ASHRAE FUNDAMENTALS IP CH 27-2013 Heat Air and Moisture Control In Building Assemblies-Examples.pdf

1、27.1CHAPTER 27 HEAT, 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-Poin

2、t) 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 m

3、uch 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 adjace

4、nt 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 t

5、hroughengineering 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 as

6、Chapter 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 mech

7、anical 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 forrefrig

8、erant 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 pr

9、actice 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 relative

10、ly 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 chara

11、cter-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 pri

12、nciples, 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

13、applications 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 use

14、d. 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 limitat

15、ions 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 TRANSFE

16、RONE-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 who

17、le-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 indo

18、or 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 Definiti

19、onRSystem, 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 resist

20、ances (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(independent of thickness) rather than thermal resistance (thickness

21、-dependent), then calculate the resistance.The conductivity k of expanded polystyrene is 0.20 Btuin/hft2F.For 6 in. thickness,Rfoam= x/k = 6/0.20 = 30.0 hft2F/BtuTo calculate the systems R-value in the example, sum the R-values ofthe system components only, disregarding indoor and outdoor air films.

22、RSystem= 0.62 + 0.06 + 0.62 + 30.0 + 0.45 = 31.75 hft2F/BtuThe assembly R-value (RAssembly) consists of the systems R-value plusthe thermal resistance of the interior and exterior air films.RAssembly= Ro+ RSystem+ Ri= 32.6 hft2F/BtuThe walls UAssembly-factor is 1/RAssembly, or 0.031 Btu/hft2F.Roof A

23、ssembly 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 assemblie

24、s 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 fa

25、sten-ers are not addressed in this example.Using UAssembly= 1/RAssembly, the UAssembly-factor is 0.030 Btu/hft2F.AtticsDuring sunny periods, unconditioned attics may be hotter thanoutdoor air. Peak attic temperatures on a hot, sunny day may be 20to 80F above outdoor air temperature, depending on fac

26、tors 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 t

27、hrough 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 loca

28、l conditionsand 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 transfe

29、r is 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, an

30、d (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 p

31、rocedures 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 persi

32、sts 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

33、the 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 f

34、or 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 2.0 Btu/hft2throughan uninsulated concrete basement floor with a temperature differ-ence of 20F between the basement f

35、loor and the air 6 in. above it.A U-factor of 0.10 Btu/hft2F is sometimes used for concretebasement floors on the ground. For basement walls below grade,the temperature difference for winter design conditions is greaterthan for the floor. Test results indicate that, at the mid-height of theElement R

36、, hft2F/Btu1. Outdoor air film 0.172. Vinyl siding (hollow backed) 0.623. Vapor-permeable felt 0.064. Oriented strand board (OSB), 7/16 in. 0.625. 6 in. expanded polystyrene, extruded (smooth skin) 30.06. 0.5 in. gypsum wallboard 0.457. Indoor air film 0.68Total 32.6Fig. 1 Structural Insulated Panel

37、 Assembly (Example 1)Fig. 2 Roof Assembly (Example 2)Element R, hft2F/Btu1. Indoor air film 0.922. 4 in. concrete, 120 lb/ft3and k = 8 0.53. 3 in. cellular polyisocyanurate (gas-impermeable facers)28.24. 1 in. mineral fiberboard 2.945. 3/8 in. built-up roof membrane 0.336. Outdoor air film 0.25Total

38、 33.1Heat, Air, and Moisture Control in Building AssembliesExamples 27.3below-grade portion of the basement wall, the unit area heat loss isapproximately twice that of the floor.For small concrete slab floors (equal in area to a 25 by 25 fthouse) in contact with the ground at grade level, tests indi

39、cate thatheat loss can be calculated as proportional to the length of exposededge rather than total area. This amounts to 0.81 Btu/h per linearfoot of exposed edge per degree temperature difference betweenindoor air and the average outdoor air temperature. This value canbe reduced appreciably by ins

40、talling insulation under the groundslab and along the edge between the floor and abutting walls. Inmost calculations, if the perimeter loss is calculated accurately, noother floor losses need to be considered. Chapters 17 and 18 containheat transfer and load calculation guidance for floors on grade

41、andat different depths 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

42、available, but the database 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

43、 to application of con-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 CALCULAT

44、IONThe following examples 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

45、 Chapter 25. The parallel-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 moderatelydifferen

46、t from those of adjacent 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

47、oftenconducted using 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

48、 not take into account 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 wa

49、lls canbe calculated 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 16 in. on center (OC), the fraction of insulated cavity may beas low