comparative study on energy performance of common

International Journal of Architecture, Engineering and ConstructionVol 6, No 2, June 2017, 1-11

Comparative Study on Energy Performance of Common

Commercial Building Wall Systems

Yiang Xiao1,∗ and Ali M. Memari2

1Department of Civil Engineering, The Pennsylvania State University, University Park, PA, United States

2Department of Architectural Engineering, The Pennsylvania State University, University Park, PA, United States

Abstract: Building sustainability and energy performance have become major focus areas for building owners and design profes-sionals. Wall systems, an important building component with great impact for such criteria, are of interest. In order for designprofessionals to make better decisions and select a wall system that satisfies sustainable design and energy efficiency objectives,they need to have a better understanding of each wall system available, as well as how they compare with other alternatives. Thisstudy considers previously conducted work relevant to the topic and provides a comparison of some of the most commonly usedwall systems of commercial buildings in terms of energy performance. The selected wall systems include glass curtain wall, brickveneer, and precast concrete panel with strip windows. The study was carried out using LCA software Athena Impact Estimator forBuildings considering geographical locations. The results obtained from computer software analysis were evaluated and presentedin a comparative fashion, which can be used to help select an optimal wall system for a commercial building project. The resultsshowed that in general, brick veneer consumes the least amount of energy among the three wall systems considered in this study,while glass curtain wall the most. The study also showed that geographical location indeed has an impact on the embodied energyof a building wall system. This study shows that different commercial building wall systems can have very different levels of energyconsumption. During design, one should consider all phases of the life cycle as well as factors related to geographical locations inorder to select the optimal system and the materials involved.

Keywords: Commercial building, wall systems, embodied energy, carbon emission, life cycle assessment, sustainability

DOI: http://dx.doi.org/10.7492/IJAEC.2017.007

1 INTRODUCTION

Realization of the issues caused by global warming and the in-creasing global energy consumption has led to increased consid-erations of sustainability in various industries, including buildingdesign. Current residential and commercial building energy con-sumption is around 40 percent of total energy consumption inthe US (U.S. Department of Energy 2012) . Due to populationgrowth and increased energy demand, this number has increasedby about 25 percent during the past 35 years, and it has beenpredicted that the percentage will continue to grow to approxi-mately 45 percent (U.S. Department of Energy 2012). Therefore,great attention needs to be paid to effective measures that canreduce building energy consumption in order to achieve higherlevel of sustainability. This is particularly important to designprofessionals when they design new building projects.One of the main interests of design professionals when con-

sidering a building’s energy performance is the building’s wallsystem. There exists many different types of building wall sys-tems, each with its own advantages and disadvantages in en-ergy performance, structural behavior, aesthetics, etc. There-

fore, a comparison method is needed to aid design professionalsmake decisions when selecting the most suitable wall system fora project. Some studies are reported on certain commonly usedresidential building wall systems (Memari et al. 2012; Memariet al. 2014; Broun et al. 2014); Comparison of such wall sys-tems has led to some conclusions drawn in terms of energy per-formance. According to these studies, some wall systems havemore desirable energy performance than others, and that geo-graphical location and climate condition can affect the perfor-mance. However, there is not much readily available informationon similar topics related to commercial buildings. To be morespecific, such a comparison among commonly used commercialbuilding wall systems is not readily available in the open liter-ature, while wall and envelope manufacturers may have unpub-lished mockup test results. According to a U.S. DOE Study,during the past five years, residential and commercial buildingshave been responsible for 21.8% and 18.7% of the total US en-ergy consumption (U.S. Department of Energy 2012). Thus,it is reasonable that commercial buildings deserve equal, if notmore, attention. Comparative studies on energy performance ofdifferent commercial building wall or envelope systems can pro-

*Corresponding author. Email: [email protected]

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http://dx.doi.org/10.7492/IJAEC.2016.019

mailto:[email protected]

Xiao and Memari/International Journal of Architecture, Engineering and Construction 6 (2017) 1-11

vide better understanding of energy related attributes of suchsystems and lead to energy saving.This study mainly aimed to evaluate selected commonly uti-

lized commercial building wall and envelope systems. For eachsystem, the focus was comparison of the chosen wall systemsthrough computer-aided embodied energy and life cycle analy-ses. Such analysis would help better understanding of sustain-able performance of each wall system in a comparative fashion,which could be of value to design professionals in selecting wallsystems for commercial buildings.This study exclusively focused on the following three com-

monly used wall systems used in commercial building projects:glass curtain wall, masonry brick veneer, and precast concretecladding panels with strip windows. In addition, despite thefact that there are many criteria for building sustainability, onlyevaluation of embodied energy related aspects were of interestin this study.

