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Development of Lightweight Steel Framed Construction Systems for Nearly-Zero Energy Buildings

Buildings

June 2022
12(7):929

DOI:10.3390/buildings12070929

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CC BY 4.0

Authors:

Marija Jelcic Rukavina

University of Zagreb Faculty of Civil Engineering

Davor Skejic

University of Zagreb

Anton Kralj

Faculty of Civil Engineering, University of Zagreb

Tomislav Ščapec

University of Zagreb

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Light steel frame (LSF) building systems offer high structural resilience, lower costs due to fast prefabrication, and high ability to recycle and reuse. The main goal of this paper was to provide state-of-the-art main components for such systems with the intention to be implemented for use in nearly-zero energy buildings (NZEBs). A brief historical outline of the development of LSF systems was given, and the key parameters affecting the design and use of LSF systems were discussed. The influence of the individual components of the LSF system (steel studs, sheathing boards, and insulation materials) was then thoroughly discussed in light of relevant research on energy efficiency and other important properties (such as sound protection and fire resistance). Web of Science and Scopus databases were used for this purpose, using relevant key words: LSF, energy efficiency, sheathing boards, steel studs, insulation, etc. Several research gaps were identified that could be used for development and future research on new LSF systems. Finally, based on the analysis of each component, an innovative LSF composite wall panel was proposed which will be the subject of the authors’ future research. Conducted preliminary analysis showed low thermal transmittance of the system and indicates the path of its further research.

DUCLOS iron demountable buildings (France patent, 1890) [13]. Figure 1. DUCLOS iron demountable buildings (France patent, 1890) [13].

…

Advantages of CFS in comparison with current traditional construction methods [25].

…

Thermophysical properties of different insulation materials [58,59].

…

Comparison of different LSF wall panel configurations.

…

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Citation: Jelˇci´c Rukavina, M.; Skeji´c,

D.; Kralj, A.; Šˇcapec, T.; Milovanovi´c,

B. Development of Lightweight Steel

Framed Construction Systems for

Nearly-Zero Energy Buildings.

Buildings 2022,12, 929. https://

doi.org/10.3390/buildings12070929

Academic Editor: Cinzia Buratti

Received: 17 May 2022

Accepted: 28 June 2022

Published: 30 June 2022

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4.0/).

buildings

Review

Development of Lightweight Steel Framed Construction

Systems for Nearly-Zero Energy Buildings

Marija Jelˇci´c Rukavina 1, Davor Skeji´c 2,* , Anton Kralj 2, Tomislav Šˇcapec 1and Bojan Milovanovi´c 1

1Department of Materials, Faculty of Civil Engineering, University of Zagreb, Fra Andrije Kaˇci´ca-Mioši´ca 26,

10000 Zagreb, Croatia; marija.jelcic.rukavina@grad.unizg.hr (M.J.R.); tomislav.scapec@grad.unizg.hr (T.Š.);

bojan.milovanovic@grad.unizg.hr (B.M.)

Department for Structures, Faculty of Civil Engineering, University of Zagreb, Fra Andrije Kaˇci´ca-Mioši´ca 26,

10000 Zagreb, Croatia; anton.kralj@grad.unizg.hr

*Correspondence: davor.skejic@grad.unizg.hr

Abstract:

Light steel frame (LSF) building systems offer high structural resilience, lower costs due to

fast prefabrication, and high ability to recycle and reuse. The main goal of this paper was to provide

state-of-the-art main components for such systems with the intention to be implemented for use in

nearly-zero energy buildings (NZEBs). A brief historical outline of the development of LSF systems

was given, and the key parameters affecting the design and use of LSF systems were discussed.

The inﬂuence of the individual components of the LSF system (steel studs, sheathing boards, and

insulation materials) was then thoroughly discussed in light of relevant research on energy efﬁciency

and other important properties (such as sound protection and ﬁre resistance). Web of Science and

Scopus databases were used for this purpose, using relevant key words: LSF, energy efﬁciency,

sheathing boards, steel studs, insulation, etc. Several research gaps were identiﬁed that could be

used for development and future research on new LSF systems. Finally, based on the analysis of each

component, an innovative LSF composite wall panel was proposed which will be the subject of the

authors’ future research. Conducted preliminary analysis showed low thermal transmittance of the

system and indicates the path of its further research.

