ASHRAE FUNDAMENTALS IP CH 26-2017 Heat Air and Moisture Control In Building Assemblies-Material Properties.pdf

1、26.1CHAPTER 26HEAT, AIR, AND MOISTURE CONTROL IN BUILDING ASSEMBLIESMATERIAL PROPERTIESINSULATION MATERIALS AND INSULATING SYSTEMS 26.1Apparent Thermal Conductivity. 26.1Materials and Systems . 26.3AIR BARRIERS 26.5WATER VAPOR RETARDERS 26.6DATA TABLES. 26.7Thermal Property Data 26.7Surface Emissivi

2、ty and Emittance Data. 26.7Thermal Resistance of Plane Air Spaces . 26.7Air Permeance Data. 26.7Water Vapor Permeance Data . 26.12Moisture Storage Data. 26.13Soils Data. 26.13Surface Film Coefficients/Resistances . 26.16Codes and Standards 26.19HIS chapter contains material property data related to

3、theTthermal-, air-, and moisture-related performance of buildingassemblies. The information can be used in simplified calculationmethods as applied in Chapter 27, or in software-based methods fortransient solutions. Heat transfer under steady-state and transientconditions is covered in Chapter 4, an

4、d Chapter 25 discusses com-bined heat, air, and moisture transport in building assemblies. Forinformation on thermal insulation for mechanical systems (includ-ing insulation used in a range of temperatures), see Chapter 23. Forinformation on insulation materials used in refrigerant piping sys-tems a

5、nd cryogenic or low-temperature applications, see Chapters10 and 47 of the 2014 ASHRAE HandbookRefrigeration. Forproperties of materials not typically used in building construction,see Chapter 33 of this volume.Density and thermal properties such as thermal conductivity,thermal resistance, specific

6、heat capacity, and emissivity for long-wave radiation are provided for a wide range of building materials,insulating materials, and insulating systems. Air and moistureproperties (e.g., air permeance, water vapor permeance or perme-ability, capillary water-absorption coefficients, sorption isotherms

7、)are given for several materials, with a brief description of how touse the tabulated data. Data on soil thermal conductivity, air cavityresistances, and surface film coefficients, which are also importantwhen considering performance of building assemblies, are alsoprovided.1. INSULATION MATERIALS A

8、ND INSULATING SYSTEMSThe main purpose of using thermal insulation materials is toreduce conductive, convective, and radiant heat flows. When prop-erly applied in building envelopes, insulating materials do at leastone of the following: Increase energy efficiency by reducing the buildings heat loss o

9、rgainControl surface temperatures for occupant comfortHelp to control temperatures within an assembly, to reduce thepotential for condensationModulate temperature fluctuations in unconditioned or partly con-ditioned spacesThe primary property of a thermal insulation material is a lowapparent thermal

10、 conductivity. Additional functions may be served,such as providing support for a surface finish, impeding watervapor transmission and air leakage into or out of controlled spaces,reducing damage to structures from exposure to fire and freezingconditions, and providing better control of noise and vi

11、bration.These functions, of course, should be consistent with the capabilitiesof the materials.ASTM Standard C168 defines terms related to thermal insulatingmaterials.1.1 APPARENT THERMAL CONDUCTIVITYThe primary property of a thermal insulation material is a lowapparent thermal conductivity, though

12、selection of the appropriatematerial for a given application also involves consideration of theother performance characteristics mentioned previously.Thermal conductivity (symbol k, in Europe) is a property of ahomogeneous, nonporous material. Thermal insulation materials arehighly porous, however,

13、with porosities typically exceeding 90%. Asa consequence, heat transmission involves conduction in the solidmatrix but mainly gas conduction and radiation in the pores (evenconvection can occur in larger pores). This is why the term apparentthermal conductivity is used. That property is affected by

14、structuralparameters such as density, matrix type (fibrous or cellular), andthickness. Each sample of a given insulation material has a uniquevalue of apparent thermal conductivity for a particular combinationof temperature, temperature difference, moisture content, and age, avalue that is not repre

15、sentative for other conditions. For more details,refer to ASTM Standards C168, C177, C335, C518, C976, andC1045.Influencing ConditionsDensity and Structure. Figure 1 shows the variation of theapparent thermal conductivity with density at one mean temperature(i.e., 75F) for a number of insulation mat

16、erials used in buildingenvelopes. For most mass-type insulations, there is a minimum thatnot only depends on the type and form of the material but also ontemperature and direction of heat flow. For fibrous materials, the val-ues of density at which the minimum occurs increase as the fiberdiameter or

