sustainability
Article Thermal Behavior and Measures to Prevent Condensation of a Newly Developed External Wall Panel
Goopyo Hong, Suk-Won Lee, Ji-Yeon Kang and Hyung-Geun Kim *
SH Urban Research Center, Seoul Housing & Communities Corporation, Seoul 06336, Korea; [email protected] (G.H.); [email protected] (S.-W.L.); [email protected] (J.-Y.K.) * Correspondence: [email protected]; Tel.: +82-2-3410-8510
Received: 31 December 2018; Accepted: 4 February 2019; Published: 11 February 2019
Abstract: An external wall panel (EWP) as a novel alternative to provide spatial flexibility and improve the performance of external walls was developed. The purpose of this study was to analyze the thermal performance of this EWP. A simulation analysis was carried out to scrutinize whether it was vulnerable to condensation, considering South Korea’s weather conditions, and find countermeasures to prevent this. Results indicated that the indoor surface temperature with the measures of added insulation materials and an inserted thermal-breaker was over 16.5 ◦C and that these methods could prevent condensation. In addition, this study assessed unsteady-state thermal characteristics, linear thermal transmittance, and the effective thermal transmittance of EWP. Effective thermal transmittance was estimated in consideration of the heat transmittance of EWP and the linear thermal transmittance of its slabs and its connection parts. The thermal characteristics of the building envelope are needed to analyze effective thermal transmittance and linear thermal transmittance-associated thermal bridges.
Keywords: external wall panel; thermal performance; thermal transmittance; condensation
1. Introduction According to statistical data in 2017, South Korea had 17.12 million housing units, and 10.37 million (approximately 61%) of them were apartment units [1]. Due to the mass supply of apartment complexes in a relatively short period of time, only a limited number of fixed and standardized planar designs have been used in South Korea [2]. Because 95% of these apartments have a bearing wall system, residents are not allowed to change their residential space. Thus, they cannot experience spatial diversity. In reality, there is a limit to accommodating the diverse lifestyles and needs of residents. In addition, it is not easy to maintain and repair facilities. Apartments built 30 years ago or less are being demolished for rebuilding due to the degradation of facilities and external walls. Bearing wall systems rather than column-beam systems are often used to construct apartments due to their economic advantages with respect to materials and construction [3–6], although column-beam systems are also considered for spatial flexibility. In recent years, South Korea has witnessed an increase in apartments built with column-beam systems, enabling long-term usage through a structure with spatial flexibility, easy maintenance, and easy repair [7,8]. External walls of apartments built with a column-beam system or a bearing wall system do not function as structural materials to support the load of a building. For an effective design of flexible space and easy repair for future building improvement, it is essential to systemize building components and develop mutually compatible systems [9]. It is possible to use buildings over the long term and in an efficient way by developing an external wall with easy
Sustainability 2019, 11, 912; doi:10.3390/su11030912 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 912 2 of 14 reparability and improved performance. With this background, this study developed an EWP that could be detached and attached as a novel alternative to provide spatial flexibility and improve the performance of external walls. This EWP was developed for external insulation to minimize thermal bridging effects, and its module systems could provide easy constructability. External wall design and selection of its materials are important elements of the building envelope. The external wall is a non-structural material, a protective covering that is fixed on the outdoor surface of a building and used to protect against moisture and thermal variations while providing aesthetic purposes [10]. The envelope of a building is a factor that has the greatest influence on the environment inside the building [11]. The building envelope separates the indoor environment that is air-conditioned by heating and cooling systems from the external environment that is not air-conditioned [12]. It protects indoor space from weather conditions and large fluctuations in temperature. A proper building envelope provides thermal comfort for occupants and conserves building energy at the same time [13,14]. Several studies have investigated the thermal performance of external walls or claddings. Bojic et al. reported the thermal behaviors of residential buildings by varying the thickness and positions of thermal insulation to walls. They found out that the largest reduction of the cooling demand was observed when thermal insulation was located either at the outer or indoor side of the wall [15]. In addition, Bojic and Loveday have investigated the thermal effect of layer distribution and the relative thickness of building insulation on building envelopes. In their study, the location of insulation in the wall can significantly influence thermal behaviors in buildings [16]. A similar study regarding thermal performance by differentiating the location of insulation materials in building envelopes was also reported [17,18]. Another important parameter that can have an impact on the thermal response of building envelopes is thermal mass. Kontoleon and Bikas showed that solar absorptivity throughout building envelopes influences the time delay of peak temperatures in buildings [19,20]. Other studies performed simulations to investigate the impact of the thermal characteristics of the external walls to reduce thermal bridges in building envelopes. Various insulation materials were applied to the walls for the evaluation of the energy performance [21,22]. Moreover, the condensation effect on the thermal behaviors in the external walls was investigated. Morgan, McGowan, and Flick explored the condensation of the external wall on manufacturing facility and decided to create an inner space to achieve anti-condensation effects [23]. Gorrell also observed the water problems of precast concrete panels as a cladding due to the inadequate separation and insulation between panels and adjacent cladding components [24]. Therefore, the purpose of the present study was to evaluate thermal performance of the EWP developed in this study and determine whether it was vulnerable to condensation, considering South Korea’s weather conditions [25,26]. A simulation analysis was carried out to examine the possibility of condensation and find countermeasures to prevent it. In addition, this study assessed the linear thermal transmittance, effective thermal transmittance, and unsteady-state thermal characteristics of the EWP.