Keywords:

lightweight steel frame (LSF); nearly-zero energy buildings (NZEB); sheathing boards;

insulation materials

1. Introduction

As the building sector accounts for about 40% of total ﬁnal energy consumption in

the European Union and is responsible for 36% of total carbon dioxide (CO

) emissions,

energy efﬁciency measures in buildings are increasingly recognized as a tool to achieve

sustainable energy supply, reduce greenhouse gas emissions, and promote the compet-

itiveness of European economies [

]. Therefore, European legislation has established

a cross-cutting framework with targets for high energy performance of buildings based

on two key documents: the Energy Efﬁciency Directive (EED) [

] and the Energy Per-

formance of Buildings Directive (EPBD) [

]. The EPBD has established several new or

strengthened requirements, such as the obligation for all new buildings to be a ‘nearly-zero

energy building’ (NZEB) by the end of 2020 [

]. In general, an NZEB is a building with

a very high energy performance, a signiﬁcant portion of which should be met by energy

from renewable sources, including energy from renewable sources generated on-site or

nearby [

]. The requirements of the EED and EPBD are primarily aimed at reducing heat

losses through the building envelope, since heat losses through the building envelope

account for a large share of the total energy consumption of buildings. While thermal

performance of the building envelope is only one of the many factors inﬂuencing the energy

needs of the speciﬁc building, i.e., geometry, orientation, climate conditions, thermal mass,

HVAC systems, etc., it is the parameter most easily inﬂuenced by the building industry.

Buildings 2022,12, 929. https://doi.org/10.3390/buildings12070929 https://www.mdpi.com/journal/buildings

Buildings 2022,12, 929 2 of 19

Poor thermal performance of the building envelope causes large heat losses, making the

beneﬁts of highly efﬁcient HVAC systems negligible.

Light steel frame (LSF) structures are already recognized worldwide (especially in the

United States of America (USA), Australia, and Japan [

]) as the most suitable building

system with the possibility of achieving low energy demand. This is due to the great

ﬂexibility of the system, the possibility of innovation, and the increased use of modern

insulating materials as an essential part of the construction with different construction

techniques to increase the effectiveness of energy saving. In addition, LSF structures have

other signiﬁcant advantages that make them a popular alternative to traditional masonry

and mortar structures. The advantages of LSF structures compared to conventional building

systems include light weight, short construction time, reduced labor and cost, reusability

and recyclability, industrial quality control, structural resistance, etc. [

]. For example,

being lighter and thinner than conventional building envelope elements, they provide

the same net ﬂoor area while decreasing the gross heated volume. Moreover, all these

advantages have an impact on the reduced carbon footprint of a building, thus offering

a potential for the use of LSF structures in European countries, especially for low-rise

buildings such as residential houses and buildings blocks.

Considering the potential of LSF buildings for meeting the NZEB requirements of

EED and EPDB, and as the heat losses are easiest to inﬂuence, this paper reviews the

development of such systems throughout history, presents the current state-of-the-art,

evaluates the types of their components, and identiﬁes research gaps that could lead to

further development of such systems for use in NZBs. The Web of Science and Scopus

databases were used to ﬁnd the available insulation, sheathing, and face sheets used in LSF

construction and relevant research on the energy efﬁciency of such systems.

2. Historical Development of Light Steel Framed Construction Systems

Early usage of LSF constructions can be found during the “Gold Rush” in the mid-19th

century [

], and they were considered as the ﬁrst prefabricated houses [

]. At ﬁrst, they

were built of wood, but for ﬁre safety reasons, the wood was replaced by an iron skeleton

and sheathed with iron plates.

According to [

], 1850 is the year when the use of cold-formed steel (CFS) members

in construction started. This was the opportunity for steel to gain a foothold in the con-

struction industry [

]. The ﬁrst example of a highly detailed prefabricated building was

the “DUCLOS iron demountable buildings”, as shown in Figure 1. They were made of iron

bars with bolted and riveted joints and had cladding of metal plates. This was a patent by

France Duclos in 1890 in France [12].

Buildings 2022, 12, x FOR PEER REVIEW 2 of 20

mass, HVAC systems, etc., it is the parameter most easily influenced by the building in-

dustry. Poor thermal performance of the building envelope causes large heat losses, mak-

ing the benefits of highly efficient HVAC systems negligible.

