Document Xz9KQnx4Z7wo5MqRYVMew81gw

384 CHAPTER 22 1965 Guide And Data Book The presence of water or ice in insulation has a marked effect on conductivity, because their conductivities are many times as great as that of the insulation. A. serious moisture condition arises when the temperature of part of the insula tion is below tiie dewpoint temperature of the surrounding air and, consequently, the water vapor from the air condenses in the insulation. While the effect of moisture on the conduc tivity of the insulation is not too serious so long as' the mois ture exists in the vapor phase, the conductivity is greatly increased by the presence of condensed moisture. Since moisture, tends to migrate toward the colder surface, at low temperatures it will condense and may freere. The effect of moisture on conductivity is difficult to determine accurately, because, in malting a thermal conductivity test, a tempera ture difference across a sample is maintained and the moisture in the sample tends to migrate from the warmer to the colder side. It is possible for convection and air infiltration currents to occur in or through some insulations and cause an increase in the heat transfer across these insulations. Low-density loose-fill,- and low-density fibrous types of insulations ' are most susceptible to increased heat transfer effect by convec tion and air infiltration. When convection occurs an increase of temperature drop across the insulation, the height of the insulation, or thickness or width of the insulated space, all tend to influence the amount of convection. However, the effect of convection in or through insulation can be' mini mized by careful design of the insulated structure.1 - - Recent tests show that the effectiveness of fibrous batt type insulation is seriously impaired if installed vertically with air spaces on both sides, because of the air interchange through the insulation-between the two spaces. However, when an impermeable membrane is applied to one surface, infiltration appears to be negligible and conductivities measured by hot plate tests are generally applicable. Factors which may affect the conductivity of insulation by wutsing settling or other dimensional rhangra, include vibra^ turn, temperature extremes, and various mechanical forces. Reference 8 Hiaensswa in detail the factors affecting the thermal conductivity and other thermal properties of soils. These factors include the effects of temperature, density, moisture, and soil characteristics. Four diagrams are pre sented in Reference 8 to aid in estimating the thermal con ductivity of any soil- The specific heat of soil is also discussed. Tests were made on 19 different soils, which represented a wide textural variety, including gravel, sand, sandy loam, silt loam, and ,day, as well as some crushed rocks and a fibrous, peat. Moisture contents in tests varied from air-dried value, to those greater than the optimum moisture content; densities varied-from a loosely-poured condition to the maximum density obtainable by heavy ramming. Table 1 of Chapter 24 gives thermal conductivity values of soils in approximate order of decreasing values. THERMAL CONDUCTANCE The methods of calculating an overall coefficient of heat transmission require knowledge of the thermal conductivity and thickness of .homogeneous elements, the thermal con ductance of non-homogeneoua elements, surface conductances of both sides of the construction, and the conductances of any contained air spaces. Accurate values of these items must-be used to obtain satisfactory;results. Procedures used for calcu-; lating thermal conductance and resistance, and definitions of heat transfer, terms and,symbols are given in .Chapter-;24i Sometimes parallel heat-flow paths of different- resistances occur in the a*TM* construction. In such cases'determination of the overall thermal transmittance by means of.the Guarded Table 1 .... Variation in Surface Conductance Coefficient for Vertical Surfaces with Different Temperatures of Surrounding Surface Surrounding Surface Tempou^uru 75 F 70 f 69 F 60 F SO F Convection--Btu per (hr) (sq ft). 6.6 6.6 6.6 6.6 6.6 Radiation--Btu per (hr) (sq ft).. 4.4 8.6 9.6 17.0 24.9 Total--Btu per (hr) (sq ft) 11.0 15.2 16.2 23.6 31.5 Hot Box method described in ASTM Designation C-236 is recommended. Surface Conductance The surface conductance of a wall is'the combined heat transfer to or from the wall by radiation,'convection, and con duction. Each of the three portions making up the total may vary, independently of the others, thus affecting the.