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Journal of Building Physics

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DOI: 10.1177/1744259115572130

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Lightweight steel-framedthermal bridgesmitigation strategies:A parametric study

Claudio Martins, Paulo Santos and Lus Simoes da Silva

AbstractIn building applications (e.g. industrial, offices and residential), the use of lightweightsteel-framed structural elements is increasing given its advantages, such as exceptionalstrength-to-weight relation, great potential for recycling and reuse, humidity shape stabi-lity, easy prefabrication and rapid on-site erection. However, the high thermal conductiv-ity of steel presents a drawback, which may lead to thermal bridges if not well designedand executed. Furthermore, given the high number of steel profiles and its reducedthickness, it is not an easy task to accurately predict its thermal performance in labora-tory and even less in situ. In a previous article, the authors studied the importance offlaking heat loss in lightweight steel-framed walls. This article discusses several thermalbridges mitigation strategies to improve a lightweight steel-framed wall model, whichincrease its thermal performance and reduce the energy consumption. The implementa-tion of those mitigation strategies leads to a reduction of 8.3% in the U-value, compara-tively to the reference case. An optimization of the wall module insulation layers is alsoperformed (e.g. making use of new insulation materials: aerogel and vacuum insulationpanels), which combined with the mitigation approaches allows a decrease of 68% in theU-value, also relatively to the reference case. Some design rules for lightweight steel-framed elements are also presented.

KeywordsLightweight steel frame, lightweight steel-framed walls, thermal transmittance, thermalbridges, mitigation strategies, parametric study, design rules

ISISE, Department of Civil Engineering, Faculty of Science and Technology, University of Coimbra, Coimbra,

Portugal

Corresponding author:

Paulo Santos, ISISE, Departamento de Engenharia Civil, Faculdade de Ciencias e Tecnologia da Universidade

de Coimbra Polo II, Rua Lus Reis Santos, 3030-788 Coimbra, Portugal.

Email: [email protected]

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Introduction

The demands to reduce the energy consumption in buildings increased in recentdecades, as a result of a greater concern towards sustainable construction. Spaceconditioning systems, heating and cooling are among the main responsible forenergy consumption, which heavily depend on the climate and the region wherethe building is located (Santos et al., 2011). Improvements of energy efficiency ofthe buildings lead to reduced energy consumption, also decreasing costs during theoperational phase. This requires improving the building construction systems andusing new technologies.

In recent years, several alternatives to traditional structural systems for buildingshave emerged, for example, lightweight steel framing. Lightweight steel-framed(LSF) construction uses as a basic component a structure made of cold-formedsteel profiles. Usually, these elements are prismatic and have thin-walled cross sec-tions. LSF as a structural system presented a significant growth in recent years andis successfully used in many types of buildings (e.g. industrial, office and residentialbuildings). These systems present many advantages, such as cost efficiency, reducedweight, exceptional resistance in relation to its mass, excellent stability of shape incase of humidity, rapid on-site erection, easy prefabrication and great potential forrecycling and reuse.

However, LSF also presents some drawbacks (Santos et al., 2012). The highthermal conductivity of steel can create thermal bridges, which if not correctlyaddressed during the design stage can significantly penalize the thermal behaviourand energy efficiency of the building. Thermal bridges may also result in construc-tive pathologies and reduced levels of comfort and healthy conditions associatedwith the occurrence of condensation phenomena driven by localized temperaturedrops inside construction components. This is particularly important in buildingswhere the relative humidity (RH) may be high and may decrease the durability ofthe materials. Another potential drawback of LSF construction system is the lowthermal mass and consequent reduced thermal inertia, leading to higher daily tem-perature fluctuations, originating higher discomfort to the occupants and higherenergy consumption. This is particularly relevant for climates with higher dailytemperature amplitudes, for example, Mediterranean climates.

As mentioned before, thermal bridges can penalize the thermal behaviour andenergy efficiency of steel buildings if not correctly addressed, increasing energyconsumption and costs during the operational phase. The total impact of thermalbridges on the heating energy demand is significant, reaching 30%, and it is greaterthan for the cooling energy demand (Erhorn-Klutting and Erhorn, 2009).

Significant work to assess and improve the thermal behaviour of constructivesolutions with steel structures was undertaken. Kosny and Christian (1995) showedthat the use of continuous exterior thermal insulation is an effective way to improvethe thermal performance and reduce the thermal bridges. In addition, the increasedspacing between profiles allows an increase of the thermal resistance (higher R-value). The improvement in R-value caused by the increased spacing of the steel

2 Journal of Building Physics



profiles may reach 20% with 13mm of expanded polystyrene (EPS) and about15% with 25mm of EPS (Kosny and Christian, 1995).

Hoglund and Burstrandb (1998) studied an efficient way to reduce heat flow byincreasing the heat flux path through the reduction of the area of the steel profile,with the insertion of slots in the web stud. Furthermore, they also concluded thatthe flanges of the steel stud act as heat collectors. If the flange length decreases, theU-value will also decrease. Blomberg and Claesson (1998) performed a similarstudy, which also concluded that one of the most efficient ways to decrease theheat flow is to use slotted steel studs. The study shows that the thickness of a stan-dard steel profile has to decrease by a factor of 6 to achieve the equivalent thermalproperties of a slotted steel profile. Furthermore, the heat flow through a profiledecreases as the number of narrow slots increases.