Light steel frame (LSF) structures are already recognized worldwide (especially in

the United States of America (USA), Australia, and Japan [6]) as the most suitable building

system with the possibility of achieving low energy demand. This is due to the great flex-

ibility of the system, the possibility of innovation, and the increased use of modern insu-

lating materials as an essential part of the construction with different construction tech-

niques to increase the effectiveness of energy saving. In addition, LSF structures have

other significant advantages that make them a popular alternative to traditional masonry

and mortar structures. The advantages of LSF structures compared to conventional build-

ing systems include light weight, short construction time, reduced labor and cost, reusa-

bility and recyclability, industrial quality control, structural resistance, etc. [1,7]. For ex-

ample, being lighter and thinner than conventional building envelope elements, they pro-

vide the same net floor area while decreasing the gross heated volume. Moreover, all these

advantages have an impact on the reduced carbon footprint of a building, thus offering a

potential for the use of LSF structures in European countries, especially for low-rise build-

ings such as residential houses and buildings blocks.

Considering the potential of LSF buildings for meeting the NZEB requirements of

EED and EPDB, and as the heat losses are easiest to influence, this paper reviews the de-

velopment of such systems throughout history, presents the current state-of-the-art, eval-

uates the types of their components, and identifies research gaps that could lead to further

development of such systems for use in NZBs. The Web of Science and Scopus databases

were used to find the available insulation, sheathing, and face sheets used in LSF con-

struction and relevant research on the energy efficiency of such systems.

2. Historical Development of Light Steel Framed Construction Systems

Early usage of LSF constructions can be found during the “Gold Rush” in the mid-

19th century [8], and they were considered as the first prefabricated houses [9]. At first,

they were built of wood, but for fire safety reasons, the wood was replaced by an iron

skeleton and sheathed with iron plates.

According to [10], 1850 is the year when the use of cold-formed steel (CFS) members

in construction started. This was the opportunity for steel to gain a foothold in the con-

struction industry [11]. The first example of a highly detailed prefabricated building was

the “DUCLOS iron demountable buildings”, as shown in Figure 1. They were made of

iron bars with bolted and riveted joints and had cladding of metal plates. This was a patent

by France Duclos in 1890 in France [12].

Figure 1. DUCLOS iron demountable buildings (France patent, 1890) [13].

Buildings 2022,12, 929 3 of 19

With the rise of the lightweight construction industry, insulation materials became very

important. The ﬁrst use of insulation materials in lightweight construction was recorded in

1897 when an American chemical engineer, Charles Corydon Hall, manufactured rock wool

from limestone and began commercial production of rock wool in Alexandria (Indiana,

USA) at his factory called the Crystal Chemical Works. Rock wool was a very popular

insulation material for lightweight structures [

]. Later, rock wool was replaced by asbestos

and was touted as the best alternative [

]. Although it was available, insulation was not

considered necessary until the 1920s, when public awareness of the importance of thermal

insulation was raised. The increasing popularity and use of lighter building materials

and the gradual introduction of air conditioning contributed to a higher need for thermal

insulation [16].

A notable achievement in LSF design was made by architect Howard Z. Fisher, who

developed the “House of the Future” in Chicago in 1933. The house was built entirely

of steel frame construction with the knowledge of leaders in the railroad and automobile

industries. He also founded “General House”, opening up a completely untapped ﬁeld

of residential construction. Additionally in 1933, “Armco Steel Corporation” introduced

the ﬁrst standing seam metal roof panel. A signiﬁcant increase in the development of such

houses was recorded in the mid-20th century by the “Lustron houses”. During World War

II, Lustron used roof, siding, and sheet metal products to develop warehouse buildings,

and in 1940, “Lustron Homes” built and sold nearly 2500 fully equipped steel-framed

houses [17].

In addition to residential construction, cold-formed steel has also proven successful in

commercial and industrial applications. One example is the Virginia Baptist Hospital, built

around 1925 in Lynchburg (Virginia, USA). The walls were masonry but the ﬂoors were

framed with double back-to-back cold-formed steel sections. After observations during

the renovation, it was conﬁrmed that these beams were still supporting loads after more

than 80 years [

]. This also shows that the service life of this type of construction can be

considerable, as shown in Figure 2a,b. Figure 2a shows a postcard from 1933 advertising a

“Stran-Steel house”, while Figure 2b shows the same house today.

