Fir sheathing. % in............................................................. ......................... -- Waterproof paper**...... ......................................--..... .......1---------------------------------1* Pine lap siding........ ..................................................................... ---- Sugar cane fiberboard. % in.------- ----------- ---- -............-................................... Brick masonry, 4 in------------------ ----------------------------------------- ---------.........-- Foil-surfaced reflective insulation, double-faced-- .......................... -............ Roll roofing--smooth, 40 to 65 lb per roll 108 sq ft................ -........ -......... Duplex or laminated papers, 30-30-30-.......................... -................................... Duplex or laminated papers. 30-60-30......... ............. -......................................... Duplex paper, coated with metallic oxides------------------- -----------------------... Insulation backup paper, treated....................-..................................................... Plaster, wood lath--. ............... --............................... -........................................-- Plaster. 3 coats of lead and oil...... ................. . ................. ..............-- ....... 2c Plaster, 2 coats of aluminum paint.... ..... .......................:.............. ---------------Plaster, fiberboard or gypsum lath...................... .......................... ................. ..-- Plywood. 2 coats of asphalt paint --.................. ------------------- --.---- Plywood, 2 coats of aluminum paint -....................................... ...........--... Gypsum lath with metallic aluminum backing.---------------------.--------------Insulating Jatb and sheathing, board type..----------------------- -.--------------- -- Insulating sheathing, surface-coated------------------------------------------------------Insulating cork blocks. 1 in-------- ....__________ --__________ -----------Mineral wool, unprotected. 4 in...................--...... ................... ............... -- Sheathing paper, asphalt impregnated, glossy......... -..................... -- 14.7 2.9 49.1 4.9 3.4 12.5 1.1 0.08 to 0.13 0.13 to 0.17 1.37 to 2.58 0.52-0.86 0.52-1.29 0.86-3.42 11.00 3.68 to 3.84 1.15 19.73 to 20.57 2.67 to 2.74 0.43 1.29 0.09-0.39 25.68 to 34.27 3.03 to 4:36 6.19 29.07 0.17-2.05 * Calculating Vapor and Heat Transfer Through Walls, by L. G. Miller (Heating ani Ventilating, Vol. 35, No. 11, November, 1933). * light weight slaters felt used to keep ram from drifting through. Not'used as a vapor barrier. ,e How to Overcome Condensation in Building Walls and Attics, by L. V. Teesdale (Beating and VentHa ting. VoL 36, No. 4 April, 1939). Class B. For uses where only a moderate degree of water-vapor resistance or high water resistance is required. . Class C. For uses where only a moderate degree of water resistance is required. Class D. For uses where high permeability to water vapor is required. Detail requirements in these specifications are given as follows: Class A 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 nave a minimum water resistance of 24 hr, and a maxi mum water-vapor permeability (permeance) of 4 grams per square meter per 24 hr, (i.e., 0.576 grain per (sq ft) (hr) for a calculated pressure differential of 1 in. Hg). Class B 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 maxi mum water-vapor permeability (permeance) of 6 grams per square meter per 24 hr, (i.e., 0.864 grain per (sq ft) (hr) for a calculated pressure differential of 1 in. Hg). Class C 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 8 hr. Heat Transmission Coefficients of Building Materials 209 Class D paper shall have a minimum tensile strength in each direction of 20 lb per inch width. The paper shall have a minimum water resistance of 10 min, a mini mum water-vapor permeability of 35 grams per square meter per 24 hr, (te, 5.04 grains per (sq ft) (hr) for a calculated pressure differential of 1 m. Hg). The specified method of test for permeance is the following: The test specimen having an area of at least 50 square centimeters, shall be sealed on the mouth of a dish containing calcium chloride. The seal shall be made with wax composed of 60 percent refined amorphous wax and 40percent of refined crystalline paraffin wax. The dish shall be exposed to an atmosphere of 73 F 3.5 deg, and 50 2 percent relative humidity, until a.constant rate of gain in the weight of the diah is attained. The average constant weight of gain for at least four test specimens shall be reported as the water-vapor permeability (permeance) of the material in terms of grams per square meter, per 24 hrs. Both sides of the material, in equal number, shall be exposed towards the calcium chloride. Surface Condensation Control Since surface condensation is caused by water vapor coming into contact with the surfaces having temperatures below its dew-point temperature, the obvious remedy is, first, to reduce as far as practicable the dew-point temperatures of the surrounding vapors, and, second, to increase the tem peratures of the surfaces with which these vapors may come in contact. The control of the dew-point is usually an operating problem. It may be lowered by giving attention to source of the moisture, and eliminating it before it comes in contact or mixes with the air in the space. It may also be effectively reduced by ventilation, or by some moisture absorption proc ess. The control of vapor formation and its elimination from the space as soon as possible, are the first requirements in most condensation prob lems. They are often the complete remedy. The temperatures of the inside room surfaces with which the vapor comes in contact may be increased by adding insulation to outside walls, by double glazing of windows, by circulation of warmer air over the surface, or perhaps by direct heating of the surfaces. The most expedient method of overcoming surface condensation difficulty will depend upon special conditions surrounding the problem. This is a construction rather than an operating problem. Control of Condensation Within Structure Since condensation within the structure is really surface condensation transfeirod to the interior parts of the structure, the same precautions as to humidity control should be observed as for surface condensation. In addition to this, however, the structure must be built to prevent vapor from reaching the interior sections of a wall. A wall which is apt to have a cold interior section should have a vapor-resisting material on its warm surface. There are many types of materials and methods of construction which may be used to vapor-proof the interior surfaces of cold walls. Vapor resistant membrane materials are often built into the wall near the warm surface. In wood frame walls they may be applied to the inside surface of the studs. They are sometimes attached to the warm side of insulating materials, or they may be applied on the cold side of plaster base materials. There are several types of vapor resistant papers in combination with metal foils which may be used. To be effective, these vapor resistors or barriers, as they are commonly called, should have a reasonably high resistance to /