ASHRAE FUNDAMENTALS IP CH 25-2013 Heat Air and Moisture Control In Building Assemblies-Fundamentals.pdf

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1、25.1CHAPTER 25HEAT, AIR, AND MOISTURE CONTROL IN BUILDING ASSEMBLIESFUNDAMENTALSTerminology and Symbols . 25.1Environmental Hygrothermal Loads and Driving Forces . 25.2HEAT TRANSFER . 25.5Steady-State Thermal Response 25.5Transient Thermal Response . 25.8AIRFLOW . 25.9MOISTURE TRANSFER . 25.10Moistu

2、re Storage in Building Materials . 25.10Moisture Flow Mechanisms 25.12COMBINED HEAT, AIR, AND MOISTURE TRANSFER 25.14SIMPLIFIED HYGROTHERMAL DESIGN CALCULATIONS AND ANALYSES 25.15Surface Humidity and Condensation 25.15Interstitial Condensation and Drying . 25.15TRANSIENT COMPUTATIONAL ANALYSIS 25.16

3、Criteria to Evaluate Hygrothermal Simulation Results 25.16ROPER design of space heating, cooling, and air-conditioningPsystems requires detailed knowledge of the building envelopesoverall heat, air, and moisture performance. This chapter discussesthe fundamentals of combined heat, air, and moisture

4、movement asit relates to the analysis and design of envelope assemblies. Guid-ance for designing mechanical systems is found in other chapters ofthe ASHRAE Handbook.Because heat, air, and moisture transfer are coupled and closelyinteract with each other, they should not be treated separately. Infact

5、, improving a building envelopes energy performance maycause moisture-related problems. Evaporation of water and removalof moisture by other means are processes that may require energy.Only a sophisticated moisture control strategy can ensure hygienicconditions and adequate durability for modern, en

6、ergy-efficientbuilding assemblies. Effective moisture control design must dealwith all hygrothermal loads (heat and humidity) acting on thebuilding envelope.TERMINOLOGY AND SYMBOLSThe following heat, air, and moisture definitions and symbols arecommonly used.A building envelope or building enclosure

7、 provides physicalseparation between the indoor and outdoor environments. A build-ing assembly is any part of the building envelope, such as wallassembly, window assembly, or roof assembly, that has boundaryconditions at the interior and the exterior of the building. A buildingcomponent is any eleme

8、nt or material within a building assembly.HeatSpecific heat capacity c is the change in heat (energy) of unitmass of material for unit change of temperature in Btu/lbF.Volumetric heat capacity c is the change in heat stored in unitvolume of material for unit change of temperature, in Btu/ft3F.Heat f

9、lux q, a vector, is the time rate of heat transfer through a unitarea, in Btu/hft2.Thermal conductivity k in Europe, the Greek letter (lambda)is used is a material property defined by Fouriers law of heat con-duction. Thermal conductivity is the parameter that describes heatflux through a unit thick

10、ness of a material in a direction perpendic-ular to the isothermal planes, induced by a unit temperature differ-ence. (ASTM Standard C168 defines homogeneity.) Units areBtuin/hft2F (preferred) or Btu/hftF. Materials can be isotropicor anisotropic. For anisotropic materials, the direction of heat flo

11、wthrough the material must be noted. Thermal conductivity must beevaluated for a specific mean temperature, thickness, age, and mois-ture content. Thermal conductivity is normally considered an intrin-sic property of a homogenous material. In porous materials, heatflow occurs by a combination of con

12、duction, convection, radiation,and latent heat exchange processes and may depend on orientation,direction, or both. When nonconductive modes of heat transfer occurwithin the specimen or the test specimen is nonhomogeneous, themeasured property of such materials is called apparent thermalconductivity

13、. The specific test conditions (i.e., sample thickness,orientation, environment, environmental pressure, surface tempera-ture, mean temperature, temperature difference, moisture distribu-tion) should be reported with the values of apparent thermalconductivity. The symbol kapp(or app) is used to deno

14、te the lack ofpure conduction or to indicate that all values reported are apparent.Materials with a low apparent thermal conductivity are called insu-lation materials (see Chapter 26 for more detail).Thermal resistivity ruis the reciprocal of thermal conductivity.Units are hft2F/Btuin.Thermal resist

15、ance R is an extrinsic property of a material orbuilding component determined by the steady-state or time-averagedtemperature difference between two defined surfaces of the materialor component that induces a unit heat flux, in ft2hF/Btu. When thetwo defined surfaces have unequal areas, as with heat