17、 cell size; see Figure 2 (Lotz 1969) and mean tempera-ture increase.Structural factors also include compaction and settling of insula-tion, air permeability, type and amount of binder used, additives thatinfluence the bond or contact between fibers or particles, and typeand form of the radiation tra

18、nsfer inhibitor, if any. In cellular mate-rials, most factors that influence strength also control the apparentthermal conductivity: size, shape, and orientation of cells, and thick-ness of cell walls. As Figures 1 and 2 suggest, a specific combinationof cell size, density, and gas composition in th

19、ose materials producesoptimum thermal conductivity.The preparation of this chapter is assigned to TC 4.4, Building Materialsand Building Envelope Performance.26.2 2017 ASHRAE HandbookFundamentals Temperature. At most normal operating temperatures, theapparent thermal conductivity of insulating mater

20、ials generallyincreases with temperature. The rate of change varies with materialtype and density. Some materials have an inflection in the curvewhere the blowing agent changes phase from gas to liquid. Theapparent thermal conductivity of a sample at one mean temperature(average of the two surface t

21、emperatures) only applies to the mate-rial at the particular thickness tested. Further testing is required toobtain values suitable for all thicknesses.Insulating materials that allow a large percentage of heat transferby radiation, such as low-density fibrous and cellular products,show the greatest

22、 change in apparent thermal conductivity with tem-perature and surrounding surface emissivity.The effect of temperature on structural integrity is unimportantfor most insulation materials in low-temperature applications. Atvery low temperatures, however, some polymeric compounds mayundergo glass tra

23、nsition, which is characterized by a markedincrease in thermal conductivity. For urethanes and butyl-basedcompounds, this occurs at approximately 40F, but for siliconesthe glass transition temperature is more in the range of 130F,which is not normally encountered in building applications. In anycase

24、, decomposition, excessive linear shrinkage, softening, or othereffects limit the maximum suitable temperature for a material.Moisture Content. The apparent thermal conductivity of insu-lation materials increases with moisture content. If moisture con-denses in the insulation, it not only reduces th

25、ermal resistance, butit may also physically damage the system, because some insulationmaterials deteriorate with exposure to water. Most materials wouldbe damaged if moisture were allowed to freeze in the material,because water expands when it freezes. The increase in apparentthermal conductivity de

26、pends on the material, temperature, mois-ture content, and moisture distribution. Section A3 of the CIBSEGuide A (CIBSE 2006) covers thermal properties of building struc-tures affected by moisture.Thickness. Radiant heat transfer in pores of some materialsincreases the measured apparent thermal cond

27、uctivity. For low-den-sity insulation (e.g., 0.35 lb/ft3), the effect becomes more pro-nounced with installed thickness) (Pelanne 1979). The effect onthermal resistance is small, even negligible for building applications.No thickness effect is observed in foam insulation.Age. As mentioned previously

28、, most heat transfer in insulationmaterials at temperatures encountered in buildings and outdoorsoccurs by conduction through air or another gas in the pores (Lander1955; Rowley et al. 1952; Simons 1955; Verschoor and Greebler1952). In fact, heat transfer in dry insulation materials can be closelyap

29、proximated by combining gas conduction with conductionthrough the matrix and radiation in the pores, each determined sep-arately. If air in the pores of a cellular insulation material is replacedby a gas with a different thermal conductivity, the apparent thermalconductivity changes by an amount app

30、roximately equal to the dif-ference between the thermal conductivity of air and the gas. Forexample, replacing air with an inert gas can lower the apparent ther-mal conductivity by as much as 50%. Cellular plastic foams with ahigh proportion (i.e., more than 90%) of closed cells retain theblowing ag

31、ent for extended periods of time. Newly produced, theyhave apparent thermal conductivities of approximately 0.15 Btuin/hft2F at 75F. This value increases with time as air diffuses intothe cells and the gas gradually dissolves in the polymer or diffusesout. Diffusion rates and increase in apparent th

32、ermal conductivitydepend on several factors, including permeance of cell walls to thegases involved, foam age, temperature, geometry of the insulation(thickness), and integrity of the surface facing or covering provided.Brandreth (1986) and Tye (1988) showed that aging of unfaced poly-urethane and p

33、olyisocyanurate is reasonably well understood analyt-ically and confirmed experimentally. The dominant parameters forminimum aging areClosed-cell content 90%, preferably 95%Small, uniform cell diameter 0.04 in.Small anisotropy in cell structureHigh densityIncreased thicknessHigh initial pressure of

34、blowing agent in the cellsPolymer highly resistant to gas diffusion and solubilityFig. 1 Apparent Thermal Conductivity Versus Density of Several Thermal Insulations Used as Building InsulationsFig. 2 Variation of Apparent Thermal Conductivity with Fiber Diameter and Density(Lotz 1969)Heat, Air, and

