ASHRAE AB-10-009-2010 Modeling the Hygrothermal Behavior of Field-Tested Walls Exposed to South Carolina Conditions.pdf

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1、424 ASHRAE TransactionsABSTRACTThis ASHRAE project looked at the nature, significance and control of solar-driven vapor diffusion in wall systems. The proj-ect combined experimental and simulation work to provide an in-depth characterization of the phenomena occurring during inwards vapor diffusion

2、in insulated wall assemblies. This paper presents the modeling of the results of a field study that was performed over a period of 2 years and where possible occur-rence of solar driven diffusion was documented for different wall assemblies. The specifics of the modeling such walls under climatic co

3、nditions are presented and discussed. The verifica-tion of the modeling results against the field testing results once more demonstrated the importance of obtaining the complete description of the wall systems, along with any degradation that occurred during the test period. Excellent agreement was

4、found for the wall case when accurate material properties and descrip-tion of wall systems was available. It is important to use material property data of the specific wall system during the validation phase of the hygrothermal modeling.INTRODUCTIONThe work presented here was performed in the framew

5、ork of ASHRAE project 1235 entitled “The nature, significance and control of solar-driven diffusion in wall systems”, initiated by ASHRAE Technical Committee 4.4. The verification of modeling capacities was part of the project. Modeling is used to predict the hygrothermal transport during solar-driv

6、en moisture transport. This paper presents work done to test and demonstrate the ability of the Oak Ridge National Laboratory (ORNL) advanced hygrothermal model (MOISTURE-EXPERT), developed by Karagiozis (2001, 2004), to predict complex hygrothermal processes that are involved in the mois-ture redis

7、tribution processes when wetting (water saturation), and drying with and without the presence of solar radiation, takes place in walls. Researched performed by Derome (2009) and Carmeliet (2009) at both the small scale and intermediate scale level further contributed to the knowledge base. The physi

8、cal processes present are: water storage, liquid transport, vapor transport, evaporation, and condensation. A field study Karagiozis (2009a) provided data that were compared against model MOISTURE-EXPERT predictions.The purposes of this activity of the ASHRAE TRP-1235 research project were:To use ad

9、vanced hygrothermal modeling and compare with experimental data developed from the ASHRAE project. This benchmark activity is an important one as the intent for this ASHRAE project was to use modeling for the development of climate specific recommenda-tions for the impact of solar driven moisture. T

10、he intent was to enhance confidence towards the implementation of the model to undertake broader parametric studies for the ASHRAE 1235 project. To evaluate further and identify performance character-istics that are specifically present in solar driven mois-ture transport mechanisms. Specifically to

11、 address the question of whether there are some mechanisms present that need to be accounted for. The aim of any numerical modeling is obviously to repre-sent reality, but this is challenging as one cannot easily model the physical enclosure exactly (each crack, twist, and imper-fection). Our knowle

12、dge of needed material properties is always incomplete and the properties are variable, and our ability to model every hygrothermal mechanisms is somewhat Modeling the Hygrothermal Behavior ofField-Tested Walls Exposed to South Carolina ConditionsAchilles Karagiozis, PhD Dominique Derome, PhDMember

13、ASHRAEJan Carmeliet, PhD Andre DesjarlaisAchilles Karagiozis is a distinguished research and development engineer and Andre Desjarlais is a group leader at the Building Technology Center, Oak Ridge National Laboratory, TN. Dominique Derome is a group leader at the Wood Laboratory, Swiss Federal Labo

14、ratories for Materials Testing and Research EMPA, Dbendorf, Switzerland. Jan Carmeliet is chair of building physics at the Swiss Federal Institute of Technology ETH Zrich and head of the Building Science and Technologies Laboratory, EMPA, Dbendorf, Switzerland.AB-10-0092010, American Society of Heat

15、ing, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission

16、.2010 ASHRAE 425limited. Constraints of time and resources oblige us to do the following:be as complex and comprehensive as possible when accuracy is required, andbe as complex and comprehensive as needed when rela-tive accuracy is sufficient.We believe that the benchmark results presented in this p

17、aper show that the verification objectives were met. HYGROTHERMAL MODELThe heat, air and moisture transport phenomena, in the presence of solar driven processes in wall systems, is complex. Many simultaneous transport processes may be present in all climatic conditions. To accurately capture the the

18、rmal and moisture movement required that the software tool to be used in the parametric analysis had to have the following qualifications:a. At least 2-dimensional analysis capabilityb. Includes heat transportc. Includes vapor transportd. Includes liquid transporte. Includes air transportf. Includes