2 BACKGROUND AND LITERATUREREVIEW

2.1 Embodied Energy and Life Cycle Assessment

Embodied energy is generally defined as the amount of energyrequired to produce one unit weight of a usable material (Ashby2013), some requiring fossil fuels, while others need electricityin their production. In order to make embodied energy of differ-ent materials comparable, conventionally all energy sources areconverted to oil equivalent (Ashby 2013).A life cycle assessment (LCA) is a study on a product’s en-

ergy consumption, carbon emission, impact, etc. throughout itslife cycle. For buildings, the main stages of a life cycle are usu-ally defined as material manufacturing, construction, use andmaintenance, and end of life (Bayer et al. 2010). As opera-tional energy of a building during the use phase can be reducedby current building technologies, embodied energy during thematerials manufacturing and construction phase becomes theprimary part of an LCA (Bayer et al. 2010).Performing an LCA of a building is important when sustain-

ability is of interest to design professionals. The results of suchassessments can help design professionals gain more practicalknowledge of the building materials so that better decisions canbe made when selecting materials and systems for each compo-nent of a building. This is particularly useful when the buildingproject is developed to obtain green building certification fromcommon rating systems like Leadership in Energy and Environ-mental Design (LEED). In the most current version LEED v4,one major section of credits is devoted for materials and re-sources, which is worth more than 10 percent of the total points(U.S. Green Building Council 2015).Many methods have been developed in the industry to calcu-

late the embodied energy of a building. However, traditionallytwo methods have been considered the most popular and widelyused: process analysis and input-output (I-O) analysis (Memariet al. 2014). Process analysis would firstly require a bill of ma-terials involved. Then, a material energy intensity database isutilized. The database contains values of energy per unit massfor various materials. The total embodied energy is simply com-puted by multiplying the energy intensity by the amount of ma-terial needed. However, one major drawback of this method is

that numerous assumptions are usually needed because of lack ofinformation(Bayer et al. 2010). The I-O analysis uses nationalenergy data model of the economy. The database is divided intodifferent sectors, each with a respective direct energy intensityand total energy intensity. It then needs to be determined whichsector the material of interest belongs to in order to estimate theembodied energy. However, this method usually neglects someenergy consumption during the production phase, which causesgreat inaccuracy for some materials (Bayer et al. 2010). There-fore, if applicable, it is generally preferred to use modern analy-sis software that include more comprehensive data and producemore accurate results.

2.2 Review of Some Recent LCA Studies onBuildings

LCA studies vary in scope, objective, and methods, whichgreatly affect the quality of the results. Wallhagen et al. (2011)performed some basic LCA calculations on an office building inSweden. The study was on a whole building scale, and eachmajor component was evaluated in terms of embodied energyand CO2 emission. Wu et al. (2012) conducted a similar studyon an office building in China, and the scope of the study wasalso on the entire building. Both embodied energy and carbonemission were analyzed, but the difference in the analysis wasbased on each phase of the building’s life cycle instead of eachsystem of the building. Kua and Wong (2012) also performed awhole-building LCA study on a multi-story commercial buildingin Singapore (Kua and Wong 2012). Similarly, each life cyclephase was investigated in terms of embodied energy and green-house gas emission. Many of these types of studies focused onthe scope of the whole building of interest, and all phases of thebuilding’s life cycle were analyzed. However, a full-scale LCA isusually complex and time-consuming, and accurate results maynot be obtainable (Ashby 2013). In addition, these studies onlylooked at one particular building of the authors’ interest, and theobjectives were not to provide a comparison of different buildingsystems.Guggemos and Horvath (2005) and Johnson (2006) conducted

two comparative studies on building framing systems. Bothstudies only focused on concrete and steel frames, and the build-ings analyzed were proposed models instead of real buildings.Neither study conducted a full life cycle assessment. In additionto energy and CO2, Guggemos and Horvath (2005) also assessedseveral other air pollutants. Robertson et al. (2012) conducteda comparative LCA on timber and reinforced concrete used inmid-rise office building construction. Bribián et al. (2011) com-pared different building materials in terms of energy and envi-ronmental impact. Rossi et al. (2012) acknowledged the impactof location on LCA by investigating residential buildings in threelocations in Europe. Cabeza et al. (2014) comprehensively re-viewed several recent LCA’s on buildings and evaluated severalcomputer software commonly used in such tasks. However, noneof these studies had particular focus on the wall systems on thebuildings.Ottelé et al. (2011) studied and compared different green fa-

cade and living wall systems. Stazi et al. (2012) assessed solarwall systems that can be used to improve building sustainability.However, these two studies were on newly emerged building en-velope systems with sustainable attributes instead of currently

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widely used wall systems. Memari et al. (2014) performed acomparative LCA on several commonly used wall systems ofresidential buildings. Both embodied energy and multi-hazardresistance were analyzed. Location factor was also taken intoconsideration. Kim (2011) compared a transparent compositefacade system and a glass curtain wall system. However, the au-thor acknowledged that climate and location were not consideredin the study but they could affect the performance of the sys-tems. Due to the complex nature of such analyses, limitations inlife cycle assessment should be recognized. Therefore, althoughvarious LCA studies have been carried out on topic of interestto the building industry, there are not many comparative lifecycle assessment studies on common commercial building wallsystems.

