1、27.1CHAPTER 27HEAT, AIR, AND MOISTURE CONTROL IN BUILDING ASSEMBLIESEXAMPLESHEAT TRANSFER 27.1One-Dimensional Assembly U-Factor Calculation . 27.1Two-Dimensional Assembly U-Factor Calculation . 27.3MOISTURE TRANSPORT . 27.7Wall with Insulated Sheathing 27.7Vapor Pressure Profile (Glaser or Dew-Point
2、) Analysis. 27.8TRANSIENT HYGROTHERMAL MODELING 27.10AIR MOVEMENT. 27.11HERMAL and moisture design as well as long-term perfor-Tmance must be considered during the planning phase of build-ings. Installing appropriate insulation layers and taking appropriateair and moisture control measures can be mu
3、ch more economicalduring construction than later. Design and material selection shouldbe based on Building useInterior and exterior climateSpace availabilityThermal and moisture properties of materialsOther properties required by location of materialsDurability of materialsCompatibility with adjacen
4、t materialsPerformance expectations of the assemblyDesigners and builders often rely on generic guidelines and pastbuilding practice as the basis for system and material selection.Although this approach may provide insight for design decisions,selections and performance requirements should be set th
5、roughengineering analysis of project-specific criteria. Recent develop-ments have increased the capabilities of available tools and methodsof thermal and moisture analysis.This chapter draws on Chapter 25s fundamental information onheat, air and moisture transport in building assemblies, as well asC
6、hapter 26s material property data. Examples here demonstrate cal-culation of heat, moisture, and air transport in typical assemblies.For design guidance for common building envelope assemblies andconditions, see Chapter 44 of the 2011 ASHRAE HandbookHVACApplications.Insulation specifically for mecha
7、nical systems is discussed in Chap-ter 23. For specific industrial applications of insulated assemblies, seethe appropriate chapter in other ASHRAE Handbook volumes. Inthe 2010 ASHRAE HandbookRefrigeration, for refrigerators andfreezers, see Chapters 15, 16, and 17; for insulation systems forrefrige
8、rant piping, see Chapter 10; for refrigerated-facility design,see Chapters 23 and 48; for trucks, trailers, rail cars, and containers,see Chapter 25; for marine refrigeration, see Chapter 26. For envi-ronmental test facilities, see Chapter 37 in the 2002 ASHRAE Hand-bookRefrigeration.Engineering pra
9、ctice is predicated on the assumption that perfor-mance effects can be viewed in functional format, where discreteinput values lead to discrete output values that may be assessed foracceptability. Heat transfer in solids lends itself to engineering anal-ysis because material properties are relativel
10、y constant and easy tocharacterize, the transport equations are well established, analysisresults tend toward linearity, and, for well-defined input values, out-put values are well defined. Airflow and moisture transport analysis,in contrast, is difficult: material properties are difficult to charac
11、ter-ize, transport equations are not well defined, analysis results tendtoward nonlinearity, and both input and output values include greatuncertainty. Air movement is even more difficult to characterize thanmoisture transport.Engineering makes use of the continuum in understanding fromphysical prin
12、ciples, to simple applications, to complex applications,to design guidance. Complex design applications can be handled bycomputers; however, this chapter begins by presenting simpler exam-ples as a learning tool. Because complex applications are built upfrom simpler ones, understanding the simpler a
13、pplications ensuresthat a critical engineering oversight of complex (computer) applica-tions is retained. Computers have facilitated widespread use of two-and three-dimensional analysis as well as transient (time-dependent)calculations. As a consequence, steady-state calculations are lesswidely used
14、. Design guidance, notably guidance regarding use of airand vapor barriers, faces changes in light of sophisticated transientcalculations. ASHRAE Standard 160 creates a framework for usingtransient hygrothermal calculations in building envelope design.However, designers should recognize the limitati
15、ons of these tools,as discussed in the following sections, and the need for continuedadvancements in the methods of analysis and understanding of heatand moisture migration in buildings.The following definitions pertain to heat transfer properties ofenvelope assemblies (see Chapter 25).HEAT TRANSFER
16、ONE-DIMENSIONAL ASSEMBLY U-FACTOR CALCULATIONWall Assembly U-FactorThe assembly U-factor for a building envelope assembly deter-mines the rate of steady-state heat conduction through the assembly.One-dimensional heat flow through building envelope assemblies isthe starting point for determining whol
17、e-building heat transmit-tance.Example 1. Calculate the system R-value RSystem, assembly total resistance(RAssembly), and UAssembly-factor of the sandwich panel assembly shownin Figure 1; assume winter conditions when selecting values for airfilms from Table 3 in Chapter 26.Solution: Determine indoo