Note that whenever material is removed from the web (thermal slotted studs),there is a consequent reduction of the mechanical resistance. This may not becritical for non-bearing panels but could be relevant in the other cases (Veljkovicand Johansson, 2006). Salhab and Wang (2008) proposed an equivalent thicknessmethod to predict the mechanical resistance of cold-formed thin-walled channelsections with perforated webs under compression. These authors performed aparametric study using finite element simulations. They concluded that perforat-ing the web can significantly reduce the column strength, and the most relevantparameters are ratio of the plate width to the thickness, ratio of the perforatedwidth to the gross width and ratio of the solid width to the total width of the per-foration zone.

Good design rules also have a very important role, as they can mitigate and/oravoid the thermal bridges. Strategies that lead to improvements are (1) keeping thefacxade geometry as simple as possible, (2) avoiding the interruption of the insulatinglayer, (3) joining the insulation layers at full width at junctions of building elements,(4) using a material with the lowest possible thermal conductivity whenever theinterruption of the insulation layer is unavoidable and (5) installing openings, suchas doors and windows, in contact with the insulation layer (Santos et al., 2012).

The utilization of new insulation materials also allows a great thermal perfor-mance improvement of walls with lower thicknesses. Aerogel blankets are one ofthe most promising thermal insulation materials in recent years. They have a ther-mal conductivity 22.5 times lower than that of conventional mineral wool(Baetens et al., 2011). Another promising material is vacuum insulation panels(VIPs), which have a thermal resistance 58 times higher than other conventionalinsulation materials (Alam et al., 2011). VIPs can achieve good results with lowthickness, but they still have some drawbacks: they are expensive, fragile, difficultto adapt at the building site (cannot be cut or drilled), have a high cost and exhibitdecreasing thermal properties through time (Baetens et al., 2010).

In a previous study by the authors (Santos et al., 2014), the effect of flankingthermal losses in a LSF modular wall was studied. It was concluded that heat fluxvalues can change from 222% (external surface) to +50% (internal surface) whenthe flanking heat loss was set to zero as a reference case, for a thermal

Martins et al. 3



transmittance equal to 0.30W/(m2 K). In this article, the authors make use of thesame reference validated three-dimensional (3D) finite element method (FEM)detailed model in order to study the importance of thermal bridges in LSF modularwalls and to assess the potential improvements on the wall thermal performancedue to the implementation of several thermal bridges mitigation strategies. First, adescription of the modular LSF reference wall materials, geometry and dimensionsis presented. Next, the experimental set-up and measurements that led to the valida-tion of the 3D detailed FEM model are briefly described. Subsequently, a para-metric study is carried out, which implements single and combined thermal bridgesmitigation approaches and optimizes the U-value using different insulation materi-als. Finally, the results are discussed leading to the proposal of the best strategies toimprove the U-value, and some conclusions are presented.

Description of the modular LSF reference wall

The modular reference wall is composed of a steel structure containing galvanizedsteel cold-formed studs with different cross-sectional shapes: C (1003 403 103 1mm), U (753 403 1mm) and Z (753 253 1mm). Each wall module is 1.2mwide and 2.49m high. Figure 1 illustrates the assembled steel structure containedin the wall, and Figure 2 shows the enclosed materials and thickness. The modularwall has two parts, the main exterior body, fixed, and interior part, detachable,linked by three horizontal steel connections (see Figure 2). Table 1 shows the com-position of the wall, including the thickness and thermal conductivity (l) of eachmaterial layer assembled in the wall.

Table 1. Wall materials and properties.

Material (from outer to inner surface) Thickness (mm) l (W/(m K))

Finish external thermal insulationcomposite systems (ETICS) coating

4 0.750

EPS (ETICS insulation) 40 0.040Windtight and water-resistant membrane Neglected in thermal computationsOriented strand board (OSB) 11 0.130Stone wool 40 0.034Aira 166 0.922Steel frames (C: 1003 403 103 1 mm,U: 753 403 1 mm and Z: 753 253 1 mm)

175 50.000

Stone wool 40 0.034Wood 15 0.180OSB 11 0.130Plasterboard 13 0.250Total thickness (mm) 325

aSolid equivalent thermal conductivity.




Experimental set-up

The experimental set-up was the same as described in a previous article by theauthors (Santos et al., 2014). Nevertheless, a brief description and some additionalinformation are presented here. The experiments were performed using a test cham-ber heated by a split-type air conditioner that allows to set the temperature at 31 C.Three standard wall modules (1.20m wide) and one smaller module (0.39m wide)were placed in the chamber gantry (Figure 3). The flanking heat losses between thewall modules and the chamber gantry are minimized with a continuous extrudedpolystyrene (XPS) insulation layer 10 cm thick along its entire perimeter. L-shapedsteel fixing elements were used to fix the modular walls to the gantry. Figure 3 pre-sents a sketch of the mobile gantry, where the wall tested specimen was assembled,which illustrates the steel structure of the wall modules, the surrounding XPS andthe L fixing elements. Figure 4 presents a photograph of the test chamber andmobile gantry.

The heat flux passing through each of the tested walls was monitored in certainkey points using heat flux sensors (precision of 65%). The internal and externalsurface temperatures were measured using PT100 surface temperature sensors withan accuracy of 60.4 C. The heat flux and surface temperature sensors werelocated near and between steel studs (Santos et al., 2014). Note that in order toincrease the reliability of the measurements, 11 experimental tests were performed.Furthermore, four points on the wall surface were monitored between the steel

Figure 1. Steel structure of an LSF wall module.LSF: lightweight steel-framed.

Martins et al. 5



Figure 2. Wall module materials.

Figure 3. LSF wall test specimen and mobile gantry geometry.LSF: lightweight steel-framed.




studs and four more points on the vicinity of the steel studs. The experimental val-ues presented in section Verification and validation are mean values of the mea-sured data.