2.3 Common Wall Systems of CommercialBuildings

Although there are many types of wall systems currently used incommercial building design, only the following three ubiquitoussystems were analyzed in this study:

1. Glass curtain wall2. Masonry brick veneer3. Precast concrete cladding panels with strip windows

A glass curtain wall is usually a thin aluminum-framed wallwith glass in-fills (Vigener and Brown 2012). The framing is at-tached to the structural system, and does not bear any verticalloads other than its self-weight (Morris 2013). A typical glasscurtain wall is shown in Figure 1.

Figure 1. Typical glass curtain wall facade

Conventionally, glass curtain walls are categorized basedon fabrication/construction methods: stick-built and unitized(Morris 2013). A stick-built glass curtain wall requires all com-ponents delivered to site in pieces and assembled on site. Thisresults in higher site labor costs because most of the work isdone at the construction site. Another issue is that the out-door work environment for assembly can induce damages fromweather conditions. On the other hand, a unitized glass curtainwall is assembled in the shop, then shipped to the constructionsite. The advantage of this type is that the indoor manufactur-ing shop has a controllable environment that ensures the correcthumidity and temperature. In addition, this would improve thedurability of the wall system (Morris 2013).According to Vigener and Brown (2012), glass curtain wall can

be further categorized into three system types. The first type is

pressure-equalized rain screen system. This is expected to mini-mize the forces that drive water across a barrier with the use of apressure-equalization chamber that eliminates the pressure dif-ference across the system. To achieve this, weep holes functionas vents that allow air to flow between the exterior and glaz-ing pocket. The second system is called water-managed system,which is similar to pressure-equalized system, but the differenceis that there is no air barrier created. This can lead to pressuredifference between the glazing pocket and the interior, whichcauses water penetration and leaks. In this case, weep holeswould be used to drain out water that enters the system. Thelast and also the least common type is face-sealed barrier wall,which requires perfect seals at all member connections and doesnot perform ideally in long term (Vigener and Brown 2012).To optimize thermal performance for glass curtain wall sys-

tem, there are several issues that require close attention duringthe design process. The framing system usually uses aluminum,which is a material with high thermal conductivity. In orderto reduce heat loss, the common practice is to provide thermalbreaks that use material with low thermal conductivity. As forthe opaque area of the wall, the lack of interior air layer canresult in drastic changes in temperature and humidity undercertain circumstances. Therefore, insulation and air/vapor bar-riers are often necessary in such areas. At the wall perimeter,insulation is also needed to prevent energy loss and possible con-densation problems (Vigener and Brown 2012). In terms of sus-tainability, durability of the wall system is important. However,issues like condensation, dirt, thermal and structural deforma-tion, exposure to water, and environmental degradation can eas-ily cause damage to the glazing and framing members (Vigenerand Brown 2012). Incorporation of systems with well-designedthermal breaks and high R-values also improves the system’sperformance. In addition, the aluminum used in frames is con-ventionally recycled at the end of service life of the wall system(Vigener and Brown 2012).The second common wall type of interest in this study is ma-

sonry brick veneer. It is a wall system made of exterior masonryunits laid in mortar and functions as a cladding material. Aninterior backup wall, commonly steel framed wall, is needed toprovide lateral out-of-plane support to the veneer (Weber 2013).An example of masonry brick veneer is shown in Figure 2.

Figure 2. Typical masonry brick veneer facade

Water penetration can often occur in masonry wall systems asmasonry is porous and can allow water to flow through. Waterabsorption of masonry units and mortar also contributes to wa-ter penetration in such wall systems (Weber 2013). Therefore,