18、r and outdoor air film resistances fromTable 3 in Chapter 26, and thermal resistance of all components fromTable 1 in that chapter. If any elements are described by conductivityThe preparation of this chapter is assigned to TC 4.4, Building Materialsand Building Envelope Performance.Symbol Definitio
19、nRSystem, CSystemSystem resistance (conductance); surface-to-surface resistance (conductance) for all materials in wall, including parallel paths for framingRAssembly, UAssemblyAssembly resistance (transmittance); air-to-air thermal resistance (transmittance), equal to system value plus film resista
20、nces (conductances)UWholeAssembly thermal transmittance, including thermal bridges (i.e., UAssemblyplus bridge conductances)Note: For all code applications that call for U, UWholeshould be used.27.2 2013 ASHRAE HandbookFundamentals (SI)(independent of thickness) rather than thermal resistance (thick
21、ness-dependent), then calculate the resistance.The conductivity k of expanded polystyrene is 0.035 W/(mK). For150 mm thickness,Rfoam= x/k = 0.150/0.035 = 4.29 (m2K)/WTo calculate the systems R-value in the example, sum the R-values ofthe system components only, disregarding indoor and outdoor air fi
22、lms.RSystem= 0.107 + 0.011 + 0.07 + 4.29 + 0.079 = 4.56 (m2K)/WThe assembly R-value (RAssembly) consists of the systems R-value plusthe thermal resistance of the interior and exterior air films.RAssembly= Ro+ RSystem+ Ri= 4.70 (m2K)/WThe walls UAssembly-factor is 1/RAssembly, or 0.21 W/(m2K).Roof As
23、sembly U-FactorExample 2. Find the U-factor of the commercial roof assembly shown inFigure 2; assume summer conditions when selecting values for air filmsfrom Table 3 in Chapter 26.Solution: The calculation procedure is similar to that shown in Exam-ple 1. Note the U-factor of nonvertical assemblies
24、 depends on the direc-tion of heat flow i.e., whether the calculation is for winter (heat flowup) or summer (heat flow down), because the resistances of indoor airfilms and plane air spaces in ceilings differ, based on the heat flowdirection (see Table 3 in Chapter 26). The effects of mechanical fas
25、ten-ers are not addressed in this example.Using UAssembly= 1/RAssembly, the UAssembly-factor is 0.17 W/(m2K).AtticsDuring sunny periods, unconditioned attics may be hotter thanoutdoor air. Peak attic temperatures on a hot, sunny day may be 10to 45 K above outdoor air temperature, depending on factor
26、s such asshingle color, roof framing type, air exchange rate through vents,and use of radiant barriers. Therefore, simple one-dimensional solu-tions cannot be offered for attics. Energy efficiency estimates can beobtained using models such as Wilkes (1991).Basement Walls and FloorsHeat transfer thro
27、ugh basement walls and floors to the grounddepends on the following factors: (1) the difference between theair temperature in the room and that of the ground and outside air,(2) the material of the walls or floor, and (3) the thermal conduc-tivity of surrounding earth. The latter varies with local c
28、onditionsand is usually unknown. Because of the great thermal inertia ofsurrounding soil, ground temperature varies with depth, and thereis a substantial time lag between changes in outdoor air tempera-tures and corresponding changes in ground temperatures. As aresult, ground-coupled heat transfer i
29、s less amenable to steady-state representation than above-grade building elements. However,there are several simplified procedures for estimating ground-coupled heat transfer. These fall into two main categories: (1) thosethat reduce the ground heat transfer problem to a closed-form solu-tion, and (
30、2) those that use simple regression equations developedfrom statistically reduced multidimensional transient analyses.Closed-form solutions, including Latta and Boileaus (1969) pro-cedure discussed in Chapter 17, generally reduce the problem to one-dimensional, steady-state heat transfer. These proc
31、edures use simple,“effective” U-factors or ground temperatures or both. Methods differin the various parameters averaged or manipulated to obtain theseeffective values. Closed-form solutions provide acceptable results inclimates that have a single dominant season, because the dominantseason persists
32、 long enough to allow a reasonable approximation ofsteady-state conditions at shallow depths. The large errors (percent-age) that are likely during transition seasons should not seriouslyaffect building design decisions, because these heat flows are rela-tively insignificant compared to those of the
33、 principal season.The ASHRAE arc-length procedure (Latta and Boileau 1969) isa reliable method for wall heat losses in cold winter climates. Chap-ter 17 discusses a slab-on-grade floor model developed by onestudy. Although both procedures give results comparable to tran-sient computer solutions for