To measure the air temperature inside the wall air gap, a PT100 needle tempera-ture sensor with the same precision of the surface temperature sensors was used atmid-height. In order to monitor the ambient air temperature inside or outside ofthe test chamber, thermo-hygrometers (accuracy of63%) were used. A data-loggerrecorded all sensor measurements, except the thermo-hygrometers that have anincorporated data-logger.

Numerical model

In this section, the 3D FEM model verification and validation procedures arebriefly described. Most of these procedures were previously presented in anotherstudy performed by the authors aimed at studying the importance of flanking ther-mal losses in LSF walls (Santos et al., 2014).

3D FEM model

A detailed numerical model of the tested wall was assembled using ANSYS CFXfinite element software. The model of the LSF wall structure comprised about308,000 nodes, a number that was found to be sufficient to provide good conver-gence and beyond which the results did not change.

The thermal boundary conditions used were 30.8 C for interior temperatureand 18.4 C for exterior temperature. The obtained average weighted film coeffi-cients for the interior and exterior surfaces were equal to 6.98 and 5.97W/m2 K,respectively. These values are slightly lower than the one prescribed by the stan-dard EN ISO 6946:2007 (2007) that prescribes for horizontal heat flow and indoorconditions a film coefficient of 7.69W/(m2 K).

4.04 m

2.48 m

2.87 m

Figure 4. Test chamber and mobile gantry.

Martins et al. 7



The air gap inside the wall was modelled in two ways. In the first approach, theair was modelled considering the equivalent thermal conductivity of a solid as pre-scribed by EN ISO 6946:2007 (2007), expression (1) and the thermal resistance val-ues presented in Table 2 (horizontal heat flow) of the same standard.

In the second approach, the air was directly modelled as a fluid using a compu-tational fluid dynamics (CFD) model. The CFD and the still air layer give similarresults, with an overall thermal transmittance difference of 0.0046W/(m2 K). Moredetails may be found in Santos et al. (2014). Since the equivalent thermal conduc-tivity approach is less time-consuming, it was used in the parametric study.

Verification and validation

Verification against EN ISO 10211. EN ISO 10211:2007 (2007) establishes the specifi-cations to be followed when modelling thermal bridges in buildings and providesseveral test cases (two-dimensional (2D) test case: two and 3D test case: two) toevaluate the precision of the numerical algorithms to compute heat flows and sur-face temperatures. In order to assess the reliability of the models using the FEMANSYS algorithm, the authors modelled all the reference test cases prescribed inAnnex 2 of EN ISO 10211:2007 (2007). Figure 5 illustrates the temperature distri-bution obtained for the 3D test reference cases, while Table 2 presents the obtainedvalues. The surface temperature differences vary between 0.003 C and 0.044 C,evidencing an excellent accuracy of the 3D FEM models. Similar procedures wereimplemented for the 2D test cases, and analogous conclusions were found (not illu-strated). These results ensure not only the accuracy of the applied algorithm butalso the authors skills to use it.

Verification against simplified models. In Santos et al. (2014), the authors made someadditional verifications in order to confirm the accuracy of the 3D FEM wallmodel, making use of a 2D FEM algorithm (THERM software) and also an analy-tical expression for homogeneous layers. The obtained results showed a maximumdifference of 0.001W/(m2 K) in the thermal transmittance of the wall.

Table 2. Temperature obtained for the 3D test reference cases of EN ISO 10211:2007 (2007).

Reference (C) Obtained (C) Difference (C)

Test reference case 3Minimum surface temperature onenvironment a

11.32 11.276 0.044

Minimum surface temperature onenvironment b

11.11 11.127 0.017

Test reference case 4Highest temperature on external surface 0.805 0.809 0.003

3D: three-dimensional.




Figure 5. Temperature distribution obtained for the 3D test reference cases of EN ISO10211:2007 (2007): (a) test reference case 3 and (b) test reference case 4.

Martins et al. 9



Validation against measured data. The validation of the numerical model was per-formed by comparison with experimental measurements described in Santos et al.(2014). Figure 6 presents some additional information in order to compare theresults of the numerical simulation with the experimental measured data for tem-peratures (wall surface and air). The thermal transmittance values are alsodisplayed.

The numerical model predicts values slightly higher than the measured ones.These values are presented in Table 3, including the differences in percentage hav-ing as reference the experimental data. The accuracy of the numerical results variesbetween +0.7% (interior surface temperature near steel studs) and +3.8% (exter-ior surface temperature between steel studs).

The 3D model produced an overall thermal transmittance of 0.235W/(m2 K).This value, although slightly higher than the experimental value, 0.214W/(m2 K), isconsidered acceptable based on the uncertainties involved in the systems, for exam-ple, precision of the heat flux sensor (65%), quantification of the steel stud influ-ence zone and imperfections of the wall assemblies. Although flanking heat lossesexists in real buildings, given the usual adjacency to other construction componentsand the subsequent lateral heat exchange, they were not considered in the validatednumerical model, leading to a 3D FEM simplified model, with adiabatic conditions

0.150

0.170

0.190

0.210

0.230

0.250

0.270

0.290

0.310

15.0

17.0

19.0

21.0

23.0

25.0

27.0

29.0

31.0

Experimental Numerical

U-v

alue

[W/(

mK

)]

Tem

pera

ture

[C]

Air cavity temp.

Internal ambient temp.

External ambient temp.

Internal surface temp. (near steel studs)

External surface temp. (near steel studs)

Internal surface temp. (between steel studs)

External surface temp. (between steel studs)

U-value

Figure 6. Temperature and thermal transmittance values: experimental versus numericalapproaches.




in the wall edges. In addition, the reference standards for laboratory experimentaltests (e.g. hot box apparatus) also prescribe null or reduced flanking thermal losses.