16、 flux throughmaterials of nonuniform thickness, an appropriate mean area andmean thickness must be given. Thermal resistance formulas involv-ing materials that are not uniform slabs must contain shape factors toaccount for the area variation involved. When heat flux occurs byconduction alone, the th

17、ermal resistance of a layer of constant thick-ness may be obtained by dividing the materials thickness by its ther-mal conductivity. When several modes of heat transfer are involved,the apparent thermal resistance may be obtained by dividing thematerials thickness by its apparent thermal conductivit

18、y. When aircirculates within or passes through insulation, as may happen in low-density fibrous materials, the apparent thermal resistance is affected.Thermal resistances of common building and insulation materialsare listed in Chapter 26.Thermal conductance C is the reciprocal of thermal resistance

19、.Units are Btu/hft2F.Heat transfer or surface film coefficient h is the proportionalityfactor that describes the total heat flux by both convection and radi-ation between a surface and the surrounding environment. It is theheat transfer per unit time and unit area induced by a unit tempera-ture diff

20、erence between the surface and reference temperature in thesurrounding environment. Units are Btu/hft2F. For convection tooccur, the surrounding space must be filled with air or another fluid.If the space is evacuated, heat flow occurs by radiation only. In thecontext of this discussion, indoor or o

21、utdoor heat transfer or sur-face film coefficient hior hodenotes an interior or exterior surfaceThe preparation of this chapter is assigned to TC 4.4, Building Materialsand Building Envelope Performance.25.2 2013 ASHRAE HandbookFundamentalsof a building envelope assembly. The heat transfer film coef

22、ficientis also commonly known as the surface film conductance.Thermal transmittance U is the quantity equal to the steady-state or time-averaged heat flux from the environment on the oneside of a body to the environment on the other side, per unit temper-ature difference between the two environments

23、, in Btu/hft2F.Thermal transmittance is sometimes called the overall coefficientof heat transfer or U-factor. Thermal transmittance includes ther-mal bridge effects and the surface heat transfer at both sides of theassembly.Thermal emissivity is the ratio of radiant flux emitted by a sur-face to tha

24、t emitted by a black surface at the same temperature.Emissivity refers to intrinsic properties of a material. Emissivity isdefined only for a specimen of the material that is thick enough to becompletely opaque and has an optically smooth surface.Effective emittance E refers to the properties of a p

25、articular ob-ject. It depends on surface layer thickness, oxidation, roughness, etc.AirAir transfer Mais the time rate of mass transfer by airflowinduced by an air pressure difference, caused by wind, stack effect,or mechanical systems, in lbm/s.Air flux ma, a vector, is the air transfer through a u

26、nit area in thedirection perpendicular to that unit area, in lb/ft2h.Air permeability kais an intrinsic property of porous materialsdefined by Darcys Law (the equation for laminar flow throughporous materials). Air permeability is the quantity of air fluxinduced by a unit air pressure difference thr

27、ough a unit thickness ofhomogeneous porous material in the direction perpendicular to theisobaric planes. Units are in lb/fthin. Hg or lb/ftsin. Hg.Air permeance Kais the extrinsic quantity equivalent to the timerate of steady-state air transfer through a unit surface of a porousmembrane or layer, a

28、 unit length of joint or crack, or a local leakinduced by a unit air pressure difference over that layer, joint andcrack, or local leak. Units are lb/ft2hin. Hg for a layer, lb/fthin.Hg for a joint or crack, and lb/hin. Hg for a local leak.Moisture Moisture content w is the amount of moisture per un

29、it volume ofporous material, inlb/ft3.Moisture ratio X (in weight) or (in volume) is the amount ofmoisture per unit weight of dry porous material or the volume ofmoisture per unit volume of dry material, in percent.Specific moisture content is the ratio between a change in mois-ture content and the

30、corresponding change in driving potential (i.e.,relative humidity or suction).Specific moisture ratio is the ratio between a change in moistureratio and the corresponding change in driving potential (i.e., relativehumidity or suction).Water vapor flux mv, a vector, is the time rate of water vaportra

31、nsfer through a unit area, in lb/ft2h.Moisture transfer Mmis the moisture flow induced by a differ-ence in suction or in relative humidity, in lb/h.Moisture flux mm, a vector, is the time rate of moisture transferthrough a unit area, in lb/ft2h.Water vapor permeability pis the steady-state water vap

32、orflux through a unit thickness of homogeneous material in a directionperpendicular to the isobaric planes, induced by a unit partial watervapor pressure difference, under specified conditions of tem-perature and relative humidity. Units are lb/fthin. Hg. Whenpermeability varies with psychrometric c