-total conductance. The heat transfer by- radiation between two surfaces is controlled-by the character of the surfaces (eniis- sivity), the temperature' difference between them, and the solid angle through which they tee each other. The heat trans fer by convection and conduction is controlled by the rough ness of the surface, by air movement, and temperature dif ference between the air and the surface. The importance of the effect of temperature of surrounding surfaces on the surface'conductance, due to the effect of radi ation, is illustrated in Table 1, which applies to a vertical surface at 80 F, with ambient air at.70 F and with radiation exchange corresponding to an effective emissivity of 0.83. In many cases, because the thermal resistance (reciprocal of conductance) of the internal parts of the .wall is high compared with the surface resistance, the surface factors are of minor importance. The surface resistances of single glass'windows,' constitute almost the entire resistance and are therefore very, important. An analysis of various factors affecting surface conductedce'and the difference between surface and air tem peratures will be found in Reference 9/(See Chapter 14 of the 1964 Guide And Data Book.) The convection part'of the surface conductance is affected markedly by air movement. This'is illustrated by. Fig. 1 of. Chapter 24; On smooth surfaces, surface length1' also signifi cantly affects the convection part'of conductance;'the average value decreases as the surface length increases. Moreover,; observations11 of- the magnitude of low temperature radiant energy received from outdoor surroundings show that only under certain conditions'may toe out-of-doors be treated as a black'body radiating at. air temperature! Because of these factors, the selection of surface conduct-' ance coefficients'for'a practical building'becomes a matter of judgment. 'Surface conductances' are shown, in Table 2 of Chapter 24, and are applicable'to ordinary building materials. In special where surface'conductances become impor tant factors in the overall' ratos'of heat transfer, more se-' lective coefficients may be required.' Principles and data given in Chapter 4 may beapplied in such cases. FACTORS AFFECTING HEAT TRANSFER ACROSS AIR SPACES - The:.transfer of heat-across an. air space- involves the boundary surfaces as well as the intervening air, and depends markedly,on the orientation of the air space and the direction of heat flow. The coefficients given for air space conductance represent the total conductance from one surface bounding the air space to the other.. The total conductance is the sum Thermal Insulation and Water Vapor Barriers 385 of a component due to radiation and a component due to convection and conduction combined. These components may roy independently of each other. lhe radiation portion of the coefficient is affected by the temperature of the two boundary surfaces, and by their re spective surface emissivities , the combined effect of which is expresed by means of the effective emissivity E of the air The radiation component is not affected by the thick ness of the space or by its orientation or direction of heat flow. The heat transfer by convection and conduction combined is markedly affected by the orientation of the air space and fiie direction of heat flow, by the temperature difference across the spflP" and in some cases by the thickness of the space. It is also affected to a small extent, by the mean temperature of its surfing. For air spaces usually employed in building construc tion, the radiation and convection-conduction components may vary independently of each other. Table 3 of Chapter 24 gives the thermal conductances and resistances of air spaces of uniform thickness and moderately smooth surfaces, based on experimental measurements con ducted at the National Bureau of Standards.11 Although the conductances of air spaces vary to some extent with thinlrnass in the range over j in., average values are tabulated for } in. and 4 in., for all except horizontal spaces with heat flow down ward. The error involved by averaging is usually less than 10 percent, and is less than 6 percent in most eases. For more exact values, Reference 12 may be consulted. For narrow air spaces, which may be defined as those for which the product of the cube of the thteknpsn of the space in inches times the temperature difference (Fahrenheit degrees) across the space is les than 3 for heat flow horizontally or downward, or less than 1 for heat flow upward, the conductance is the sum of the radiative heat transfer coefficient And that for conduction alone through air, since convection is practically suppressed. The radiation component can be com puted by methods outlined in Chapter 4. The conduction component can be computed using the conductivity of air at the appropriate mean temperature (see Chapter 5). The effects of different mean temperatures, temperature differences, and effective emissivities are indicated in Table 3 of Chapter 24. To obtain a high thermal resistance with reflective insula