2. Development of External Wall Panel (EWP)
2.1. Introduction of the Developed EWP As an envelope applicable to multi-unit housings, this study developed a detachable EWP with flexibility and easy maintenance. It can be prefabricated at a workshop and installed on site. This panel was developed to ensure long-term usage by making EWPs attachable and detachable to improve the performance of external walls. The EWP was made of floor units and units as shown in Figure1. Upper slabs and lower slabs were connected by the floor units of EWP. Units of EWP were installed in a vertical direction from each floor. Floor units and units were fabricated with the same materials. From the outside to the inside, the EWP consisted of 30 mm of extruded cement panel, a 25 mm insulation board, and an aluminum frame for the exterior side, followed by 2 sheets of 85 mm glass wool, another Sustainability 2018, 10, x FOR PEER REVIEW 3 of 15 Sustainability 2019, 11, 912 3 of 14
Sustainability 2018, 10, x TableFOR PEER 1. Material REVIEW layers of the developed external wall panel (EWP). 3 of 15 10 mm insulation board, and a 9 mm cellulose fiber-reinforced cement board for the interior side as Type Layers shown in Table1. Table 1. Material layers of the developed external wall panel (EWP). Floor Unit/Unit ECP 1 (35 mm) + PIR Board 2 (25 mm) + Glass Type Table 1. Material layers of the developedLayersWool external(85 mm+85 wall mm) panel (EWP). Floor Unit/Unit ECP+ PIR 1 Board(35 mm) 2 (10 + mm)PIR Board+ CRC 2Board (25 mm) 3 (9 mm)+ Glass Type1 ECP: extruded cement panel. Layers 2 PIR Board: polyisocyanurateWool (85 mm+85 board. mm) 3 CRC Board: cellulose fiber- ECP 1 (35reinforced mm) + PIR cement Board+ PIR board. 2Board(25 mm) 2 (10 + Glassmm) Wool+ CRC (85 Board mm + 3 85 (9 mm) mm) Floor Unit/Unit 2 3 1 ECP: extruded cement panel.+ PIR 2Board PIR Board:(10 mm)polyisocyanurate + CRC Board board.(9 mm) 3 CRC Board: cellulose fiber- 1 2 3 ECP: extruded cement panel. PIR Board:reinforced polyisocyanurate cement board. board. CRC Board: cellulose fiber-reinforced cement board.
Figure 1. Floor unit and unit of the EWP. Figure 1. Floor unit and unit of the EWP. As shown shown in in Figure Figure 2,2, floor floor units units were were installed installed by by fixing fixing them them to to each each slab, slab, and and units units could could be assembledbe assembledFigure with 1. Floor with a tilt-in aunit tilt-in method and method unit from of fromthe the EWP. interior the interior side. side.Depending Depending on the on size the and size weight and weight of the ofEPW, the itEPW, couldAs it couldshownbe transported be in transported Figure and2, floor andinstalled units installed wereusing using installed smal small-scalel-scale by fixingequipment. equipment. them to The each The use useslab, of of modularizedand modularized units could units units be assembledmakes it easy with to ainstall tilt-in them method on site.from In Inthe South South interior Kore Korea, sia,de. the the Depending external external wall wallon the is is constructed constructedsize and weight by by concrete concrete of the EPW, and and itisis couldinstalled be inside insidetransported insulation, insulation, and installedas as with with most using apartments. small-scale The Theequipment. advantage advantage The of of usethis this ofmethod method modularized is is that that it itunits can makesshortenshorten it the theeasy construction to install them period. on site. On On theIn the South other other Korehand, hand,a, in inthe general, general, external installing installing wall is constructed outs outsideide insulation insulation by concrete requires requires and theisthe installed installation installation inside of scaffolding insulation, or as gondola with most and apartments. so on. The advantage of this method is that it can shorten the construction period. On the other hand, in general, installing outside insulation requires the installation of scaffolding or gondola and so on.
Figure 2. Transportation and installation of the developed EWP using small-scale equipment.
2.2. Basic Performance Experiment of EWP (Methods and Results) Figure 2. Transportation and installation of the developed EWP using small-scale equipment. After fabricating the EWP, an actual-size mock-up experiment was carried out to evaluate its environmental and structural performance as a basic building envelope. Performance evaluation items Figure 2. Transportation and installation of the developed EWP using small-scale equipment.