34、cold climates, their results for warmerU.S. climates differ substantially.Research conducted by Dill et al. (1945) and Hougten et al.(1942) indicates a heat flow of approximately 6.3 W/m2through anuninsulated concrete basement floor with a temperature differenceof 11 K between the basement floor and
35、 the air 150 mm above it. AU-factor of 5.7 W/(m2K) is sometimes used for concrete basementfloors on the ground. For basement walls below grade, the temper-ature difference for winter design conditions is greater than for theElement R, (m2K)/W1. Outdoor air film 0.0302. Vinyl siding (hollow backed) 0
36、.1073. Vapor-permeable felt 0.0114. Oriented strand board (OSB), 11 mm 0.115. 150 mm expanded polystyrene, extruded (smooth skin) 5.286. 13 mm gypsum wallboard 0.0797. Indoor air film 0.120Total 5.70Fig. 1 Structural Insulated Panel Assembly (Example 1)Fig. 2 Roof Assembly (Example 2)Element R, (m2K
37、)/W1. Indoor air film 0.162. 100 mm concrete, k = 0.7 0.073. 75 mm cellular polyisocyanurate (gas-impermeable facers)4.974. 25 mm mineral fiberboard 0.525. 9 mm built-up roof membrane 0.066. Outdoor air film 0.04Total 5.81Heat, Air, and Moisture Control in Building AssembliesExamples 27.3floor. Test
38、 results indicate that, at the mid-height of the below-gradeportion of the basement wall, the unit area heat loss is approxi-mately twice that of the floor.For small concrete slab floors (equal in area to a 7.5 by 7.5 mhouse) in contact with the ground at grade level, tests indicate thatheat loss ca
39、n be calculated as proportional to the length of exposededge rather than total area. This amounts to 1.4 W per linear metreof exposed edge per degree temperature difference between indoorair and the average outdoor air temperature. This value can bereduced appreciably by installing insulation under
40、the ground slaband along the edge between the floor and abutting walls. In most cal-culations, if the perimeter loss is calculated accurately, no otherfloor losses need to be considered. Chapters 17 and 18 contain heattransfer and load calculation guidance for floors on grade and at dif-ferent depth
41、s below grade.The second category of simplified procedures uses transient two-dimensional computer models to generate ground heat transfer data,which are then reduced to compact form by regression analysis(Mitalas 1982, 1983; Shipp 1983). These are the most accurate pro-cedures available, but the da
42、tabase is very expensive to generate. Inaddition, these methods are limited to the range of climates and con-structions specifically examined. Extrapolating beyond the outerbounds of the regression surfaces can produce significant errors.Guide details and recommendations related to application of co
43、n-cepts for basements are provided in Chapter 44 of the 2011 ASHRAEHandbookHVAC Applications. Detailed analysis of heat transferthrough foundation insulation may also be found in the BuildingFoundation Design Handbook (Labs et al. 1988).TWO-DIMENSIONAL ASSEMBLY U-FACTOR CALCULATIONThe following exam
44、ples show three methods of two-dimensional,steady-state conductive heat transfer analysis through wall assem-blies. They offer approximations to overall rates of heat transfer(U-factor) when assemblies contain a layer composed of dissimilarmaterials. The methods are described in Chapter 25. The para
45、llel-path method is used when the thermal conductivity of the dissimilarmaterials in the layer are rather close in value (within the same orderof magnitude), as with wood-frame walls. The isothermal-planesmethod is appropriate for materials with conductivities moderatelydifferent from those of adjac
46、ent materials (e.g., masonry). The zonemethod and the modified zone method are appropriate for materialswith a very high difference in conductivity (two orders of magnitudeor more), such as with assemblies containing metal.Two-dimensional, steady-state heat transfer analysis is oftenconducted using
47、computer-based finite difference methods. If theresolution of the analysis is sufficiently fine, computer methods pro-vide better simulations than any of the methods described here, andthe results typically show better agreement with measured values.The methods described here do not take into accoun
48、t heat storagein the materials, nor do they account for varying material properties(e.g., when thermal conductivity is affected by moisture content ortemperature). Transient analysis is often used in such cases.Wood-Frame WallsThe assembly R-values and U-factors of wood-frame walls canbe calculated
49、by assuming either parallel heat flow paths throughareas with different thermal resistances or by assuming isothermalplanes. Equation (15) in Chapter 25 provides the basis for the twomethods.The framing factor expresses the fraction of the total buildingcomponent (wall or roof) area that is framing. The value depends onthe specific type of construction, and may vary based on local con-struction practices, even for the same type of construction. For studwalls 400 mm on center (OC), the fraction of insulated cavity maybe as low as 0.75, where the fraction of studs, plate
