ASHRAE FUNDAMENTALS IP CH 25-2017 Heat Air and Moisture Control In Building Assemblies-Fundamentals.pdf

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

2、 Storage in Building Materials 25.10Moisture Flow Mechanisms . 25.11COMBINED HEAT, AIR , AND MOISTURE TRANSFER. 25.14SIMPLIFIED HYGROTHERMAL DESIGN CALCULATIONS AND ANALYSES. 25.14Surface Humidity and Condensation . 25.14Interstitial Condensation and Drying 25.14TRANSIENT COMPUTATIONAL ANALYSIS . 25

3、.15Criteria 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 mois

4、ture 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 interactclosely with each other, they should not be treated separately. F

5、orexample, improving a building envelopes energy performance maycause moisture-related problems. Conversely, evaporation of waterand removal of moisture by other means are processes that requireenergy. Only a sophisticated moisture control strategy can ensurehygienic conditions and adequate durabili

6、ty for modern, energy-efficient building assemblies. Effective moisture control design mustdeal with all hygrothermal (heat and humidity) loads acting on thebuilding envelope.1. FUNDAMENTALS1.1 TERMINOLOGY AND SYMBOLSThe following heat, air, and moisture definitions, properties, andsymbols are commo

7、nly used.A building envelope or building enclosure provides physicalseparation between the indoor space and the outdoor environment. Abuilding assembly is any part of the building envelope, such as awall, window, or roof assembly, that faces the interior and exterior ofthe building. A building compo

8、nent is any element, layer, or mate-rial within a building assembly.HeatSpecific heat capacity c is the change in heat (energy) of a unitmass of material for a unit change of temperature in Btu/lbF.Volumetric heat capacity c is the change in heat stored in a unitvolume of material for a unit change

9、of temperature, in Btu/ft3F.Heat flux q, a vector, is the time rate of heat transfer through a unitarea, in the direction perpendicular to that area, in Btu/hft2.Thermal conductivity k in Europe, the Greek letter (lambda) isused is a material property describing ability to conduct heat, and isdefine

10、d by Fouriers law of heat conduction. Thermal conductivity isthe property that describes heat flux through a unit thickness of a ma-terial in a direction perpendicular to the isothermal planes, inducedby a unit temperature difference. (ASTM Standard C168 defineshomogeneity.) Units are Btuin/hft2F (p

11、referred) or Btu/hftF.For anisotropic materials, the direction of heat flux in the materialmust be noted. Thermal conductivity must be evaluated for a specificmean temperature, thickness, age, and moisture content. Thermalconductivity is normally considered an intrinsic property of a homog-enous mat

12、erial. In porous materials, heat flow occurs by a combina-tion of conduction, convection, and radiation, and may depend onorientation, direction, or both. When nonconductive modes of heattransfer occur within the specimen or the test specimen is nonhomo-geneous, the measured property of such materia

13、ls is called apparentthermal conductivity. The specific test conditions (e.g., samplethickness, orientation, environment, environmental pressure, surfacetemperature, mean temperature, temperature difference, moisturedistribution) should be reported with the values of apparent thermalconductivity. Th

14、e symbol kapp(or app) is used to denote the absenceof pure 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 conductiv

15、ity.Units are hft2F/Btuin.Thermal resistance R is an extrinsic property that describes theresistance of a material layer or assembly to heat transfer. It is deter-mined by the steady-state or time-averaged temperature difference(between two defined surfaces of a material layer within a buildingassem

16、bly) that induces a unit heat flux, in ft2hF/Btu. When thetwo defined surfaces have unequal areas, as with heat flux throughmaterial layers of nonuniform thickness, an appropriate mean areaand mean thickness must be given. Thermal resistance formulasinvolving materials that are not uniform slabs mus

17、t contain shapefactors to account for the area variation involved. When heat fluxoccurs by conduction alone, the thermal resistance of a layer of con-stant thickness may be obtained by dividing the materials thicknessby its thermal conductivity. When several modes of heat transfer areinvolved, the a

18、pparent thermal resistance may be obtained bydividing the materials thickness by its apparent thermal conductiv-ity. When air circulates within or passes through insulation, as mayhappen in low-density fibrous materials, the apparent thermal resis-tance is affected. Thermal resistances of common bui

19、lding and insu-lation materials are listed in Chapter 26.Thermal conductance C is the reciprocal of thermal resistance.Units are Btu/hft2F.Heat transfer or surface film coefficient h is the value that de-scribes the total heat flux by both convection and radiation betweena surface and the surroundin

20、g environment. It is defined as the heattransfer per unit time and unit area induced by a unit temperaturedifference between the surface and the reference temperature in thesurrounding environment. Units are Btu/hft2F. For convection toThe preparation of this chapter is assigned to TC 4.4, Building

21、Materialsand Building Envelope Performance.25.2 2017 ASHRAE HandbookFundamentals occur, the surrounding space must be filled with a fluid, usually air.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 coe

22、fficient hior horelates to an interior or exterior surfaceof a building envelope assembly. The heat transfer film coefficientis 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

23、 oneside of a body to the environment on the other side, per unit tem-perature difference between the two environments, in Btu/hft2F.Thermal transmittance is sometimes called the overall coefficientof heat transfer or U-factor. Average thermal transmittance differsfrom clear-wall transmittance, in t