The new numerical model with adiabatic conditions in the edges (called ModelA in the next section) predicted a U-value of 0.301W/(m2 K). The differencebetween the basic configuration (0.235W/(m2 K)) and the one with adiabaticboundary conditions is quite high even with an XPS insulation layer around thetest wall. This is due to the flanking thermal losses that are mainly originated bythe steel plates used to fix the wall panels to the test gantry. The importance offlanking thermal losses in the thermal performance of LSF walls was analysed indetail by Santos et al. (2014).

Parametric study for the mitigation of thermal bridges

Single thermal bridges mitigation strategies

With the purpose of improving the thermal performance of the wall module pre-sented in Figure 2, several thermal bridges mitigation strategies were implementedbased on the validated model. The various models and results are described in thefollowing subsections.

As mentioned before, the reference wall module, Model A (Figure 2), presentsadiabatic conditions in the edges, which are common conditions for all models ana-lysed in this parametric study and are ideal conditions prescribed for laboratoryexperimental set-ups.

Thermal break rubber strip. The first improvement strategy consists of a thermalbreak rubber strip (l=0.037W/(m K)) inserted between the vertical steel stud andthe oriented strand board (OSB) panel on the outer surface of Model B (Figure 7).Two rubber thicknesses were used: 5mm (Model B1) and 10mm (Model B2).

Models B1 and B2 lead to a decrease of 1.9% and 3.5% in the U-value, respec-tively, corresponding to 0.2954 and 0.2906W/(m2 K) for the U-value. These solu-tions provide a small thermal performance improvement. However, this can beconsidered a good option, since this upgrading is easy and affordable to imple-ment. Figure 8 shows the heat flux in the exterior face of Models A, B1 and B2,

Table 3. Experimental and numerical obtained values.

Air cavitytemperatures (C)

Surface temperatures (C) U-value(W/(m2 K))

Near steel studs Between steel studs

Internal External Internal External

Experimental 25.41 29.66 18.45 29.60 18.19 0.214Numerical 25.91 29.86 19.01 30.20 18.88 0.235Differences + 2.0% + 0.7% + 3.1% + 2.0% + 3.8% + 9.8%

Martins et al. 11



where the reduction of the heat flux is visible, mainly in the zone of the verticalprofiles.

Vertical male or female studs. The second mitigation strategy uses vertical female ormale studs that have connections in the web, between wall modules (Figure 9), allother characteristics of Model A being maintained. This approach has two differ-ent connection sizes: 15mm (Model C1) and 25mm (Model C2).

In Models C1 and C2, the vertical female or male connection profiles do not sig-nificantly improve the U-value, causing a slight increase of 0.4% and 0.2% in theU-value, respectively, corresponding to 0.3021 and 0.3018W/(m2 K) for the U-value. These findings may be justified by the increased amount of steel area insidethe wall, leading to a slightly higher heat transfer due to an increased steel surfacewithin the wall. Although this solution does not provide an improved thermal per-formance, it results in increased shear strength at the junction between modules,allowing them to work together against the perpendicular actions in the wall (e.g.wind forces). It also increases assembly speed of the walls due to the couplingcapacity between the modules. Models C1 and C2 show a similar heat flux as forModel A, as expected (not illustrated).

Vertical slotted steel studs. The third thermal bridge mitigation approach is the use ofslotted steel profiles (only vertical, as illustrated in Figure 10), preserving the othercharacteristics of Model A. Two different slotted areas were modelled. In the first(Model D1), 336.6 cm

2 of the vertical web area was removed (14% of the web massin the C studs and 19% in the U and Z profiles). In the second (Model D2), a

Figure 7. Model B with rubber strips: (a) location of the rubber strips in the wall and (b)dimensions of the rubber strips.




reduction of 673.2 cm2 of the vertical web area was considered, which represents28% of mass in the web of the C profiles and 37% in the U and Z profiles.

The use of vertical slotted steel profiles, in Models D1 and D2, provides an iden-tical benefit as the rubber solution, improving by 3.2% and 3.5% the U-value,respectively, corresponding to 0.2913 and 0.2904W/(m2 K) for the U-value. Thesevalues are lower due to the fact that the wall has external insulation. In case ofwalls with insulation in the same plane of the steel (batt thermal insulation), theimprovement is higher. Note that doubling the slotted area only yields to a reduc-tion of 0.3% in the overall thermal transmittance value of the wall. This is mainlybecause the number of slots was kept the same, confirming the trend identified byBlomberg and Claesson (1998).

Slotted steel studs. To evaluate the thermal advantages of having all the steel studsslotted, instead of only the vertical profiles (Model D), a new model was created, as

Figure 8. Heat flux of Models A and B external surface view: (a) Model A: reference case, (b)Model B1: 5-mm rubber and (c) Model B2: 10-mm rubber.

Figure 9. Model C: vertical female or male studs: (a) normal stud, (b) indented stud: 15 mmand (c) indented stud: 25 mm.

Martins et al. 13



illustrated in Figure 11. Model D3 uses the same profile types as Model D2 (Figure11). This model shows that it is possible to reduce by 4.54% the U-value with allthe steel profiles slotted, leading to a U-value of 0.2874W/(m2 K).

Fixing bolts instead of horizontal steel plate connection. In order to reduce the thermalbridges created by the horizontal steel connections, nine bolts were modelled, repla-cing these elements. Figure 12 shows the location of the bolts in the wall.

Model E shows that it is possible to reduce by 2.1% the U-value by removingthe horizontal steel connections of Model A and replacing them by bolts. A U-value of 0.2949W/(m2 K) was obtained.