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several measures need to be taken to prevent potential dam-age caused by presence of moisture. Typically, drainage cavityshould be placed behind the veneer to allow water to flow freelydown to the base, from where water can be further redirectedto the exterior of the wall. Moisture barrier is also requiredon the interior wall to prevent water from further penetration.Furthermore, condensation can potentially occur when air in thecavity contacts the fenestrations and other openings on the wall,which means cavity seals are generally required (Weber 2013).Masonry also has very little insulating value, and therefore, in-sulation is needed in the drainage cavity or within the interiorbackup wall (Weber 2013).During construction phase, brick veneer walls require protec-

tion from rain and snow. Insufficiently protected walls experi-ence issues such as efflorescence and distress in the future. Windscreens and enclosures are also needed sometimes for additionalprotection (The Brick Industry Association 2005). However,once the wall system enters use phase, a brick veneer systemrequires very little maintenance as compared to other claddingsystems (e.g., metal panels). If periodical inspections are per-formed, damage can be reduced or even prevented (The BrickIndustry Association 2005). A typical service life for a brick ve-neer wall is at least 100 years (Weber 2013). In terms of firesafety, masonry walls benefit from the material’s inherent fireresistance (Weber 2013).In addition, brick masonry has several sustainability related

attributes. The material’s durability property allows the wallsystem to perform satisfactorily with minimal maintenance andrefurbishment if proper details are used. Brick masonry alsoproduces less on-site construction waste than many other ma-terials, and after use, it can largely be salvaged and recycledfor new construction. Furthermore, in general brick masonry isfabricated by using materials acquired in vicinity of the manu-facturing plant, which reduces energy input and cost (The BrickIndustry Association 2005).The third wall system used in this study is precast concrete

cladding panels with strip windows. It’s the most common useof precast concrete for building envelope systems. Like othertypes of building exterior (curtain) wall systems, cladding pan-els do not bear any vertical loads; instead, they simply enclosethe space within the building. The self-weight and lateral loadsare transferred to and supported by the main structural frames(Gaudette 2009). These cladding panels are typically attachedto the building at floor levels, and between each level of panelsare strip windows. Figure 3 shows a typical wall system of thiskind.Similar to masonry brick veneer wall system, the thermal per-

formance of precast concrete cladding panel depends on theinsulation installed in the drainage cavity or within the inte-rior backup wall, which is generally steel stud wall. In termsof moisture protection, the common practices are uses of seal-ers or coatings to prevent water from penetrating the barrier.As for safety, precast concrete panels can cause serious damagewhen connections are compromised in case of fire. Durabilityand maintenance could be undesirable depending on the finishesand shapes of cladding panels. Some finishes might result in vul-nerability to water penetration, while others might lead to morelikely deterioration of concrete or reinforcing steel. In addition,the relative complexity of installation can also cause damageand reduce durability. Improvement can be achieved by incorpo-

rating surface treatment and enhanced concrete mix (Gaudette2009).

Figure 3. Typical precast concrete cladding panelfacade with strip windows

Nevertheless, precast concrete cladding has many benefits.Since typical precast concrete panels are manufactured in a con-trolled plant environment, high quality and uniformity can beensured and production is rarely interfered by weather condi-tions and poor craftsmanship. Precast concrete also has a sat-isfactory resistance against explosions, vehicles, projectiles, andsevere wind conditions (National Precast Concrete Association2014a). Moreover, precast concrete panels can have outstandingR-value insulation that leads to energy savings if a proper insu-lation layer is added. The wide availability of manufacturers alsomakes precast concrete cladding easily obtainable in most loca-tions in North America. In addition, precast concrete panels areconsidered economical because their long-term costs are not ashigh as other materials (National Precast Concrete Association2014b).

3 MODELING AND ANALYSIS

3.1 Plan of Study

In this comparative study on life cycle assessment (LCA) of dif-ferent commercial building wall systems, three commonly usedwall systems (glass curtain wall, masonry brick veneer, and pre-cast concrete panels with strip windows) were selected for analy-sis. These three systems were further investigated by looking atcomponents and materials involved in each system. Athena Im-pact Estimator for Buildings, which is a commercially availablecomputer software, was chosen to analyze the three wall sys-tems. Firstly, a simple commercial building was modeled, whereinputting the major components and materials of each wall sys-tem into the software enabled conducting an LCA study. Toimprove the quality of the results that depend on variation ofsome parameters and geographic locations, several locations inthe US were chosen for this analysis. After the results becameavailable, they were interpreted, graphed, and tabulated to fa-cilitate the comparison among the three wall systems. Fromsuch results, some conclusions on sustainability and energy per-formance could be drawn to help identify the system that mayhave a relatively more satisfactory performance under the as-sumed conditions.