Sustainability 2018, 10, x FOR PEER REVIEW 4 of 15
2.2. Basic Performance Experiment of EWP (Methods and Results)
SustainabilityAfter 2019fabricating, 11, 912 the EWP, an actual-size mock-up experiment was carried out to evaluate4 of its 14 environmental and structural performance as a basic building envelope. Performance evaluation items included a thermal cycling test, air leakage test, static and dynamic water penetration test, includedstructural a performance thermal cycling test, test, story air displacement leakage test, statictest, and and residual dynamic strain water test. penetration For the thermal test, structural cycling performancetest, heating test,was storymaintained displacement in the test,chamber and residual at 24 °C strain for one test. hour, For the according thermal cyclingto the temperature test, heating ◦ wascondition maintained of AAMA in the chamber (American at 24 ArchitecturalC for one hour, Manufacturers according to the temperatureAssociation) condition501.5. After of AAMA the ◦ (Americantemperature Architectural was kept at Manufacturers82 °C for two hours, Association) it was slowly 501.5. lowered After the to temperature -18 °C for 2 washours, kept which at 82 wasC ◦ forthen two maintained hours, it for was 2 slowlyhours. After lowered that, to the−18 temperC forature 2 h, was which increased was then to 24 maintained °C for one for hour. 2 h. This After 8- ◦ that,hour thecycle temperature was repeated was three increased times toin 2424 hours.C for one The hour. air leakage This 8-h test cycle was was conducted repeated by three keeping times the in 2 24pressure h. The at air + 7.6 leakage kgf/m test2 (the was standard conducted test pressure) by keeping and the measuring pressure the at +7.6 air leakage kgf/m from(the the standard specimen. test pressure)The water and penetration measuring test the was air leakageconducted from by the keeping specimen. 30.4 Thekgf/m water2 of positive penetration pressure test was and conducted spraying 2 2 by204 keepingL/m2 for 30.4 15 kgf/mminutes,of and positive measuring pressure water and sprayingpenetration. 204 The L/m storyfor 15displacement min, and measuring test was waterconducted penetration. by imposing The storythe displacement displacement of test 9.4 was mm(L/300) conducted both by left imposing and right the horizontally displacement and of 9.4examining mm (L/300) if the both functions left and and right appearances horizontally of andall pa examiningrts have problems. if the functions The residual and appearances strain test of was all partsconducted have problems.by keeping The the positive residual pressure strain test (75%) was and conducted negative by pressure keeping (150%) the positive of the pressuredesigned (75%)wind andload negativeof the specimen pressure for (150%) 10 seconds of the designed and then wind removing load of the the pressure, specimen and for 10measuring s and then the removing residual thestrain pressure, of each andpart. measuring the residual strain of each part. Figure3 shows3 shows flowchart flowchart of the of experiment the experiment to evaluate to eachevaluate performance each performance item. The experimental item. The methodsexperimental and criteriamethods are and shown criteria in Tableare shown2. The in performance Table 2. The of performance the building envelopeof the building that connected envelope EWPthat connected with windows EWP waswith evaluatedwindows was in accordance evaluated within accordance the standard with test the method standard for test the method structural for performancethe structural of performance external windows, of external curtain windows, walls, and curtain doors. walls, Figure and4 shows doors. the Figure installation 4 shows of thethe experimentalinstallation of subject the experimental and its performance subject and testing. its performance testing.
Figure 3. Mock-up test.
Figure 3. Mock-up test. SustainabilitySustainability2019 2018, 11, 10, 912, x FOR PEER REVIEW 5 of 5 15 of 14
(a) Installation (b) EWP and window
(c) Air leakage test (d) Structural performance test
Figure 4. Mock-up test. Figure 4. Mock-up test.
Table 2. Test methods and results for performance evaluation
Items Test Methods Allowance Measured Results Hot cycle (+82 ◦C) Thermal cycling AAMATable 2. 501.5-07 Test methodsCold and cycle results (−18 for◦ C)performanceNo evaluation damage No damage test 3rd cycle Items Test Methods Allowance Measured Results 50% of design Pre-load test ASTM E330-14 Hot cycle (+82 °C) No damage No damage Thermal cycling wind load ASTMAAMA E283-04 501.5-07 Cold cycle (-18 °C) No damage No damage Air leakagetest +7.6 kgf/m2 1.09 CMH/m2 1.06 CMH/m2 (for window) 3rd cycle 50% of design wind Pre-load test ASTM E330-14 No damage No damage load Sustainability 2019, 11, 912 6 of 14
Table 2. Cont.
Items Test Methods Allowance Measured Results ASTM E 331 (uniform Static air 20% of design No uncontrolled water Water penetration difference)/AAMA No leakage wind load leakage 501.1-05 (dynamic pressure) 1. 100% maximum displacement test (+50% → +100% → 1. L/175 (=12.43 mm) Structural −50% → −100%) ASTM E 330-14 2. No permanent 1.53 mm (OK) performance 2. 150% residual damage strain test (+75% → +150% → −75% → −150%) L/300 Story No wall components AAMA 501.4-09 (high-occupancy No deflection displacement may fall off assembly) 75% and 150% of positive and 2L/1000 1.33 mm (positive) Residual strain ASTM E330-02 negative design (=4.35 mm) 1.31 mm (negative) wind load Full specimen area: 4064 mm (W) × 3405 mm (H), PVC sliding window: 1800 mm (W) × 2085 mm (H)
As shown in Table2, the mock-up test results confirmed that the EWP met the basic environmental and structural performance requirements as a building envelope. Therefore, its thermal performance was evaluated next.