Combined mitigation strategies for thermal bridges

To improve the wall thermal performance, several models were created combiningthe solutions previously presented, as shown in the next sections.

Combination of rubber strip, vertical slotted steel profiles and bolted connections. Model F isthe combination of the improvements introduced in Model B2 with 10-mm rubberstrips, Model D2 with vertical slotted steel profiles and Model E with bolted con-nections. Figure 13(a) shows the details of the wall module.

In this model, an improvement of 7.6% in the U-value was achieved, corre-sponding to a U-value of 0.2782W/(m2 K). Figure 13 shows the heat flux of ModelF, which compared with Figure 8 (Models A and B), shows a reduction of heatflux, especially in the vicinity of the vertical steel studs.

Combination of rubber strip, slotted steel profiles and bolted connections. Model G is similarto Model F but with all the steel structure slotted in the web. This model providesthe best result of the U-value, 0.2762W/(m2 K), corresponding to an improvement

Figure 10. Model D: vertical slotted steel profiles: (a) steel structure, (b) slotted web area:336.6 cm2 and (c) slotted web area: 673.2 cm2.




of 8.3%. The heat flux on the external surface of the wall is similar to Model F(Figure 13(b)).

Results: overview and discussion

Table 4 presents an overview of the obtained thermal transmittance values for allmodels and the thermal transmittance difference (DU) between each model and thereference model (Model A). All models have adiabatic edges conditions and arevariations of Model A. Four decimal places are used in the presentation of U-

Figure 11. Model D3: all structural steel profiles slotted (except horizontal connections).

Martins et al. 15



values, to increase the accuracy in the comparison analysis. Figure 14 presents anillustration of the results for each wall model and shows the obtained percentage ofU-value reduction between each model and the reference one (Model A).

Figure 13. Model F: 10-mm rubber strip, vertical slotted steel profiles and nine bolts: (a) wallmaterials and (b) heat flux: external surface view.

Figure 12. Model E: nine bolts instead of horizontal steel plate connections.




Tab

le4.

Par

amet

ric

study

for

the

mitig

atio

nofth

erm

albri

dge

s:ove

rvie

wofm

odel

san

dre

sults.

Model

Model

des

crip

tion

U-v

alue

(W/(

m2K

))(D

U)

Without

hori

zonta

lst

eelco

nnec

tions

Rubbe

rst

rip

Ver

tica

lfe

mal

eor

mal

est

uds

Slott

edst

eel

pro

files

Singl

est

rate

gies

Aa

0.3

011

()

B1

P5

mm

0.2

954

(21.9

%)

B2

P10

mm

0.2

906

(23.5

%)

C1

P15

mm

0.3

021

(+0.4

%)

C2

P25

mm

0.3

018

(+0.2

%)

D1

P14%

Only

vert

ical

0.2

913

(23.2

%)

D2

P28%

Only

vert

ical

0.2

904

(23.5

%)

D3

P28%

0.2

874

(24.5

%)

EP

9bolts

0.2

949

(22.1

%)

Com

bin

edst

rate

gies

FP

9bolts

P10

mm

P28%

Only

vert

ical

0.2

782

(27.6

%)

GP

28%

0.2

762

(28.3

%)

a Ref

eren

cem

odel

.

Martins et al. 17



The best solution is the one that combines rubber strips (10mm), slotted steelprofiles and bolted connections, identified as Model G. The U-value was reducedfrom 0.3011W/(m2 K), in the reference Model A, to 0.2762W/(m2 K), correspond-ing to a reduction of 8.3% in the U-value, which represents a good improvementin the thermal performance of the wall module, considering that the thermal insu-lation thicknesses and materials are the same. It is noted that since the wall withoutsteel profiles has a reduction of 11.01% (reference model), the mitigation strategiesfor the thermal bridges achieve a 75% reduction of their impact.

The most efficient single mitigation strategy assessed is the inclusion of slottedsteel profiles in all structures, Model D3, which allows to obtain a thermal trans-mittance reduction of 4.54%. This can be considered a good thermal performanceimprovement since it has a small cost impact. However, the reduction of themechanical resistance of the steel structure should be taken into account mainlyfor load-bearing walls (Salhab and Wang, 2008; Veljkovic and Johansson, 2006).

With the performed studies, it can be concluded that the wall module is opti-mized in order to minimize the thermal bridges. If there is a need of improving theU-value, the strategy must be to increase the thermal insulation. With this objec-tive, several additional models were analysed and are presented in the next section.

Parametric study for U-value improvement

To achieve a U-value improvement in the LSF modular wall, many alternativesmay be considered, for example, other insulation systems, placing more insulation

Figure 14. Parametric study: overview of results for the wall models.




or insulation materials with higher thermal performance. In this study, it wasdecided to maintain the thickness of the wall. Three insulation systems were consid-ered: polyurethane (PU) foam, with a l of 0.028W/(m K); silica aerogel insulationblanket, with a l of 0.015W/(m K); and rigid VIPs, with a l of 0.007W/(m K).

Single improvement strategies

PU foam. As individual strategies to improve the U-value, two models with PUfoam were created. Model H replaces the air gap and stone wool of Model A byPU foam, as illustrated in Figure 15(a). In Model I, a PU layer is placed replacingthe air gap and the inside layer of stone wool of Model A. The stone wool layerplaced in the exterior side, present in Model A, is replaced by an air gap, as illu-strated in Figure 15(b).