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3.2 Building Modeling

The modeling and analysis of this study were performed by usingAthena Impact Estimator for Buildings, which is a commercialsoftware used to evaluate whole buildings and assemblies in ac-cordance to internationally recognized LCA methodology. Withthe information in its database, the software is able to model95% of the building stock in North America (Athena Sustain-able Materials Institute 2014). The software takes into accountmany life cycle stages and factors that would affect the overallenergy consumption of a building. These factors include mate-rial manufacturing, transportation, construction, region, build-ing type and lifespan, maintenance and renovation, demolitionand disposal (Athena Sustainable Materials Institute 2014).The building model represents a three-story commercial/office

building, which has a gross floor area of 4,181 m2 (45,000 ft2),with each floor having 1,394 m2 (15,000 ft2). The base dimen-sion of the building is 45.7 m (150 ft) by 30.5 m (100 ft). Thetotal building height is 13.7 m (45 ft). The design life of thebuilding was considered 60 years.Athena Impact Estimator for Buildings also requires operat-

ing energy consumption data to construct the building model.More specifically, annual electricity and natural gas usage needto be provided as initial inputs. On average, an office buildingin the US uses approximately 17.3 kWh of electricity per squarefoot per year (Madison Gas and Electric, 2010), which for themodeled building with 4,181 m2 (45,000 ft2) floor area, resultsin 778,500 kWh/year. Furthermore, the average natural gas con-sumption for an office building in the US is about 31.8 ft3 persquare foot per year (Madison Gas and Electric, 2010), whichresults in 1,431,000 ft3/year for the modeled building.In this study, geographical location was also taken into con-

sideration. This is because the location of a building affectsenergy consumption during transportation stage. In addition,the local climate also has an impact on the energy performanceof a building. Therefore, the following five locations in the US,each with distinct climate, were selected: Pittsburgh PA, Or-lando FL, Minneapolis MN, Seattle WA, and Los Angeles CA.Each building location was used to analyze the three commercialbuilding wall systems of interest.

3.3 Wall Modeling

After a base building model was constructed in Athena ImpactEstimator for Buildings, the wall systems could be modeled inthe software. Because the nature of this study is mainly com-parative, it is not necessary to model all exterior walls of thebuilding. Only one typical wall with dimensions of 45.7 m (150ft) by 13.7 m (45 ft) was defined for each wall system at eachgeographical location. Due to variations in the composition ofthe three chosen wall systems, some assumptions were made toobtain realistic exterior walls for the analysis.For glass curtain walls, it was assumed that 80% of the exterior

wall was viewable glazing (vision glass) and 20% was spandrelpanel (opaque glass) at each floor level and roof level. Insulationwith thickness of 50.8 mm (2 in.) was also included in the span-drel. The software uses default door dimensions of 813 mm (32in.) by 2.13 m (7 ft). Across the 45.7-meter (150-foot) length ofthe wall, three sets of double doors, or six default doors in total,were used. The doors were assumed to be aluminum exterior

doors with 80% glazing, which had already been predefined inthe software. Since the glazing already functions as windows, noadditional windows need to be specified in the model. The per-spective detail of this typical glass curtain wall system is shownin Figure 4.

Figure 4. Example glass curtain wall detailsBrick veneer walls were assumed to be backed up by steel

studs. The steel studs were set to be non-load bearinglightweight studs, with depth of 152 mm (6 in.) and spacingof 406 mm (16 in.) o.c. The sheathing used for the wall is ori-ented strand board (OSB). Standard brick units were assumed.The other envelope components involved in this wall system in-cluded 12.7 mm ( 12 in.) air barrier, 6.35 mm ( 14 in.) polyethylenevapor barrier, 50.8 mm (2 in.) extruded polystyrene insulation,12.7 mm ( 12 in.) gypsum board, and latex water based paint. Itwas assumed that there were 42 windows on the exterior wall,each with an area of 3.25 m2 (35 ft2). The windows are alldouble-panel with aluminum window frames, which is a prede-fined option in the software. Door openings were assumed thesame as those in the glass curtain wall models, in order to main-tain consistency and eliminate effects of the doors. A typicalmasonry brick veneer wall detail is similarly shown in Figure 5.

Figure 5. Example masonry brick veneer detailsPrecast concrete panel (PCP) walls also utilized steel studs,

with the exact same setup and properties as defined in the brickveneer walls. Precast insulated panels were used for this typeof wall system. The other envelope components and doors wereassumed the same as in the brick veneer models. This would,again maintain consistency of the models and eliminate effectson energy performance from minor components in the wall sys-tems. The windows in this wall system are strip windows. It wasassumed that there were 75 window panels making up the strip

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windows on the three floors, each window still with an area of3.25 m2 (35 ft2). An isometric section detail of a typical precastconcrete cladding panel wall is shown in Figure 6.

Figure 6. Example precast concrete spandrel cladding panelwith rigid insulation (image courtesy of metal studcrete)

After modeling the base building and typical walls, AthenaImpact Estimator for Buildings was able to analyze the compo-nents and materials, and produce results with regard to energyand fossil fuel consumptions. The results could be further eval-uated based on the output data the software had generated.