33、onditions, the specific per-meability defines the property at a specific condition.Water vapor permeance M is the steady-state water vaporflux by diffusion through a unit area of a flat layer, induced bya unit partial water vapor pressure difference across that layer,in lb/ft2hin. Hg.Water vapor res

34、istance Z is the reciprocal of water vapor per-meance, in ft2hin. Hg/lb.Moisture permeability kmis the steady-state moisture fluxthrough a unit thickness of a homogeneous material in a directionperpendicular to the isosuction planes, induced by a unit differencein suction. Units are lb/fthin. Hg (su

35、ction).Moisture diffusivity Dmis the ratio between the moisture per-meability and the specific moisture content, in ft2/h.ENVIRONMENTAL HYGROTHERMAL LOADS AND DRIVING FORCESThe main function of a building enclosure is separation of indoorspaces from the outdoor climate. This section describes the hy

36、gro-thermal loads acting on the building envelope. These descriptionsare used to predict their influence on the hygrothermal behavior ofbuilding assemblies, as a basis for design recommendations andmoisture control measures (Knzel and Karagiozis 2004). Coolingand heating load estimations for sizing

37、mechanical systems can befound in Chapters 17 and 18.In Figure 1, the hygrothermal loads relevant for building enve-lope design are represented schematically for an external wall. Gen-erally, they show diurnal and seasonal variations at the exteriorsurface and mainly seasonal variations at the inter

38、ior surface. Dur-ing daytime, the exterior wall surface heats by solar radiation, lead-ing to evaporation of moisture from the surface layer. Aroundsunset, when solar radiation decreases, long-wave (infrared) emis-sion may lead to cooling below ambient air temperature (under-cooling) of the exterior

39、 surface, and surface condensation mayoccur. The exterior surfaces are also exposed to moisture from pre-cipitation and wind-driven rain.Usually, several load cycles overlap (e.g., summer/winter, day/night, rain/sun). Therefore, a precise analysis of the expected hygro-thermal loads should be done b

40、efore starting to design any buildingenvelope component. However, the magnitude of loads is not inde-pendent of building geometry and the components properties.Analysis of the transient hygrothermal loads is generally based onhourly meteorological data. However, determination of local condi-tions at

41、 the envelopes surface is rather complicated and requiresspecific experience. In some cases, computer simulations are nec-essary to assess the microclimate acting on differently oriented orinclined building assemblies.Fig. 1 Hygrothermal Loads and Alternating Diurnal or Seasonal Directions Acting on

42、 Building EnvelopeHeat, Air, and Moisture Control in Building AssembliesFundamentals 25.3Ambient Temperature and HumidityAmbient temperature and humidity with respect to partial watervapor pressure are the boundary conditions always affecting bothsides of the building envelope. The climate-dependent

43、 exterior con-ditions may show large diurnal and seasonal variations. Therefore,at least hourly data are required for detailed building simulations,though monthly data may suffice in case simple calculation methodsare applicable. ASHRAE provides such meteorological data sets,including temperature an

44、d relative humidity, for many locationsworldwide (see Chapter 14). These data sets usually represent aver-age meteorological years based on long-term observations at spe-cific locations. However, data of more extreme climate conditionsmay be important to assess the risks of moisture damage. Therefor

45、e,Sanders (1996) proposed using data of the coldest or warmest yearin 10 years for hygrothermal analysis instead of data from an aver-age year. Another method to obtain a severe annual dataset concern-ing the moisture-related damage risk from several decades of hourlydata has been developed by Salon

46、vaara (2011). This method ana-lyzes the data with respect to their effect on moisture behavior oftypical building assemblies. The more severe datasets increase thesafety of risk prediction for the service life of building envelopecomponents, but they are less suitable for analyzing the long-termbeha

47、vior (performance over several years) of constructions becausethe probability of a sequence of severe years is very low. Also, notethat the temperature at the building site may differ from the meteo-rological reference data when the sites altitude differs from that ofthe station recording the data.

48、On average, there is a temperatureshift of 1.2F for every 330 ft. The microclimate around the build-ing may result in an additional temperature shift that depends on theseason. For example, the proximity of a lake can moderate seasonaltemperature variations, with higher temperatures in winter andlow

49、er temperatures in summer compared to sites without waternearby. A low-lying site experiences lower temperatures in winter,whereas city temperatures are higher year round (METEOTEST2007).Indoor Temperature and HumidityIndoor climate conditions depend on the purpose and occupationof the building. For most commercial constructions, temperature andhumidity are controlled by HVAC systems with usually well-definedset points. Indoor humidity conditions in residential buildings, how-ever, are influenced by the outdoor cli