Model H achieved a significant improvement, reducing the U-value by 46.4%,with the replacement by PU of the air gap and the stone wool layer, correspondingto a U-value of 0.1615W/(m2 K). Note that a possible side effect of filling theexisting air layer with thermal insulation material (e.g. PU foam in Model H) is thepotential increased heat flows and the consequent inner surface temperaturedecrease. However, in this case, the PU foam also fills the gap between the vertical

Figure 15. Wall models with polyurethane foam: (a) Model H: replacement of air gap andstone wool by polyurethane foam and (b) Model I: replacement of air gap and interior stonewool by polyurethane foam and replacement of exterior stone wool by an air gap.

Martins et al. 19



steel studs and the inner wood slats. Therefore, there is a decrease in the heat flowsalso in the vicinity of the steel frame as illustrated in Figure 16(a).

Although this is a good thermal performance improvement, this solution mayhave a functional drawback related with the lack of an air gap, if and wheneverthere is moisture infiltration or condensation. Hence, Model I introduces an airgap with 40mm, relatively to the previous model. This solution provides a decreaseof 37.7% in the U-value, with 0.1876W/(m2 K), which is 8.7% worse than the pre-vious model. Figure 16(b) shows that the decrease of heat flux is slightly lower thanin Model H.

Silica aerogel insulation blanket. Silica aerogel insulation blankets have a dual function(similar to stone wool) by improving both fire protection and thermal performance.In this approach, three solutions were used, with 30-mm-thick layers. In the firstmodel (Model J), the aerogel insulation was placed on the internal side, betweenthe internal stone wool and the air gap. In the second approach (Model K), theaerogel was positioned on the external side, between the external stone wool andthe air gap. In the third approach (Model L), the aerogel blankets were positionedon both sides, as illustrated in Figure 17.

The main difference between the first two models (Models J and K) is the insu-lation position on the wall, a better U-value being reached for the solution whenthe insulation blankets are placed on the internal side (Model K), 0.2295versus 0.2356W/(m2 K). As expected, Model L obtained a better U-value,

Figure 16. Heat flux: external surface view: (a) Model H: replacement of air gap and stonewool by polyurethane foam. (b) Model I: replacement of air gap and interior stone wool bypolyurethane foam and replacement of exterior stone wool by an air gap.




0.1889W/(m2 K), which represents an improvement of 37.5%. Comparing Figure18(a) to (c), it is possible to see the influence of the location of the blankets. InFigure 18(c), there is a clear better performance, as a result of having twice moreaerogel insulation blankets.

VIPs. For the VIPs, with a thickness of 30mm, the same strategies were used as forthe aerogel insulation blankets (Models M, N and O). All numerical models withVIPs gave good results for the thermal performance due to the low thermal con-ductivity of this material. Note that the thermal conductivity of the VIPs (0.007W/(m K)) was provided by the VIP manufacturer Kingspan UK (2013) and takes intoaccount the foils edge effect, being the aged value. Furthermore, according to theinformation provided by this manufacturer, it is possible to produce VIPs with thesuitable dimensions for each LSF wall module.

The main difference between the first two models (Models M and N) is the posi-tion of the panels in the wall: inner and outer side positions, respectively. A slightlybetter U-value was obtained for the solution in which the panel is placed in theinner side (Model M), 0.2032 versus 0.2048W/(m2 K).

Model O gives a good U-value, 0.1536W/(m2 K), which represents an improve-ment of 49% in relation to the reference model. Comparing Figure 19(a) to (c), itis possible to see the influence of the location of the panels. In Figure 19(a), thereis a greater uniform distribution of heat flux that occurs because the insulation is

Figure 17. Wall Model L with silica aerogel insulation blanket on both sides.

Martins et al. 21



positioned (considering the heat flow path) before the steel structure. Figure 19(c)shows the decrease of heat flux and shows that this solution is better for achievinga better thermal performance in winter and summer conditions, as the steel struc-ture is insulated on both sides.

Combined improvement strategies

Combining the presented thermal bridges mitigation strategies with the strategiesto improve thermal behaviour, a better U-value is achieved. Six models were cre-ated that combine the best mitigation solutions presented earlier. Model P com-bined the following improvements: Model B2 with rubber strips, Model D2 withvertical slotted steel profiles, Model E with bolted connections and Model H, inwhich the air gap and stone wool are replaced by PU foam (Figure 20(a)). ModelQ is similar to the previous, adopting instead the solution used in Model I: a PUlayer and an air gap with 40mm (Figure 20(b)).

Model P leads to a reduction of 57.8% in the U-value, corresponding to0.1271W/(m2 K). A clear reduction of the heat flux is visible in Figure 21(a).Model Q results in a reduction of 49.3% in the U-value, 0.1525W/(m2 K). Figure21(b) shows the heat flux in the external face, being visible a higher heat flux inrelation to the previous model. However, the thermal bridge effect due to the pres-ence of the vertical steel studs is well mitigated.

Two similar approaches were done in Models R and S. These models differ fromthe previous in the type of steel structure. In these approaches, all the steel struc-tures are slotted in the web; Figure 22(a) and (b) illustrates the models.

Figure 18. Heat flux: external surface view: (a) Model J: 30-mm silica aerogel insulation blanketon the internal side, (b) Model K: 30-mm silica aerogel insulation blanket on the external sideand (c) Model L: 30-mm silica aerogel insulation blanket on both sides.




Figure 20. Wall combined solutions with polyurethane foam and vertical slotted steel profiles:(a) Model P: nine bolts + 10-mm rubber + vertical slotted steel profiles + replacement of airgap and stone wool by polyurethane and (b) Model Q: nine bolts + 10-mm rubber + verticalslotted steel profiles + replacement of air gap and interior stone wool by polyurethane foamand replacement of exterior stone wool by an air gap.

Figure 19. Heat flux: external surface view. (a) Model M: 30-mm vacuum panel on the internalside, (b) Model N: 30-mm vacuum panel on the external side and (c) Model O: 30-mm vacuumpanel on both sides.