3. Thermal Properties of the Developed EWP
3.1. Model and Method Figure5 shows the floor plan of a household where the developed EWP is going to be installed. The floor plan had a size of 59 m2. The house had three bedrooms, one kitchen, and two bathrooms. The EWP was planned to be installed at the north and south of the plan. Figure5 shows a partial cross-section drawing of the EWP. The image on the right of Figure5 shows a simulation model to evaluate its thermal properties. It shows that the ceiling part is joined with the developed EWP and the insulated-concrete wall of an adjacent household. The insulated-concrete wall had 140 mm concrete, 110 mm insulation materials with a thermal conductivity of 0.03 W/(mK), and 9.5 mm gypsum board. The surface temperature of the interior side of the EWP and the possibility of the occurrence of condensation were assessed through this analysis model. The PHYSIBEL TRISCO program was used for the steady-state 3-dimensional heat transfer analysis [27]. SustainabilitySustainability2019 2018, 11,, 91210, x FOR PEER REVIEW 7 of 7 14 of 15
Figure 5. Floor plan, cross-section, and analysis model of the EWP. Figure 5. Floor plan, cross-section, and analysis model of the EWP. 3.2. Conditions for Heat Transfer Simulation 3.2. Conditions for Heat Transfer Simulation Table3 shows the boundary conditions for simulation. The outdoor temperature and indoor temperatureTable were 3 shows set at −the20 boundary◦C and 25 ◦conditionsC, respectively, for simulation. in accordance The with outdoor the Korean temperature Design Standardand indoor fortemperature Preventing Condensation were set at – in 20 Apartments °C and 25 [ 28°C,]. respectively, in accordance with the Korean Design Standard for Preventing Condensation in Apartments [28]. Table 3. Boundary Conditions. Table 3. Boundary Conditions. Set-Point Set-Point Relative Surface Heat Transfer TemperaturesSet-Point HumiditySet-Point Relative CoefficientSurface Heat
Outdoor −20Temperatures◦C- Humidity 23.25 W/(mTransfer2 K) Coefficient Outdoor -20 °C - 23.25 W/(m2 K) Indoor 25 ◦C 50% 9.09 W/(m2 K) Indoor 25 °C 50% 9.09 W/(m2 K)
TheThe analysis analysis model model was madewas usingmade theusing PHYSIBEL the PHYS program.IBEL program. Table4 shows Table the 4 thermal shows properties the thermal of eachproperties material. of each material. Table 4. Thermal properties of materials [29]. Table 4. Thermal properties of materials [29]. Thermal Conductivity Specific Heat Capacity Density Materials Thermal Conductivity Specific Heat Density Materials [W/(mK)] [J/(kgK)] [kg/(m2)] [W/(mK)] Capacity [J/(kgK)] [kg/(m2)] Aluminum 160.00 880 2700 AluminumCement 1.00160.00 1000 880 1800 2700 ConcreteCement 1.601.00 1000 1000 2200 1800 ConcreteGasket 1.001.60 1000 1000 11502200 SealantGasket 1.001.00 1000 1000 14501150 Gypsum board 0.18 1000 900 Sealant 1.00 1000 1450 Thermal breaker 0.30 1000 1000 ExpandedGypsum polystyrene board 0.028 0.18 1470 1000 25 900 Thermal breaker 0.30 1000 1000 Expanded polystyrene 0.028 1470 25 Sustainability 2019, 11, 912 8 of 14 Sustainability 2018, 10, x FOR PEER REVIEW 8 of 15
3.3.3.3. Results FigureFigure6 6shows shows the the results results of of the the thermal thermal performance performance analysis. analysis. TheThe wholewhole indoorindoor surface surface temperaturetemperature waswas estimatedestimated atat aa relativelyrelatively highhigh temperaturetemperature ofof approximatelyapproximately 2222 ◦°CC duedue toto thethe useuse ofof insulationinsulation materialsmaterials onon thethe interiorinterior side.side. However, thethe indoorindoor surfacesurface temperaturetemperature atat thethe connectionconnection partsparts ofof thethe EWPEWP waswas muchmuch lower.lower. BecauseBecause connectionconnection parts were installedinstalled withwith only three layerslayers ofof weatherweather stripstrip siliconesilicone gasketsgaskets withoutwithout anyany insulation,insulation, thesethese jointsjoints showedshowed thethe lowestlowest indoorindoor surfacesurface temperaturetemperature of -3.2−3.2 °C,◦C, -2.3°C,−2.3 ◦andC, and1.0 °C 1.0, respectively.◦C, respectively. Because Because boundary boundary conditions conditions were wereset at setan atindoor an indoor temperature temperature of 25 °C, of 25an ◦indoorC, an indoorhumidity humidity of 50%, of and 50%, a dew and point a dew temperature point temperature of 13.9 °C, of 13.9condensation◦C, condensation was expected was expected to occur to occurduring during winter, winter, indicating indicating the need the need for forthe theimprovement improvement of ofinsulation insulation performance. performance. AccordingAccording to the analysisanalysis model, heat loss of the EWP was measured at 68 W. ItsIts effectiveeffective thermalthermal transmissiontransmission waswas estimatedestimated toto bebe 0.720.72 W/(mW/(m2K) by considering linear thermal transmissiontransmission ratesrates at the the connection connection parts. parts. The The heat heat loss loss of ofthe the insulated-concrete insulated-concrete wall wall was was estimated estimated at 40 at W. 40 The W. Theeffective effective thermal thermal transmittance transmittance of the of the insulated insulated conc concreterete wall wall in in consideration consideration of of the the linear thermal transmittancetransmittance atat thethe ceilingceiling partpart waswas estimatedestimated toto bebe 0.420.42 W/(m W/(m22K). Given these results, the EWP hadhad aa lowerlower thermalthermal performanceperformance byby approximatelyapproximately 70%70% thanthan thatthat ofof thethe insulatedinsulated concreteconcrete wall.wall.