Buildings 2022, 12, x FOR PEER REVIEW 3 of 20

With the rise of the lightweight construction industry, insulation materials became

very important. The first use of insulation materials in lightweight construction was rec-

orded in 1897 when an American chemical engineer, Charles Corydon Hall, manufactured

rock wool from limestone and began commercial production of rock wool in Alexandria

(Indiana, USA) at his factory called the Crystal Chemical Works. Rock wool was a very

popular insulation material for lightweight structures [14]. Later, rock wool was replaced

by asbestos and was touted as the best alternative [15]. Although it was available, insula-

tion was not considered necessary until the 1920s, when public awareness of the im-

portance of thermal insulation was raised. The increasing popularity and use of lighter

building materials and the gradual introduction of air conditioning contributed to a

higher need for thermal insulation [16].

A notable achievement in LSF design was made by architect Howard Z. Fisher, who

developed the “House of the Future” in Chicago in 1933. The house was built entirely of

steel frame construction with the knowledge of leaders in the railroad and automobile

industries. He also founded “General House”, opening up a completely untapped field of

residential construction. Additionally in 1933, “Armco Steel Corporation” introduced the

first standing seam metal roof panel. A significant increase in the development of such

houses was recorded in the mid-20th century by the “Lustron houses”. During World War

II, Lustron used roof, siding, and sheet metal products to develop warehouse buildings,

and in 1940, “Lustron Homes” built and sold nearly 2500 fully equipped steel-framed

houses [17].

In addition to residential construction, cold-formed steel has also proven successful

in commercial and industrial applications. One example is the Virginia Baptist Hospital,

built around 1925 in Lynchburg (Virginia, USA). The walls were masonry but the floors

were framed with double back-to-back cold-formed steel sections. After observations dur-

ing the renovation, it was confirmed that these beams were still supporting loads after

more than 80 years [18]. This also shows that the service life of this type of construction

can be considerable, as shown in Figure 2a,b. Figure 2a shows a postcard from 1933 ad-

vertising a “Stran-Steel house”, while Figure 2b shows the same house today.

(a)

(b)

Figure 2. (a) Postcard advertising Stran-Steel house at the 1933 Century of Progress World’s Fair;

(b) “The Ensign-Seelinger Home”, a Stran-Steel house in Huntington, West Virginia is on the Na-

tional Register of Historic Homes by Dr. Kathy L. Seelinger [19].

After World War II, Lustron began building houses in an assembly line fashion using

steel plates for framing, siding, trusses, and roof tiles. Their houses were cost effective,

fast, and easy to build [17].

Figure 2.

(

) Postcard advertising Stran-Steel house at the 1933 Century of Progress World’s Fair;

(

) “The Ensign-Seelinger Home”, a Stran-Steel house in Huntington, West Virginia is on the National

After World War II, Lustron began building houses in an assembly line fashion using

steel plates for framing, siding, trusses, and roof tiles. Their houses were cost effective, fast,

and easy to build [17].

Buildings 2022,12, 929 4 of 19

As cold-formed steel became more popular as a building material, the use of gypsum

board (GB) sheathing also grew. In 1931, the Gypsum Association conducted a series of ﬁre

tests and published a manual on the ﬁre resistance of gypsum board. The United States

Gypsum Company developed a nailable stud framing system called “Trussteel”, which

was advertised as “the original stud framing system of open construction for erecting

hollow, ﬁre-resistant partitions

. . .

” This led to the 1950s, which became known as the

era of gypsum board [

] and steel stud framing for non-combustible partitions with the

introduction of self-drilling screws.

As the market grew, it was difﬁcult to specify standard products because all products

had their own span and load tables until the Metal Stud Manufactures Association (MSMA)

developed a uniﬁed catalogue. The last major development in LSF design occurred in

the 1990s, when manufacturers realized that many engineers did not understand the

complexity of AISI speciﬁcations, and as a result, an association called the Light Gauge Steel

Engineers Association was formed. In 2001, it published the North American Speciﬁcation

for the Design of Cold-Formed Steel Structural Members. AISI then formed the Committee

on Framing Standards (COFS) to develop speciﬁc standards for steel framing used in

lightweight structures.

Nowadays, product standardization is in its ﬁnal stages and new products are being

developed. The Cold-Formed Steel Engineers Institute (CFSEI) has developed design

guides for each of the COFS standards, as well as computer-aided design details (CAD),

technical notes, and online and live courses for steel design. Software developers have

released programs that leverage the provisions of the speciﬁcations and standards to

provide fast, accurate, and in some cases, complete 3D building information modelling that

incorporates the design of steel framing elements and systems.

Australian company Framecad is one of the leaders in the construction industry of CFS.