24、hat the former considers all ther-mal bridge effects in the assembly.Thermal emissivity is the ratio of radiant flux emitted by a sur-face to that emitted by a black surface at the same temperature. Emis-sivity refers to intrinsic properties of a materials surface and isdefined only for a specimen o

25、f 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 thickness, oxidation, roughness, etc.AirAir transfer Mais the time rate of mass transfer by airflowinduce

26、d 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 unit area in thedirection perpendicular to that unit area, in lb/ft2h.Air permeability kais an intrinsic property of a porous materialdefined by Darcys L

27、aw (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 material in the direction perpendicular to theisobaric planes. Units are in lb/fthin. Hg or lb/ftsin. Hg.Air

28、 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 unit air pressure difference over that layer, joint andcrack, or local leak. Units are lb/f

29、t2hin. 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 unit 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 th

30、e 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 suction).Specific moisture ratio is the ratio between a change in moistureratio an

31、d 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 the direction perpendicular to that unitarea, in lb/ft2h.Moisture transfer Mmis the moisture flow induced by a differ-ence

32、 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 the direction perpendicular to that unit area, inlb/ft2h.Water vapor permeability pis the steady-state water vaporflux through a unit thickness of homogeneous material

33、 in a direc-tion perpendicular to the isobaric planes, induced by a unit partialwater vapor pressure difference, under specified conditions oftemperature and relative humidity. Units are lb/fthin. Hg. Whenpermeability varies with psychrometric conditions, the specificpermeability defines the propert

34、y 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 resistance Z is the reciprocal of water vapor per-meance, in

35、 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 (suction).Moisture diffusivity Dmis the ratio between the mo

36、isture per-meability and the specific moisture content, in ft2/h.1.2 HYGROTHERMAL LOADS AND DRIVING FORCESThis section describes the hygrothermal loads acting on thebuilding envelope. That description is used to predict the influenceon the hygrothermal behavior of building assemblies, as a basis for

37、design recommendations and moisture control measures (Knzeland Karagiozis 2004). Cooling and heating load estimations for siz-ing mechanical systems can be found in Chapters 17 and 18.In Figure 1, the loads relevant for building envelope design arepresented schematically for an external wall. Genera

38、lly, they showdiurnal and seasonal variations at the exterior surface and mainlyseasonal variations at the interior surface. In sunny weather, theexterior wall surface heats by solar radiation, leading to evaporationof moisture from the surface layer. Around sunset, when solar radi-ation decreases,

39、long-wave (infrared) emission to the clear sky maylead to cooling of the exterior surface below the ambient air tem-perature, even below the dew-point temperature, so surface conden-sation may occur. This phenomenon is called undercooling. Theexterior surfaces are also exposed to moisture from preci

40、pitationand wind-driven rain.Usually, several load cycles overlap (e.g., summer/winter, day/night, rain/sun). Therefore, a precise analysis of the expected loadsshould be done before designing any building envelope component.However, the magnitude of the loads is not independent of buildinggeometry

41、and the components properties. Analysis of the transienthygrothermal loads is generally based on hourly meteorologicalFig. 1 Hygrothermal Loads and Alternating Diurnal or Seasonal Directions Acting on Building EnvelopeHeat, Air, and Moisture Control in Building AssembliesFundamentals 25.3data, altho

42、ugh a shorter time step may be needed. However, deter-mination of local conditions at the envelopes surface is complicatedand requires specific experience. In some cases, computer simula-tions are necessary to assess the microclimate acting on differentlyoriented, overhang-protected, or inclined bui

43、lding assemblies.Ambient Temperature and HumidityAmbient temperature and humidity, represented by the partialwater vapor pressure, are the boundary conditions always affectingboth sides of the building envelope. The climate-dependent exteriorconditions may show large diurnal and seasonal variations.

44、 There-fore, at least hourly data are required for detailed building simula-tions, though monthly data may suffice in case simple calculationmethods are applicable. Chapter 14 provides such meteorologicaldata sets, including temperature and relative humidity, for manylocations worldwide. These data

45、sets usually represent average mete-orological years based on long-term observations at specific loca-tions. However, data of more extreme climate conditions may beimportant to assess the risks of moisture damage. Therefore, Sanders(1996) proposed using data of the coldest or warmest year in 10 year

46、sfor hygrothermal analysis instead of data from an average year.Another method to obtain a severe annual data set concerning themoisture-related damage risk starting from several decades of hourlydata has been developed by Salonvaara (2011). This method ana-lyzes the data with respect to their effec

47、t on moisture behavior of typ-ical building assemblies. The more severe data sets increase thesafety of risk prediction for the service life of building envelope com-ponents, but they are less suitable for analyzing the long-term behav-ior (performance over several years) of constructions because th

48、eprobability of a sequence of severe years is very low. Also, note thatthe temperature at the building site may differ from the meteorolog-ical reference data when the sites altitude differs from that of the sta-tion recording the data. On average, there is a temperature shift of1.2F for every 330 f

49、t. The microclimate around the building mayresult in an additional temperature shift that depends on the season.For example, the proximity of a lake can moderate seasonal tem-perature variations, with higher temperatures in winter and lowertemperatures in summer compared to sites without water nearby. Alow-lying site experiences lower temperatures in winter, whereascity temperatures are higher year round (METEOTEST 2007).Indoor Temperature and HumidityIndoor conditions depend on the purpose and occupation of thebuildin