4 RESULTS AND DISCUSSIONS

4.1 Comparison of Wall Systems

Fifteen wall models consisting of three wall systems in five loca-tions were made for the base building model in Athena ImpactEstimator for buildings. The software includes operational en-ergy use in the use phase of a building assembly’s life cycle. Thisenergy use was found to be significantly more intensive than anyother categories. Since it mainly depends on the annual electric-ity and natural gas consumptions, where national average valueswere input into the models, operational energy use would notcontribute to comparisons made to evaluate each specific wallsystem in different locations. Therefore, in the analysis for thisstudy, no operational energy uses were taken into consideration.Athena Impact Estimator for Buildings also provides primaryenergy data for Beyond Building Life phase, which is the energycredit a system can receive after being recycled or reused. How-ever, in this relatively simplified LCA study, this phase is not ofparticular interest and therefore was also neglected. The resultswere graphed as shown in Figures. 7-12. Note that Figure 12represents the average values of data in all five locations.

Figure 7. Embodied energy of wall systems in Pittsburgh

Figure 8. Embodied energy of wall systems in Orlando

Figure 9. Embodied energy of wall systems in Minneapolis

Figure 10. Embodied energy of wall systems in Seattle

Figure 11. Embodied energy of wall systems in Los Angeles

Figure 12. Embodied energy of wall systems - average of allfive geographic locations

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The results show that for all three wall systems productionphase and use phase are significantly more energy intensive thanconstruction phase and End of Life (EoL) phase. Such a resultwould make sense considering that manufacturing of the materi-als and maintenance of the systems both require a large amountof energy.It was found that in general, glass curtain wall systems had

the most amount of total embodied energy, and brick veneer wallsystem the least. On average, the total primary energy of curtainwall is about 1400 GJ, brick veneer is 968 GJ, and precast con-crete panel (PCP) is 1313 GJ. Additionally, the total embodiedenergy is dominated by energy used in production phase, whichis the main energy consumption phase for all wall types. Cur-tain wall has the most total embodied energy because it has themost embodied energy during production and manufacturing.This trend shows that glass and aluminum manufacturing con-sumes more energy than that of other materials considered inthis study. On average, window glass production has an em-bodied energy of around 10 MJ/kg, and aluminum mullionsabout 200 MJ/kg (Jackson 2005). Precast concrete claddingpanel walls use about 27% more energy than brick veneer wallsduring production phase. This is partially due to the use ofstrip windows in PCP wall systems, which almost doubled theamount of windows that brick veneer walls had. However, thequantity and gross area of the windows were only estimates. Forthis specific study, the window area considered is close to 40%of the total wall area. If assumptions for windows were changed,the results would be affected. Another reason PCP consumesmore energy in production than brick veneer is that cementi-tious materials like concrete require excessive energy for cementproduction, e.g., processing materials in high-temperature kilns(Memari et al. 2014). Conventional clay brick production, how-ever, does not involve such energy intensive processes. Commonclay bricks usually have an embodied energy of about 7 MJ/kgduring production phase (Jackson 2005).As for use and maintenance, the second major energy con-

sumption phase, PCP has relatively higher embodied energy.On average, it is approximately 70% more than either of theother two wall systems. The reason PCP requires such amountof energy during use phase is that it is a delicate and complexproduct and is vulnerable to damages. Its durability is not desir-able, and maintenance can be very energy intensive and costly.Brick veneer, on the other hand, consumes much less energy inuse phase. Maintenance is not intensively needed because of thematerial’s satisfactory durability. The results of this analysisshow that glass curtain walls, in general, have about the sameamount of embodied energy for use and maintenance as brickveneer walls.For construction phase, the results show that brick veneer wall

uses the most energy among the three wall types. This mightbe because brick veneer is not normally used in a panelizedfashion in construction, which results in more time-consuminglay-in-the-filed brickwork construction and more uses of energy-consuming equipment. Despite the complexity in constructionfor both curtain wall and precast concrete panel wall, they re-quire much less energy input than brick veneer.In terms of End of Life phase, glass curtain walls seem to

consume significantly more energy than the other two types ofwalls. This is because at the end of a curtain wall’s life cycle, theglass and aluminum framing are often recycled, and the process-

ing methods can consume a very large amount of energy. PCPcan be recycled as well after service life, but the reprocessingis not as energy intensive as that of a glass curtain wall. Thecomplexity in construction of PCP also means complexity in de-molition, which increases energy usage in this phase (Memariet al. 2014). However, overall, the embodied energy PCP hasduring EoL phase is still much less than that of curtain wall. Inaddition, it was found that brick veneer had the lowest embod-ied energy in this phase of a life cycle. This is because after itsservice life, brick veneer can be easily demolished and disposedto landfill or recycled, and these processes are not very energyintensive.