19、 coupling of thermal and air flow to capture natu-ral convectiong. Includes coupling of thermal, vapor and liquid moisture flow to capture thermally driven moisture transporth. Include wind-driven raini. Include evaporation/condensation and freeze/thaw latent transportj. Is a transient modelk. Inclu

20、des radiative exchange in air cavitiesl. As full functional dependencies of material properties, both on moisture content and on temperaturem. Includes hysteresis and temperature dependencies (sorp-tion, water vapor permeabilities, thermal conductivities) n. Uses solutions that are not time step dep

21、endento. Includes solar radiation, nocturnal radiation exchange and cloud interferenceBased on the qualifications required to perform the research investigation a limited number of computer models could be used. One such demonstrated model was MOIS-TURE-EXPERT developed by Karagiozis (2001a, b, c, 2

22、004). This model was used in the modeling activity.Description of the Hygrothermal ModelThe simulation code used is an advanced hygrothermal simulation model that has been used extensively to develop design guidelines and guidance to numerous heat, air and moisture transport problems. For example, t

23、he current Inter-national Council Code (ICC) building code recommendations for vapor control strategies have the scientific basis drawn from this simulation model. Other example is the recently completed ASHRAE TRP 1091 on Rainscreen and cavity ventilation wall systems that employed MOISTURE-EXPERT

24、for the parametric study (Burnett et al 2006).The moisture transport potentials used in the model are moisture content, vapor pressure and relative humidity; for energy transfer, temperature is the driving factor. This model includes functional dependencies of material properties on moisture content

25、 and temperature when the data was available (Karagiozis and Salonvaara, 1995). The model has 2-dimen-sional capabilities and a quasi-3D model exists. The equations on the left side are formulated based on Knzel (1995) thesis when cast in delta-form (see Karagiozis 1997). The governing equations are

26、 as follows:Moisture BalanceThe moisture transport balance is given as:(1)where m= dry density of porous material, kg/m3(lb/ft3)D= liquid moisture transport coefficient, kg/m s (lb/fts)u = moisture content, kgwet/kgdry, (lbwet/lbdry)T = temperature, C (F) = relative humidityp= vapor permeability, kg

27、/smPa (equivalent Perm in)Pv= vapor pressure, PaVa= velocity of air, m/s (ft/s)v= density of vapor in the air, kg/m3 (lb/ft3)t = time in seconds Air BalanceThe air mass balance is given as:(2)where a= dry density of air, kg/m3(lb/ft3)Momentum BalanceThe momentum balance (Navier Stokes equation) is g

28、iven as:(3)wherePa= air pressure, PaKa= air permeability, s/m (s/ft)a= dynamic viscosity, m/s2 (ft/s2)g = gravity, m/s2 (ft/s2)mu()t- D pPvvVa+()=at- aVa()+ 0=aVa()t- aVaVa()+ Pa 2aKa- Vaag+=2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Publi

29、shed in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.426 ASHRAE TransactionsEnergy BalanceHeat transfer in a porous media is comple

30、x. Present in the material are conduction, convection, evaporation / condensa-tion sources and radiation heat transfer. The equation govern-ing this scalar quantity is given below as: (4)where Cp= heat capacity, J/kgK (Btu/lbF)k = thermal conductivity, W/mK (Btu/(fthF)Lv= enthalpy of evaporation, J/

31、kg (Btu/lb)Lice= enthalpy of freezing, J/kg (Btu/lb)fl= liquid fraction ()The boundary conditions are defined as follows:Vapor Mass Flow at the Faces of the Geometry(5)wheremv= vapor mass flow, kg/m2s (lb/ft2s) = convective mass transfer coefficient, s/m (s/ft)Liquid Flow at the Faces of the Geometr

32、y(6)wheremliquid= liquid flow, kg/m2s (lb/ft2s)with a maximum moisture content equal to capillary moisture content of the exterior surface. The maximum flow rate is given by the predetermined wind-driven rain flux. The wind-driven rain mass flow mdriving rainavailable at the face of the geometry is

33、predicted using the ASHRAE 160 standard. Heat Flux at Surface Including Solar Radiation(7)(8)whereTeq= equivalent temperature (including shortwave solar and longwave radiation with environment)Cp,w= heat capacity of liquid water, J/kgKhc= convective heat transfer coefficient, W/m2Khr= radiative heat