Figure 6. Interior surface temperature distribution of the EWP. Figure 6. Interior surface temperature distribution of the EWP. 3.4. Measures for Improvement of Thermal Performance and Results
As shown in Table5, three countermeasures were taken to improve the thermal performance of the connection parts. The first improvement method was to install insulation materials at the connection parts. The second method was to insert a thermal breaker in the middle of the aluminum frame. The third method was to use both the first and second methods. According to the analysis results of the above three improvement methods, for the indoor surface temperature of the first and third methods with added insulation materials, the lowest temperature was measured at 16.5 ◦C. The temperature was higher than the dew point temperature of 13.9 ◦C, indicating that these two methods could prevent condensation. However, the second improvement measure with an inserted thermal breaker showed the lowest indoor surface temperature of 5.7 ◦C, lower than the dew point temperature. Thus, it is desirable to install insulation materials at connection parts on the interior side to improve insulation performance and prevent condensation. Sustainability 2018, 10, x FOR PEER REVIEW 9 of 15
Sustainability3.4. Measures 2018 for, 10 Improvement, x FOR PEER REVIEW of Thermal Performance and Results 9 of 15
Sustainability 2018, 10, x FOR PEER REVIEW 9 of 15 3.4. MeasuresAs shown for in Improvement Table 5, three of Thermal countermeasures Performance were and takenResults to improve the thermal performance of the connection parts. The first improvement method was to install insulation materials at the Sustainability3.4. Measures 2018 for, 10 Improvement, x FOR PEER REVIEW of Thermal Performance and Results 9 of 15 connectionAs shown parts. in TableThe second 5, three method countermeasures was to insert were a thermal taken tobreaker improve in the the middle thermal of performance the aluminum of the connection parts. The first improvement method was to install insulation materials at the 3.4.frame. Measures The third for Improvement method was of toThermal use both Performance the first andand Results second methods. 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The of resultsconnection of the parts. above The three second improvement method was methods, to insert fo ar thermalthe indoor breaker surface in thetemperature middle of of the the aluminum first and theframe.temperature connection The third was parts. highermethod The than was first the to dewimprovementuse pointboth thetemperatur firsmethodt ande ofsecondwas 13.9 to °C, methods.install indicating insulation According that these materials to two the methodsanalysis at the connectionthirdcould methods prevent parts. withcondensation. The added second insulation methodHowever, materials,was the to second insert the a lowestimprovement thermal temperature breaker measure in was the withmiddlemeasured an ofinserted theat 16.5 aluminum thermal°C. The temperatureresults of the was above higher three than improvement the dew point methods, temperatur for thee of indoor 13.9 °C, surface indicating temperature that these of two the methodsfirst and Sustainabilityframe.thirdbreaker2019 methods The ,showed11 third, 912 with methodthe added lowest was insulation indoorto use materials,bothsurface the firstemp thet lowestanderature second temperature of 5.7methods. °C, waslower According measured than theto at the 16.5dew analysis °C. point The9 of 14 resultscouldtemperature. prevent of the aboveThus, condensation. itthree is desirable improvement However, to install methods,the insulation second fo improvementr materials the indoor at surfaceconnection measure temperature with parts an on inserted theof the interior first thermal andside breakertemperature showed was higherthe lowest than theindoor dew pointsurface temperatur temperaturee of 13.9 of °C,5.7 indicating°C, lower that than these the two dew methods point thirdcouldto improve methods prevent insulation withcondensation. added performa insulation However,nce and materials, theprevent second the condensation. lowestimprovement temperature measure was withmeasured an inserted at 16.5 thermal°C. The Tabletemperaturetemperature. 