Martins et al. 23



Figure 21. Heat flux: external surface view: (a) Model P: nine bolts + 10-mm rubber + verticalslotted steel profiles + replacement of air gap and stone wool by polyurethane and (b) Model Q:nine bolts + 10-mm rubber + vertical slotted steel profiles + replacement of air gap and stonewool by polyurethane.

Figure 22. Wall combined solutions with polyurethane foam and slotted steel profiles: (a)Model R: nine bolts + 10-mm rubber + slotted steel profiles + replacement of air gap and stonewool by polyurethane and (b) Model S: nine bolts + 10-mm rubber + slotted steelprofiles + replacement of air gap and interior stone wool by polyurethane foam andreplacement of exterior stone wool by an air gap.




These models provide good thermal performance, the best U-value beingobtained in Model R, with a U-value of 0.1173W/(m2 K). This model improves by61.0% the U-value relatively to the reference model (Model A). Model S also pre-sents a good thermal performance, with 0.1333W/(m2 K) for the U-value, whichcorresponds to an improvement of 55.7% relatively to the reference model. Figure23(b) shows the heat flux in the external face.

The last two models, T and U, combined the improvements introduced in theprevious model. Model T combines the following: Model B2 with rubber strips,Model D2 with vertical slotted steel profiles, Model E with bolted connections andModel L in which 30-mm silica aerogel insulation blankets were placed on bothsides (Figure 24(a) shows the details). Similarly, Model U consists of Model B2,Model D2, Model E and Model O in which 30-mm vacuum panels were placed onboth sides (Figure 24(b) shows the details).

These last models provide good results for the U-value, being the best oneobtained for Model U, with a thermal transmittance value of 0.0959W/(m2 K).This model improves by 68.2% the U-value relatively to the reference Model A.Model T also presents a good thermal performance, although lower, that is, a U-value equal to 0.1406W/(m2 K), which corresponds to an improvement of 53.3%comparatively to the reference model. Figure 25 shows the heat flux distributionalong the external surface of the walls.

Figure 23. Heat flux: external surface view: (a) Model R: nine bolts + 10-mm rubber + slottedsteel profiles 28% + polyurethane and (b) Model S: nine bolts + 10-mm rubber + slotted steelprofiles 28% + polyurethane and 40-mm air gap.

Martins et al. 25



Figure 24. Wall combined solutions with aerogel or vacuum panels and slotted steel profiles:(a) Model T: nine bolts + 10-mm rubber + slotted steel profiles + 30-mm silica aerogelinsulation blankets on both sides and (b) Model U: nine bolts + 10-mm rubber + slotted steelprofiles + 30-mm vacuum panels on both sides.

Figure 25. Heat flux: external surface view: (a) Model T: nine bolts + 10-mm rubber + slottedsteel profiles + 30-mm silica aerogel insulation blanket on both sides and (b) Model U: ninebolts + 10-mm rubber + slotted steel profiles + 30-mm vacuum panel on both sides.




U-value improvements results and discussion

Table 5 and Figure 26 presents a summary of the thermal transmittance values forthe improvements of the LSF modular walls and the thermal transmittance differ-ences (DU) between each model and the reference one (Model A).

The best single strategy for U-value improvement is Model O, which adds VIPson both sides of the wall steel structure. The U-value was reduced by 49%, whichrepresents a good improvement in the thermal performance of the wall module,considering that a layer of insulation with only 60mm was added.

From the numerical simulations, for the combined strategies, it is concludedthat the best solution is the one that combines rubber strips, slotted steel profiles,bolted connections, and VIPs on both sides, identified as Model U. The U-valuewas reduced from 0.3011W/(m2 K), in the reference Model A, to 0.0959W/(m2

K).Model R also presents good performance; in this model, the following solutions

are combined: rubber strips, slotted steel profiles, bolted connections and PU. TheU-value was also significantly reduced from 0.3011W/(m2 K), in the referenceModel A, to 0.1173W/(m2 K). Although this model improves the reference modelby 61%, it presents the drawback of not having an air gap, which in case of waterinfiltration can be a problem. However, this disadvantage can be mitigated byusing a windtight and water-resistant membrane, placed between the ETICS andthe OSB.

Model S, which has an air gap with 40mm, also presents a good alternative forimprovement. This model increases the U-value by only 0.016W/(m2 K) compara-tively to the previous (Model R).

Finally, Model T decreases the U-value by 53.3%. Although this model doesnot present the best result, this is in fact also a good solution because the place-ment of aerogel insulation blankets in the wall not only decreases the U-value butalso increases the fire resistance of the wall.

The most efficient material added to the wall is the VIPs, improving by 68.2%the thermal performance. However, due to the difference of price between this solu-tion and the PU foam, which allows a reduction of 61.0% in the U-value, this sec-ond solution may also be considered a good option.

Conclusion

In this article, several thermal bridges mitigation strategies to improve an LSF wallmodule were assessed. An optimization of the wall module insulation layers wasalso performed, which combined with the mitigation approaches allows a signifi-cant improvement in the LSF wall thermal performance. The implementation ofthermal bridges mitigation strategies in a modular LSF wall was performed using3D FEM models, derived from a previously validated 3D FEM reference model(Santos et al., 2014).

Martins et al. 27



Tab

le5.

Par

amet

ric

study

for

U-v

alue

impro

vem

ent:

ove

rvie

wofm

odel

san

dre

sults.