tion used with refrigeration, multiple air layers bounded by re flective surfaces are formed and the total resistance is equal to the sum of the resistance values across each air space. Depending upon the type of reflective insulation, one or both sides may have highly reflective surfaces. Except for thick horizontal air spaces with heat flow down, little is gained thermally by the addition of a second highly reflective surface to the same air space. If an air space has only one reflective surface, it makes no appreciable difference as far as the rate ofbeat transfer is concerned as to the side on which the reflec tive surface is placed. However, it is important that conden sation be prevented from forming on a reflective surface if it is the only one and is placed on the cold side of the construction. A reflective surface placed on the warm side of an air space will usually not be a condensing surface and will, therefore, maintain the thermal resistance of the air space and, in addi tion, will act as a water vapor barrier if the material and its jomta are of adequately low permeance. 'Die emissivity of a surface is the measure of its ability to onit radiant energy, and for the same temperature and wave W 13 equal to the absorptivity (ratio of the radiant absorbed by a surface to the total radiant energy idling on it). The ratio of toe energy reflected by the surface to that falling on it is known as reflectivity, and for an opaque sunace is equal to one minus the emissivity. The emissivity. varies with the type, color, and condition of the surface.and with the wave length of the radiation. For reflective insulation used with hearing, air conditioning, and refrigeration applications, the emissivity value for long wave-length (infra-red) radiation is important, and not the value -for the shorter wave lengths of the visible spectrum. Visible brightness b not a true measure of the reflectivity for thermal radiation, because there is no definite relation between the reflectivity for light and for long wave-length radiation. Typical reflectivity and emissivity values for reflective-sur faces and building materials and toe corresponding emissivity factors for air spaces are given in Table 2 of Chapter 24. Changes in the condition of a reflective surface which may reduce its reflectivity and increase its emissivity include those caused by chemical action, dust accumulations, and the presence of condensation or frost. Chemical nhangwa include oxidation, corrosion, or tarnishing caused by air, moisture,, wet plaster, or the chemical treatment of wood spacing strips or other adjoining structural members. Surface emissivity values should be obtained by tests. PART II: WATER VAPOR BARRIERS Water vapor barriers are those materials which retard the transmission of water vapor with reasonable effectiveness under specified conditions. The permissible rate of water vapor transmission (permeance) of a barrier material depends upon design criteria for the structure or system being insulated. VAPOR TRANSMISSION THROUGH MATERIALS The principles and calculation of vapor flow through insula tion and vapor barriers are treated in more detail in Chapter 23, Moisture in Building Construction. Water vapor in air is a gas which exerts its own vapor pres sure and which can move through materials under differences in its own vapor pressure independently of the air with which it is mixed. Water vapor transmission through materials under the influence ofa water vapor pressure gradient b called vapor diffusion. The design equation commonly used b as follows: W = ?A8a-dj- (i) where W -- total weight of vapor transmitted, grains. m " permeability, grains per (square foot) (hour) (inch of mercury vapor pressure difference per inch). A -- area of cross section of the flow path, square feet. 8 time during which the transmission occurred, hours. Ap " difference of vapor pressure between. ends of- the flow path, inches of mercury. I -- length of flow path, (or of specimen), inrho* The basic units given are those now favored by the building industry. Whenever it b convenient to deal with a material of a thick ness other than the unit thickness to which ft refers, use may be made of the permeance coefficient M, where M lhe designation perm for the unit of permeance b now widely.used, and b a convenient substitute for the unit, 1 grain per (square foot) (hour) (inch of mercury vapor pressure difference)! The corresponding unit of permeability b perm-vich, inop it b the permeance of unit thickness. .The corresponding 'flow equation b: W - MAe&p (2) The resistance to water vapor flow through a material-b the reciprocal of its permeance and b equal to the thickness divided by its permeability. The overall resistance of an irisu-'' lated section to water vapor flow b the sum of-the resistances'