5. Insulation was Thus, higher it is improvement desirable than the dewto install measurespoint insulation temperatur for thematerialse of developed 13.9 at °C, connection indicating EWP andparts that results theseon the two interior of methods indoor side breakerSustainabilitySustainability showed 20182018 ,the,, , 10 10,,, , x xxxlowest FOR FORFORFOR PEER PEERPEERPEER indoor REVIEW REVIEWREVIEWREVIEW surface temperature of 5.7 °C, lower than the dew point 10 10 10 10 of of of of 15 15 15 15 to improveSustainabilityTable 5.insulation Insulation 2018, 10, x performa FORimprovement PEER nceREVIEW and measures prevent for condensation. the developed EWP and results of indoor surface 10 of 15 surfacecouldtemperature. prevent temperature. Thus, condensation. it is desirable However, to install the insulation second improvement materials at connection measure with parts an on inserted the interior thermal side breakertemperature. showed the lowest indoor surface temperature of 5.7 °C, lower than the dew point to improveImprovementImprovementImprovementImprovement insulation performaSimulationSimulationnce ModelsModels and withwith prevent ImprovedImproved condensation. TableImprovement 5. Insulation SimulationimprovementSimulation Models measures Models with Improved with for the developed ResultsResultsEWP and ofof IndoorIndoor results SurfaceSurface of indoor TemperatureTemperature surface temperature.MeasuresMeasures Thus, it is desirableInsulationInsulation to install PerformancePerformance insulation materials atResults connection of Indoor parts Surface on Temperaturethe interior side ImprovementMeasuresMeasures Measures SimulationInsulationInsulationInsulation Models Performance PerformancePerformance with Results of Indoor Surface Temperature Improvementtemperature. Improved Insulation Performance to improveTable 5.insulation Insulation performa Improvedimprovementnce Insulation and measures prevent for condensation. the developedResults EWPof Indoor and results Surface of Temperatureindoor surface Measures Simulation Models with Improvementtemperature. Performance Table 5. Insulation Improvedimprovement Insulation measures for the developedResults EWPof Indoor and results Surface of Temperatureindoor surface Measures Simulation Models with Improvementtemperature. Performance Improved Insulation Results of Indoor Surface Temperature
Measures Simulation Models with Improvement Performance Improved
➌ Alt-2
(insertion of
➍➍ Alt-3Alt-3Alt-3Alt-3 ➍ Alt-3 ➍Alt-3 Alt-3(added (added(added(added (added(added insulation +
𝜓 𝐿 𝑈 𝑙 (1) where ψ—linear thermal transmittance [W/(mK)],
𝐿 —heat conduction coefficient calculated by two-dimensional computation [W/(mK)], 2 𝑈 —thermal transmittance of computation [W/(m K)], 𝑙 —length to thermal bridge [m]. According to the heat losses shown in Table 6, the EWP had a heat loss of 68 W, the first measure (Alt-1) had a loss of 49.2 W, and the second measure (Alt-2) had a loss of 51.9 W. For the external walls (Alt-3) with additionally installed insulation materials and thermal breakers into the aluminum frame, the heat loss was 42.1 W, indicating a heat performance improvement of approximately 62%. Because there were no other improvement measures for slabs where heat bridges occurred, the linear thermal transmittances of the slabs were equally at 0.58 W/(mK). Depending on the improvement measures, the linear thermal transmittance at the horizontal and vertical connection parts ranged from 0.03 W/(mK) to 0.57 W/(mK). Heat losses of the EPW, three measures and the insulated concrete wall were calculated by using effective thermal transmittance considering linear thermal transmittance. The heat loss of the insulated concrete wall was lower than that of EPW because insulated concrete wall has no connection parts with thermal bridges, as presented in Table 6.
Table 6. Heat loss and linear thermal transmittance dependent on improvement measures.
➊ EWP ➋ Alt-1 ➌ Alt-2 ➍ Alt-3 Insulated Concrete Wall Heat loss (W) 68.0 49.2 51.9 42.1 40 Linear Slab 0.58 0.58 0.58 0.58 0.58 thermal Horizontal 0.57 0.18 0.15 0.03 0 transmittance connection parts [W/(mK)] Vertical 0.49 0.18 0.22 0.13 0 connection parts
4. Unsteady State Thermal Characteristics of the EWP To evaluate the thermal performance of the EPW, an unsteady state 3-dimensional heat transfer analysis was also carried out for models with the improvement measures. For the simulation, the Sustainability 2019, 11, 912 10 of 14
Table6 shows the heat losses and linear thermal transmittance at the slabs and connection parts of the EWP and three improvement measures. The linear thermal transmittance is the heat flow rate divided by length and by temperature that is lost through the region of a thermal bridge in the steady state [25]. The thermal bridges are situated at the junction of walls, floors, roofs and building elements are characterized by a linear thermal transmittance as the ψ-value in W/(mK) [30]. Linear transmittance can be expressed as Equation (1):
j ψ = L2d − ∑ Ujlj (1) 1
where
ψ—linear thermal transmittance [W/(mK)],
L2d—heat conduction coefficient calculated by two-dimensional computation [W/(mK)], 2 Uj—thermal transmittance of computation [W/(m K)], lj—length to thermal bridge [m].
