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

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1、25.1CHAPTER 25 HEAT, 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 . 25.10Moist

2、ure 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 25.1

3、6Criteria 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. Infac

5、t, 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, e

6、nergy-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 enclosur

7、e 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 elem

8、ent 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 J/(kgK).Volumetric heat capacity c is the change in heat stored in unitvolume of material for unit change of temperature, in J/(m3K).Heat f

9、lux q, a vector, is the time rate of heat transfer through a unitarea, in W/m2.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 thickness

10、 of a material in a direction perpen-dicular to the isothermal planes, induced by a unit temperature dif-ference. (ASTM Standard C168 defines homogeneity.) Units areW/(mK). Materials can be isotropic or anisotropic. For anisotro-pic materials, the direction of heat flow through the material mustbe n

11、oted. Thermal conductivity must be evaluated for a specificmean temperature, thickness, age, and moisture content. Thermalconductivity is normally considered an intrinsic property of ahomogenous material. In porous materials, heat flow occurs by acombination of conduction, convection, radiation, and

12、 latent heatexchange processes and may depend on orientation, direction, orboth. When nonconductive modes of heat transfer occur within thespecimen or the test specimen is nonhomogeneous, the measuredproperty of such materials is called apparent thermal conductiv-ity. The specific test conditions (i

13、.e., sample thickness, orientation,environment, environmental pressure, surface temperature, meantemperature, temperature difference, moisture distribution) shouldbe reported with the values of apparent thermal conductivity. Thesymbol kapp(or app) is used to denote the lack of pure conductionor to i

14、ndicate that all values reported are apparent. Materials with alow apparent thermal conductivity are called insulation materials(see Chapter 26 for more detail).Thermal resistivity ruis the reciprocal of thermal conductivity.Units are (mK)/W.Thermal resistance R is an extrinsic property of a materia

15、l 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 (m2K)/W. When thetwo defined surfaces have unequal areas, as with heat flux throughmaterials of nonuniform thickness

16、, 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 thermal resistance of a layer of constant thick-

17、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 conductivity. When aircirculates within or passes through

18、 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.Units are W/(m2K).Heat transfer or surface fi

19、lm 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 difference between the surface and reference tempera

20、ture in thesurrounding environment. Units are W/(m2K). 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 outdoor heat transfer or sur-face film coefficient

21、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 HandbookFundamentals (SI)of a building envelope assembly. The heat transfer film coefficientis also commonly known as the surface

22、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, in W/(m2K). Ther-mal transmittance is somet

23、imes called the overall coefficient ofheat transfer or U-factor. Thermal transmittance includes thermalbridge 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 that emitted by a black surface at the same temper

24、ature.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 particular ob-ject. It depends on surface layer

25、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 kg/s.Air flux ma, a vector, is the air transfer through a unit area in thedirection perpendicular to that u

26、nit area, in kg/(sm2).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 through a unit thickness ofhomogeneous porous mate

27、rial in the direction perpendicular to theisobaric planes. Units are in kg/(Pasm) or s.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 unit length of joint or crack, or a local leakinduced by a u

28、nit air pressure difference over that layer, joint andcrack, or local leak. Units are kg/(Pasm2) for a layer, kg/(Pasm)for a joint or crack, or kg/(Pas) for a local leak.Moisture Moisture content w is the amount of moisture per unit volume ofporous material, inkg/m3.Moisture ratio X (in mass) or (in

29、 volume) is the amount ofmoisture per unit mass 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 corresponding change in driving potential (i.e.,relative humidity or suctio

30、n).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 vaportransfer through a unit area, in kg/(sm2).Moisture transfer Mmis the moisture

31、flow induced by a differ-ence in suction or in relative humidity, in kg/s.Moisture flux mm, a vector, is the time rate of moisture transferthrough a unit area, in kg/(sm2).Water vapor permeability pis the steady-state water vaporflux through a unit thickness of homogeneous material in a directionper

32、pendicular to the isobaric planes, induced by a unit partial watervapor pressure difference, under specified conditions of tem-perature and relative humidity. Units are kg/(Pasm). Whenpermeability varies with psychrometric conditions, the specific per-meability defines the property at a specific con

33、dition.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 kg/(Pasm2).Water vapor resistance Z is the reciprocal of water vapor per-meance, in ( m2 sPa)/kg.Moisture

34、 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 kg/(Pasm) (suction).Moisture diffusivity Dmis the ratio between the moisture per-meability and t

35、he specific moisture content, in m2/s.ENVIRONMENTAL HYGROTHERMAL LOADS AND DRIVING FORCESThe main function of a building enclosure is separation of indoorspaces from the outdoor climate. This section describes the hygro-thermal loads acting on the building envelope. These descriptionsare used to pre

36、dict 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 mechanical systems can befound in Chapters 17 and 18.In Figure 1, the hygrothermal l

37、oads 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 interior surface. Dur-ing daytime, the exterior wall surface heats by solar radiation, le

38、ad-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 surface, and surface condensation mayoccur. The exterior surfaces are also exposed

39、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 before starting to design any buildingenvelope component. However, the magnitude of l

40、oads 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 the envelopes surface is rather complicated and requiresspecific experience. In som

41、e 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 Building EnvelopeHeat, Air, and Moisture Control in Building AssembliesFundamentals

42、 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 exterior con-ditions may show large diurnal and seasonal variations. Therefore,at l

43、east 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 and relative humidity, for many locationsworldwide (see Chapter 14). These data sets u

44、sually 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. Therefore,Sanders (1996) proposed using data of the coldest or warmest yearin 10 years for h

45、ygrothermal 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 Salonvaara (2011). This method ana-lyzes the data with respect to their effect on moistur

46、e 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-termbehavior (performance over several years) of constructions becausethe probability of a s

47、equence 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. On average, there is a temperatureshift of 0.65 K for every 100 m. The microclimate

48、around thebuilding may result in an additional temperature shift that dependson the season. For example, the proximity of a lake can moderateseasonal temperature variations, with higher temperatures in winterand lower temperatures in summer compared to sites without waternearby. A low-lying site exp

49、eriences 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 climate and by occupant behav-ior. Moisture release in an average household is highly variable.According to San