Model

Model

des

crip

tion

U-v

alue

(W/(

m2K

))(D

U)

Without

hori

zonta

lst

eelco

nnec

tions

Rubber

Slott

edst

eelpro

files

Poly

ure

than

efo

amA

eroge

lV

IPs

Without

air

gap

and

stone

wool

With

40-m

mai

rga

pan

dw

ithout

stone

wool

Singl

est

rate

gies

Aa

0.3

011

()

HP

0.1

615

(246.4

%)

IP

0.1

876

(237.7

%)

JP

Inte

rnal

side

onl

y0.2

295

(223.8

%)

KP

Exte

rnal

side

onl

y0.2

356

(221.8

%)

LP

0.1

889

(237.3

%)

MP

Inte

rnal

side

only

0.2

032

(232.5

%)

NP

Exte

rnal

side

onl

y0.2

048

(232.0

%)

OP

0.1

536

(249.0

%)

Com

bin

edst

rate

gies

PP

P10

mm

P28%

Only

vert

ical

P0.1

271

(257.8

%)

QP

0.1

525

(249.3

%)

RP

28%

P0.1

173

(261.0

%)

SP

0.1

333

(255.7

%)

TP

0.1

406

(253.3

%)

UP

0.0

959

(268.2

%)

VIP

:va

cuum

insu

lation

pan

el.

a Ref

eren

cem

odel

.




From this work, several remarks can be highlighted. The mitigation of thermalbridges caused by the steel structure is best accomplished by the introduction ofthermal break strips and the introduction of slotted steel profiles. The parametricstudy shows that the thermal transmittance of the wall can be reduced by up to8.3%, corresponding to 75% of the total impact of the steel thermal bridges. Notethat these values could be even higher if there is no continuous thermal insulationlayer of ETICS (40mm).

Furthermore, the following design guidance may be provided:

Introduce at least one-third of continuous thermal insulation; If the previous condition is verified, then some thermal bridges mitigation

strategies could be very much reduced or even irrelevant (e.g. male or femalestuds, thin rubber strips, fixing bolts instead of steel plate connections andslotted steel profiles);

When selecting or designing thermal profiles, choose the ones with highernumber of narrow slots since they are more efficient than the ones withlarger slots;

Whenever possible, try to use two layers of perpendicular steel studs, avoid-ing trespassing the entire wall cross section with two parallel steel profiles.

In the performed evaluation of the wall module, it is clear that the air gapcrossed by steel influences its thermal performance, and filling the air gap spacewith insulation allows a great improvement. The parametric study shows that the

Figure 26. Parametric study for U-value improvement: overview of results for the wall models.

Martins et al. 29



wall U-value can be reduced by near three times with quite simple solutions, corre-sponding to a reduction of 68%. From the economical perspective, the solution inwhich PU is added to the wall module instead of the air gap is the best solutiongiven the higher cost of aerogel and VIPs.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, author-ship and/or publication of this article.

Funding

The authors would like to thank the European Union for funding in the form of the QRENSI I&DT N. 24804 EcoSteelPanel Thermal and acoustic comfort grant.

References

Alam M, Singh H and Limbachiya MC (2011) Vacuum Insulation Panels (VIPs) for building

construction industry a review of the contemporary developments and future directions.

Applied Energy 88(11): 35923602.

Baetens R, Jelle BP and Gustavsen A (2011) Aerogel insulation for building applications: a

state-of-the-art review. Energy and Buildings 43(4): 761769.

Baetens R, Jelle BP, Thue JV, et al. (2010) Vacuum insulation panels for building

applications: a review and beyond. Energy and Buildings 42(2): 147172.

Blomberg TR and Claesson J (1998) Heat transmission through walls with slotted steel

studs. Conference proceedings by ASHRAE. In: Thermal envelopes VII, Clearwater, FL,

610 December, 621628.

EN ISO 10211:2007 (2007) Thermal bridges in building construction heat flows and

surface temperatures detailed calculations, European Committee for Standardization.

EN ISO 6946:2007 (2007) Building components and building elements thermal resistance

and thermal transmittance calculation method, European Committee for

Standardization.

Erhorn-Klutting H and Erhorn H (2009) ASIEPI Impact of Thermal Bridges on the Energy

Performance of Buildings. Stuttgart: Fraunhofer Institute for Building Physics.

Hoglund T and Burstrandb H (1998) Slotted steel studs to reduce thermal bridges in

insulated walls. Thin-Walled Structures 32: 81109.

Kingspan UK (2013) OPTIM-R External Wall System Next Generation Insulation

Solution for External Masonry Walls. Leominster: Kingspan, Low energy Low Carbon

Buildings.

Kosny J and Christian JE (1995) Thermal evaluation of several configurations of insulation

and structural materials for some metal stud walls. Energy and Buildings 22(2): 157163.

Salhab B and Wang YC (2008) Equivalent thickness of cold-formed thin-walled channel

sections with perforated webs under compression. Thin-Walled Structures 46(79):

823838.

Santos P, Martins C, Simoes da Silva L, et al. (2014) Thermal performance of lightweight

steel framed wall: the importance of flanking thermal losses. Journal of Building Physics

38(1): 8198.




Santos P, Simoes da Silva L and Ungureanu V (2012) Energy efficiency of light-weight steel-

framed Buildings, 1st ed. European Convention for Constructional Steelwork (ECCS),

Technical Committee 14-Sustainability & Eco-Efficiency of Steel Construction, no. 129.

Santos P, Simoes da Silva L, Gervasio H, et al. (2011) Parametric analysis of the thermal

performance of light steel residential buildings in Csb climatic regions. Journal of Building

Physics 35(1): 753.

Veljkovic M and Johansson B (2006) Light steel framing for residential buildings. Thin-

Walled Structures 44(12): 12721279.

Martins et al. 31



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