According to the heat losses shown in Table6, the EWP had a heat loss of 68 W, the first measure (Alt-1) had a loss of 49.2 W, and the second measure (Alt-2) had a loss of 51.9 W. For the external walls (Alt-3) with additionally installed insulation materials and thermal breakers into the aluminum frame, the heat loss was 42.1 W, indicating a heat performance improvement of approximately 62%. Because there were no other improvement measures for slabs where heat bridges occurred, the linear thermal transmittances of the slabs were equally at 0.58 W/(mK). Depending on the improvement measures, the linear thermal transmittance at the horizontal and vertical connection parts ranged from 0.03 W/(mK) to 0.57 W/(mK). Heat losses of the EPW, three measures and the insulated concrete wall were calculated by using effective thermal transmittance considering linear thermal transmittance. The heat loss of the insulated concrete wall was lower than that of EPW because insulated concrete wall has no connection parts with thermal bridges, as presented in Table6.
Table 6. Heat loss and linear thermal transmittance dependent on improvement measures.
Insulated EWP Alt-1 Alt-2 Alt-3 Concrete Wall Heat loss (W) 68.0 49.2 51.9 42.1 40 Slab 0.58 0.58 0.58 0.58 0.58 Linear thermal Horizontal transmittance 0.57 0.18 0.15 0.03 0 connection parts [W/(mK)] Vertical 0.49 0.18 0.22 0.13 0 connection parts
4. Unsteady State Thermal Characteristics of the EWP To evaluate the thermal performance of the EPW, an unsteady state 3-dimensional heat transfer analysis was also carried out for models with the improvement measures. For the simulation, the time step was 10 min, the calculation duration was 3 days, and there were five iteration cycles. Figure7 shows the 24-h temperature distribution on the coldest winter day in South Korea. The temperature distribution had a lowest level of −20 ◦C and a highest level of −10 ◦C. Figure8 shows the heat loss of each model based on 24-h temperature distribution. The EWP had a maximum heat loss of 89.8 W and a minimum heat loss of 79.2 W. In the case of Alt-1, which had added insulation at the connection parts, the maximum heat loss was 59 W, while the minimum heat loss was 51.8 W. In the case of Alt-3, which had added insulation at the connection parts and inserted thermal breakers to the aluminum frame, the maximum heat loss was estimated at 50.8 W, Sustainability 2018, 10, x FOR PEER REVIEW 11 of 15 time step was 10 minutes, the calculation duration was 3 days, and there were five iteration cycles. Figure 7 shows the 24-hour temperature distribution on the coldest winter day in South Korea. The temperature distribution had a lowest level of -20 °C and a highest level of -10 °C. Figure 8 shows the heat loss of each model based on 24-hour temperature distribution. The EWP had a maximum heat loss of 89.8 W and a minimum heat loss of 79.2 W. In the case of Alt-1, which had added insulation at the connection parts, the maximum heat loss was 59 W, while the minimum heat loss was 51.8 W. In the case of Alt-3, which had added insulation at the connection parts and Sustainability 2019, 11, 912 11 of 14 inserted thermal breakers to the aluminum frame, the maximum heat loss was estimated at 50.8 W, (which was higher by approximately 4 W than that of the existing insulated concrete wall), whereas (whichthe minimum was higher heat by was approximately estimated at 444.9 W thanW (which that of was the higher existing by insulated approximately concrete 1.7 wall), W than whereas that of thethe minimum existing heatinsulated was estimated concrete atwall). 44.9 WThese (which results was higherconfirm by that approximately if connection 1.7 W parts than have that of more the existinginsulation, insulated the disparity concrete wall).between These the results maximum confirm heat that loss if connection and the minimum parts have heat more loss insulation, can be thenarrowed. disparity between the maximum heat loss and the minimum heat loss can be narrowed. AsAs seen seen in in Figure Figure8 ,8, the the existing existing insulated insulated concrete concrete walls walls had had the the lowest lowest heat heat loss loss at at 12:00, 12:00, while while thethe developed developed EWP EWP had had the the lowest lowest heat heat loss loss at at 08:00. 08:00. Because Because the the existing existing insulated-concrete insulated-concrete walls walls hadhad aa heavy-weight heavy-weight structurestructure comparedcompared toto thethe EWP,EWP, there there was was a a difference difference of of 4 4 h hours in terms in terms of time of lagstime [31 lags]. Due [31]. to Due the lightto the weight light weig of theht EWP,of the it EWP, had a it shorter had a shorter time lag. time Thus, lag. it Thus, was subjected it was subjected to greater to influencegreater influence by outside by temperaturesoutside temperatures than the existingthan the insulated-concreteexisting insulated-concrete walls. walls.
Sustainability 2018, 10, x FOR PEER REVIEW 12 of 15 Figure 7. Outside temperature for the analysis of unsteady state thermal characteristics. Figure 7. Outside temperature for the analysis of unsteady state thermal characteristics.