4.2 Comparison of Geographical Locations

Geographical locations were taken into consideration in thisLCA study, because embodied energy varies for different loca-tions even if the buildings are identical. Material availability,construction methods, transportation, etc. will have an impacton a building’s embodied energy (Memari et al. 2014). By ana-lyzing the same three wall systems in five different geographicallocations in the US, the results can be more representative, com-prehensive, and applicable to building projects in various areasin the U.S. Therefore, the output results from Athena ImpactEstimator for Buildings were re-graphed based on geographicallocations instead of wall systems, as presented in Figures. 13-15.

Figure 13. Embodied energy of curtain wall in differentlocations

Figure 14. Embodied energy of brick veneer in differentlocations

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Figure 15. Embodied energy of precast concrete panel indifferent locations

From the figures, it can be observed that Los Angeles has thehighest embodied energy for all three wall systems, followed byOrlando, Pittsburgh, Minneapolis, then Seattle, which has thelowest embodied energy among all five geographical locations. Incomparison, Los Angeles leads Seattle by 16%-30%, dependingon the wall type, but the differences between Los Angeles andthe four other cities are generally no more than 12%. In termsof production and manufacturing energy, most cities consideredhave very similar energy consumption level. This is becausethe methods used to produce the materials are similar through-out the country, and therefore geographical location only has aminor effect on the energy consumption. However, Seattle hasa slightly lower consumption than the other cities considered,which might be due to Seattle has more widely available mate-rials and easy transportation to sites.As for use and maintenance energy, based on the analysis re-

sults Los Angeles and Orlando seem to consume the most, fol-lowed by Pittsburgh and Minneapolis. Once again, Seattle usesthe lowest embodied energy in this phase of a life cycle. Dur-ing the use phase of a building, several factors can affect theoperational energy. A few main factors to consider include thefollowing: heating, cooling, ventilation, lighting, water heating,etc. (U.S. Energy Information Administration 2016). Amongthese, heating and cooling are the main two factors that areconsiderably relevant to geographical location and climate. Inrelatively hot coastal climate regions, like Los Angeles and Or-lando, as well as regions with relatively very cold winters, likePittsburgh and Minneapolis, more intense heating and coolingwould be needed during many months of a year, which increasesthe amount of energy consumed. However, Seattle has a rel-atively mild climate, and in such regions, heating and coolingwould not be intensely and as frequently needed. In addition,geographical locations with very hot climate face issues like highair moisture content and thermal effects, and those with cold cli-mate are often subject to problems like freeze-thaw effects. Allof these are detrimental to the exterior wall system of a building,which as a result demands more energy for maintenance and re-pair. Again, in Seattle where such issues are less severe, energyconsumption during use phase is less intensive.Los Angeles also has the highest construction energy consump-

tion among the five locations. This is because when compared toother locations, Los Angeles is relatively more difficult to accessfrom building material manufacturing facilities. The long deliv-ery distance and the large span of the city cause transportation

energy use during construction phase to rise. In addition, EoLenergy is approximately the same for all five locations. The rea-son for this is that the demolition and recycling methods of thesame wall system are similar in all geographical locations.

5 RANKING AND SUMMARY

The results of the analysis by Athena Impact Estimator forBuildings are tabulated and presented in Table 1 and Table 2.Table 1 provides data of embodied energy for the three wall sys-tems in five geographical locations, and Table 2 shows data forcarbon emission of each wall system in each life cycle phase.The standard measurement of global warming potential is in kgCO2 equivalent, which means all relevant factors are convertedinto carbon dioxide. This parameter, in addition to embodiedenergy, is also often of interest in a life cycle assessment.The results in the tables show that for both embodied energy

and carbon emission, brick veneer generally has the best per-formance, followed by precast concrete panel and then curtainwall. Furthermore, geographical location indeed has an impacton energy and carbon performance, although the impacts arerelatively small. In order to compare the obtained results ina more straightforward manner, a simple ranking system wasdeveloped and is shown in Table 3.The ranking matrix developed provides a simpler presentation

that can be used to compare and evaluate the embodied energyand carbon emission of the three selected common commercialbuilding wall types. However, this matrix is only based on sim-plified wall models with many assumptions made. Therefore,the information in the matrix should only be used for generalcomparison purposes.