34、 transfer coefficient, W/m2KHygrothermal Material PropertiesAs with any simulation exercise, the quality of the comparison between the field measured data and the hygro-thermal simulations is dependent on the material properties used. In this project, material properties were measured for the key ma

35、terial elements for the exterior claddings and exterior sheathing layers, but for framing materials and interior sheath-ings data was used from Knzel (1995), ASTM (2001) and Kumaran (2001). The hygrothermal material properties of EIFS wall lamina were measured at the Oak Ridge National Laboratory. T

36、he full description of the material properties can be found in Karagiozis (2009b).FIELD MONITORINGEight wall assemblies were monitored under field condi-tions. All walls were facing south. The test facility (shown in Figure 1) was situated in Charleston (Hollywood), South Carolina, where the climate

37、 is categorized as a hot and humid climate (EERE 2009).Wall CompositionAll walls were made of 38 mm by 89 mm (nominal 2” x 4”) wood framing at 400 mm (16”) on centre, insulated with R-11 unfaced glass fiber insulation and closed on the inside with gypsum board. Cladding types included exterior finis

38、h insulation system (EIFS), clay brick masonry, and cement stucco. The types of weather resistive barrier (WRB) were liquid applied WRB, building paper (BP), spun bonded poly-olefin membrane (SBPO) and double ventilated WRB (DV WRB), i.e. a corrugated sheet. In terms of exterior sheathing, either pl

39、ywood or oriented strand board (OSB) was retained. Finally, the different interior finishes were vapor open paint, polyethylene sheet and vapor tight vinyl wall covering (VWC). Although the verification exercise presented in this paper uses mainly data from the two first walls listed, the eight wall

40、s involved in the project are listed in Table 1 for sake of completion.For year 1, the walls were constructed in August/Septem-ber of 2004 and were allow to be acclimatized until equilib-rium conditions were established. The start period for data collection was Jan. 1 2005 and continued until 1 May

41、2006. Monitoring Temperature, relative humidity and moisture content were recorded at several heights and depths through out the wall specimens as shown in Figure 2. The monitoring system includes 17 thermistors, 6 relative humidity sensors and 8 moisture content sensors. In Figure 3, a few of the s

42、ensors are displayed as instrumented in the wall assembly. The moisture contents (MC1,T1), (MC3,T3) are located 50mm (1.97 in) (below the top plate while (MC2, T2) and (MC5,T5) are located 50 mm (1.97 in) above the bottom plate. The difference between MC1 and MC2 versus MC3 and MC5 is the depth of t

43、he moisture pin at either 10 mm mCpmTt- aCpaVaT()= kT()LvpPv()Licemuf1t-+ +mv PvaPvsurf()Vav+=mliquidmdriving rain=qsurfheffTeqTsurf()VaaTmliquidCpw,TamvLv+ +=heffhrhc+=2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transac

44、tions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.2010 ASHRAE 427(0.39 in) or 3 mm (0.118 in) respectively. MC1 and MC2 are closer to the exterior tha

45、n MC3 and MC5. MC4 is located 901 mm (35.47 in) from the top plate into the sheathing board, while MC6 and MC7 are located in the top and bottom plate respectively. MC4, MC6 and MC7 are all located 3 mm (0.118 in) into the wood material. MC8 is located in the studs at 1000 mm (39.37 in) from the top

46、 plate. The relative humidity (RH) sensors are placed in the following manner:RH1 is attached to the exterior face of the foam or in contact with inner most part of the brick/mortar siding, or inner most surface of stucco, (placed in the middle of stud cavity, top).RH2 is attached to inner face of t

47、he foam or weather resistive barrier for brick/cement siding/stucco, (placed in the middle of stud cavity, top).RH3 is bored into the exterior surface of the sheathing board, (placed in the middle of stud cavity).RH4 is attached to the interior side of the sheathing board, (placed in the middle of s

48、tud cavity, top).RH5 is attached to the interior side of the sheathing board, (placed in the middle of stud cavity, bottom).RH6 is attached to the exterior surface of the gypsum board, (placed in the middle of stud cavity, top).Environmental Data The interior environment provided was dynamic condi-t

49、ions typically present in a hot and humid zone. These condi-tions ranged between at 23C (73.4 F) and 40% RH in winter and 21C (69.8 F) and 60% RH in summer, as shown in Figure 4.For recording the outdoor conditions, a weather station was constructed, where the exterior temperature, relative Figure 1 View of the field test building, Charleston (Hollywood), South Carolina. Walls 2 and 14 are mainly used for the verification study.Table 1. Main