Figure 8. Distribution of heat losses by unsteady state models. Figure 8. Distribution of heat losses by unsteady state models.
5. Discussion and Conclusion Unsteady state thermal heat transfer characteristics such as the time lag and heat loss of the EWP were evaluated in this study. Although the light weight of EWP provides better constructability, it is necessary to consider energy and indoor thermal comfort at the same time [32]. Existing heat transmittance can be estimated by conducting a one-dimensional computation of insulation performance on mixed materials. The present study adopted a three-dimensional computation concept to estimate the effective thermal transmittance of the EPW by measuring the linear thermal transmittance induced by a heat bridge at slabs as well as the linear thermal transmittance at vertical and horizontal connection parts. Effective thermal transmittance is the heat transfer unit area and temperature that is lost through the thermal bridge [33]. Effective thermal transmittance was estimated in consideration of heat transmittance of the EWP, the linear thermal transmittance of its slabs, and the linear thermal transmittance of the connection parts. It was also estimated in consideration of the size of the EWP, the area size for construction, and thermal bridge effects [34,35]. The difference between the effective thermal transmittance with multi-dimensional calculation-considered linear thermal transmittance and thermal transmittance with one- dimensional calculation is very high [36,37]. In the building sector, the thermal characteristics of the building envelope are needed to analyze the effective thermal transmittance and linear thermal transmittance concerning thermal bridges [21,33]. The effective thermal transmittance was calculated using the Equation (2). 𝑈 𝐴 ∑ 𝜓 𝑙 𝑈 (2) 𝐴 where 2 𝑈 —effective thermal transmittance [W/(m K)], 2 𝑈 —thermal transmittance calculated by one-dimensional computation [W/(m K)], A—area [m2], 𝜓—linear thermal transmittance [W/mK],
𝑙 —length to thermal bridge [m].
Sustainability 2019, 11, 912 12 of 14
5. Discussion and Conclusions Unsteady state thermal heat transfer characteristics such as the time lag and heat loss of the EWP were evaluated in this study. Although the light weight of EWP provides better constructability, it is necessary to consider energy and indoor thermal comfort at the same time [32]. Existing heat transmittance can be estimated by conducting a one-dimensional computation of insulation performance on mixed materials. The present study adopted a three-dimensional computation concept to estimate the effective thermal transmittance of the EPW by measuring the linear thermal transmittance induced by a heat bridge at slabs as well as the linear thermal transmittance at vertical and horizontal connection parts. Effective thermal transmittance is the heat transfer unit area and temperature that is lost through the thermal bridge [33]. Effective thermal transmittance was estimated in consideration of heat transmittance of the EWP, the linear thermal transmittance of its slabs, and the linear thermal transmittance of the connection parts. It was also estimated in consideration of the size of the EWP, the area size for construction, and thermal bridge effects [34,35]. The difference between the effective thermal transmittance with multi-dimensional calculation-considered linear thermal transmittance and thermal transmittance with one-dimensional calculation is very high [36,37]. In the building sector, the thermal characteristics of the building envelope are needed to analyze the effective thermal transmittance and linear thermal transmittance concerning thermal bridges [21,33]. The effective thermal transmittance was calculated using the Equation (2).
j U1d × A + ∑1 ψ × lj U = (2) e f f A where
2 Ue f f —effective thermal transmittance [W/(m K)], 2 U1d—thermal transmittance calculated by one-dimensional computation [W/(m K)], A—area [m2], ψ—linear thermal transmittance [W/mK], lj—length to thermal bridge [m]. Table7 shows the estimation results of the effective thermal transmittance of the EWP with improvement measures. It is judged that when calculating heating and cooling loads in the future, it will be necessary to estimate energy consumption more accurately through the computation of effective thermal transmittance.
Table 7. Effective thermal transmittance by improvement measures.
Insulated EWP Alt-1 Alt-2 Alt-3 Concrete Wall Effective Thermal 0.72 0.52 0.55 0.44 0.42 Transmittance [W/(m2K)]
This research introduced the newly developed EWP. To assess the possibility of its application to real apartments, this study carried out a simulation to analyze the thermal properties of the EWP and measures to prevent condensation. This study evaluated the linear thermal transmittance of EWP through a three-dimensional computation, analyzed the effective thermal transmittance of the envelope in reflection of these results, and identified its unsteady state thermal characteristics.
Author Contributions: All authors have contributed substantially. Writing and editing, G.H.; data analysis, S.-W.L.; investigation, J.-Y.K.; supervision, H.-G.K. Funding: This study was made possible by financial support from part of results a major research project conducted by the Korea Ministry of Land, Infrastructure and Transport, Residential Environment Research Project in 2019 (Project No.: 19RERP-B082173-06). Sustainability 2019, 11, 912 13 of 14
Acknowledgments: We would like to acknowledge the research group of long-life housings. Conflicts of Interest: The authors declare no conflict of interest.
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