35、Moisture Control in Building AssembliesMaterial Properties 26.3Larger proportion of polymer evenly distributed in struts and win-dows between cells Low temperatureFor laminated and spray-applied products, aging is further re-duced with higher-density polymer skins, or by well-adhered facingsand cove

36、rings with low gas and moisture permeance. An oxygen dif-fusion rate of less than 0.02 in3/1000 ft2day for a 0.001 in. thick fac-ing is one criterion used by some industry organizations formanufacturers of laminated products. Adhesion of the facing mustbe continuous, and every effort must be made du

37、ring manufacturingto eliminate or minimize the shear plane layer at the foam/substrateinterface (Ostrogorsky and Glicksman 1986).Before 1987, chlorinated fluorocarbons were commonly used ascell gas. Because of their high ozone-depleting potential, chlorofluo-rocarbons (CFCs) were phased out during t

38、he 1990s in accordancewith the Montreal Protocol of 1987. Alternatives used today are flu-orinated hydrocarbons, CO2, n-pentane, and c-pentane.Closed-cell phenolic-type materials and products, which areblown with similar gases, age differently and much more slowly be-cause of their closed-cell struc

39、ture.Other Influences. Convection and air infiltration in or throughsome insulation systems may increase heat transfer. Low-density,loose-fill, large open-cell, and fibrous insulation, and poorlydesigned or installed reflective systems are the most susceptible. Thetemperature difference across the i

40、nsulation and the height and widthof the insulated space influence the amount of convection. In somecases, natural convection may be inherent to the system (Wilkes andChilds 1992; Wilkes and Rucker 1983), but in many cases it is a con-sequence of careless design and/or construction of the insulateds

41、tructure (Donnelly et al. 1976). Gaps between board- and batt-typeinsulations lower their effectiveness. Board-type insulation may notbe perfectly square, may be installed improperly, and may be appliedto uneven surfaces. A 4% void area around batt insulation can pro-duce a 50% loss in effective the

42、rmal resistance for ceiling applicationwith R = 19 hft2F/Btu (Verschoor 1977). Similar and worseresults have been obtained for wall configurations (Brown et al.1993; Hedlin 1985; Lecompte 1989; Lewis 1979; Rasmussen et al.1993; Tye and Desjarlais 1981). As a solution, preformed joints inboard-type i

43、nsulation allow boards to fit together without air gaps.Boards and batts can be installed in two layers, with joints betweenlayers offset and staggered. The requirements of ASHRAE Stan-dard 90.1 provide additional guidance on proper installation ofinsulating materials, as does Chapter 44 in the 2015

44、 ASHRAEHandbookHVAC Applications.Measurement. Apparent thermal conductivity for insulationmaterials and systems is obtained by the measuring methods listed inASTM (2008). These methods apply mainly to laboratory measure-ments on dried or conditioned samples at specific mean temperaturesand temperatu

45、re gradient conditions. Although fundamental heattransmission characteristics of a material or system can be deter-mined accurately, actual performance in a structure may vary fromlaboratory results. Only field measurements can clarify the differ-ences. Field-test procedures continue to be developed

46、. Envelopedesign, construction, and material may all affect the procedure to befollowed, as detailed in ASTM (1985a, 1985b, 1988, 1990, 1991).1.2 MATERIALS AND SYSTEMSGlass Fiber and Mineral WoolGlass fiber is produced using recycled glass, whereas mineralwool uses diabase stone. Glass and stone are

47、 melted, after which aspinning head stretches the melt into fibers with diameter 10 m.These fall through a spray of phenol or silicon binder onto the facingsfor blankets and batts, which lie on a conveyor belt. The fiber blan-kets, batts, or boards pass a heated press where the binder hardensand the

48、 insulation gets its final density and thickness. After passingthrough the press, the blankets, batts, or boards are cut to size. Thespectrum of finished products includes loose fill; over blankets andbatts; and soft, semidense, and dense boards. Blankets cannot takeany extra load, except their own

49、weight. Dense boards are moder-ately compression resistant, with a modulus of 10, or about 5.5 to11.5 psia.Mineral wool and glass fiber may look similar, but there areimportant differences. Glass fiber consists of well-ordered, longfibers, whereas mineral wool is composed of unordered, shorterfibers. Glass is also amorphous, whereas diabase stone is crystal-line.The thermal conductivity of glass fiber is somewhat lower thanfor mineral wool (see Table 1), with lower values for higher-densityblankets in both materials. Glass and mineral fiber are very vaporperme