6 CONCLUSIONS

From the literature review and analysis of the results in this lifecycle assessment study on common commercial building wall sys-tems, several conclusions can be drawn. For all building exteriorwall systems, the production phase and use phase of a life cy-cle are the major stages in which energy is consumed. In orderto improve energy performance of a wall system, manufacturingand maintenance methods that are more energy efficient need tobe implemented. Production energy is greatly affected by theuse of windows and glazing systems. The more glass utilized,the higher level of energy consumption a wall system has. Dura-bility of the materials directly affects the need of maintenance.Less resilient wall systems will demand more energy during usephase. Construction energy usage correlates to the complexityof construction. End of Life energy is related to the methods ofrecycling, reusing, and disposal, which however could be offsetby the energy credit received during the Beyond Building Lifephase. Overall, curtain wall consumes the most energy and hasthe most carbon footprint, brick veneer is more ideal in termsof energy and carbon performance. Geographical location wasalso shown to have some effects on embodied energy and carbonemission. In regions where hot and moist climate or cold climateexists, exterior wall systems are more susceptible to damage,which increase energy consumption during use and maintenancephase. Some regions are also greatly affected by the availabil-ity of materials and the ease of transportation, since commonmethods of transportation require large amounts of fossil fuels

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and emits greenhouse gases that contribute to global warming.Therefore, when evaluating the sustainability features of a

commercial building exterior wall system, one must consider allthe relevant factors to make more reasonable decisions. How-ever, sustainability features like embodied energy and carbonemission should not be the only criteria when selecting the mostapplicable wall type. Design professionals should also considermany other important factors, like owner requirements, safety,economical design, wind and seismic resistance, thermal perfor-mance, moisture response, indoor environment, occupant health,aesthetics, etc. In order to select the optimal exterior wall sys-

tem, all applicable criteria need to be carefully evaluated.For future studies and researches, one may focus on consider-

ing more varied types of exterior wall systems that are commonlyused for commercial buildings. Height of the building may alsoaffect wall type selection, as certain wall types are not appli-cable to high-rise commercial buildings. Other more advancedsoftware can be used to obtain models that are more realistic.More accurate assumptions can also be made to improve thequality of the result data. In addition, other criteria that couldpossibly affect a building’s exterior wall system may be reason-ably included as well to investigate the effects.

Table 1. Embodied energy summary (GJ)

Pittsburgh, PA

Production Construction Use End of Life Total

Curtain wall 942 20 345 122 1428

Brick veneer 545 56 341 20 962

Precast concrete panel 697 33 577 36 1343

Orlando, FL


Curtain wall 933 69 370 122 1494

Brick veneer 553 109 346 20 1028


Minneapolis, MN


Curtain wall 916 33 330 122 1400

Brick veneer 540 48 333 20 941


Seattle, WA


Curtain wall 793 22 225 122 1161

Brick veneer 502 34 297 20 853


Los Angeles, CA


Curtain wall 917 107 369 122 1515

Brick veneer 540 150 346 20 1056


Average


Curtain wall 900 50 327 122 1400

Brick veneer 536 79 333 20 968


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Table 2. Carbon emission summary (103 kg CO2 eq.)

Pittsburgh, PA


Curtain wall 105 1.5 46 9.5 162

Brick veneer 37 4 28 1.5 71

Precast concrete panel 61 2.3 49 2.6 115

Orlando, FL


Curtain wall 104 5.2 47 9.5 166

Brick veneer 38 8 29 1.6 76


Minneapolis, MN


Curtain wall 102 2.5 44 9.5 158

Brick veneer 37 3.5 28 1.5 70


Seattle, WA


Curtain wall 93 1.4 37 9.5 141

Brick veneer 36 2.3 25 1.6 62


Los Angeles, CA


Curtain wall 102 7.9 47 9.5 166

Brick veneer 38 10 29 1.6 76


Average


Curtain wall 101 3.7 44 9.5 159

Brick veneer 37 5.6 28 1.6 71


Table 3. Common commercial building wall system ranking

Wall typeEmbodied Energy

Pittsburgh, PA Orlando, FL Minneapolis, MN Seattle, WA Los Angeles, CA Average

Curtain wall III III III II III III

Brick veneer I II I I II I

Precast concrete panel III III III II III III

Wall typeCarbon Emission

Pittsburgh, PA Orlando, FL Minneapolis, MN Seattle, WA Los Angeles, CA Average

Curtain wall 3 3 3 2 3 3

Brick veneer 1 1 1 1 1 1

Precast concrete panel 2 2 2 2 2 2

Note: I/1 = good II/2 = average III/3 = poor

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http://calculatelca.com/software/impact-estimator/

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http://precast.org/wp-content/uploads/2014/08/Precast-Concrete-Wall-Panel-Brochure.pdf

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http://buildingsdatabook.eren.doe.gov/TableView.aspx?table=1.1.1

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https://www.wbdg.org/design/env_fenestration_cw.php

https://www.wbdg.org/design/env_wall_masonry.php

comparative study on energy performance of common

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