Drawing on previous knowledge from historical developments, it offers research and de-

velopment of intelligent systems that can be incorporated into the design and construction

of CFS structures, providing fast and safe buildings for various project requirements [21].

Today, LSF structures have been developed as a fast and relatively simple system

since most of the components are prefabricated. It depends on the type of prefabricated

elements: (1) stick-framing or stick-built, (2) panelized or areal, and ﬁnally, (3) modular

or volumetric construction. From the environmental point of view, systems (1) and (2) are

better adapted to microclimatic conditions [

]. The stick-built construction method

is fully assembled at the construction site, where the elements are often pre-cut but the

connections are made on site. This method of construction is not often used for larger

buildings because it increases construction time and cost. As stick-built LSF construction is

still heavily site-based, the trend for modern buildings is more toward the modular system

because it provides better quality control and allows for precise prefabrication that would

otherwise not be possible on site. Panelized constructions are prefabricated in the factory

and later assembled on the construction site. This ensures geometric accuracy and reliability

while reducing construction time. Modular LSF structures are completely prefabricated

at the factory and often delivered to the construction site with all components, ﬁnishes,

and features.

3. Components and Its Inﬂuence on Achieving NZEBs

The composition of LSF structures in use today has not fundamentally changed over

the years and consists of three different main components: cold-formed steel sections,

sheathing boards, and insulating materials [

]. In addition, there are fastening materi-

als such as self-drilling screws, rivets, nails, staples, etc., as well as air- and water-tight

membranes and the ﬁnishing cover layers [

]. The ﬁnishing layers can be plaster, tiles,

bricks, cladding panels, etc., depending on the desired purpose and construction. Since the

load-bearing steel elements increase the heat loss through the component, it is important to

separate them from direct exposure, and here, the face layer can have a great inﬂuence.

Buildings 2022,12, 929 5 of 19

3.1. Cold-Formed Steel Members

CFS members are widely used in the construction industry because they offer many

advantages compared to other building products and materials [

]. CFS is considered the

most sustainable and increasingly popular modern building product in the construction

industry [

] and is mainly used in the commercial sector where entire buildings are

constructed. Table 1shows the advantages of CFS over conventional materials, some

of which compare directly with hot-rolled steel (HRS) as the next product in terms of

performance. CFS components are used for a variety of project requirements, whether as a

complete structure or for smaller structures where only a portion of CFS is used, such as

steel connectors, vertical deﬂection connectors, rigid connectors, and more. The different

types of CFS have speciﬁc applications in different parts of buildings, such as main walls,

shear wall panels, roof elements, etc. [

]. These types differ from each other by the shape of

the proﬁles, which are formed in cold rolling machines. Two main types of CFS are framing

members and panels (decks). Framing members are used for shell construction and are

usually in the form of C-sections, Z-sections, I-sections, angles, hat sections, T-sections, and

tube sections [

]. Of the above, C-sections are most commonly used for LSF structures. The

main function of each frame member is to support loads and provide structural strength

and stiffness. Secondary elements from CFS are joists, studs, decks, or panels. This category

of CFS is generally used for roof and ﬂoor slabs, wall panels, cladding materials, and bridge

formwork. Some basic thermophysical properties are explained in the next section.

Table 1. Advantages of CFS in comparison with current traditional construction methods [25].

Easy adaptation to project requirements Corrosion resistance

Less cost to environment

(manufacturing process)

Lower long-term cost

(maintenance and durability)

Easy of prefabrication (automation) Easy extension

Less steel with same performance No moisture-related expansion

Side effects of cooling

(manufacturing process) Recyclable

Precision (dimensional stability) Does not contribute to the ﬁre development

High strength and stiffness Safer in natural disasters (earthquake)

Fast construction (assembling) Easy erection of installations

Mold resistance Economic in transportation and handling

CFS is necessary for the modern construction industry, which requires maximum

safety and strength embedded in a supporting structure [

]. It is very beneﬁcial for

achieving thermal performance of NZEB buildings because it can be easily combined with

other components as the main structural system. CFS is usually thin in relation to its width

and can buckle at stresses below the yield stress. Such elements do not necessarily fail

when their buckling stress is reached and often continue to carry the load. In recent years,

deformation buckling has been considered as one of the most important limit states for

the design of CFS beams and columns. Since the cross-sections of CFS are relatively thin

and in some sections, the center of gravity and the sheer center do not coincide, ﬂexural

torsional buckling can be a critical factor for compression members. Web curvature is also a

known problem that often becomes critical in design. The efﬁciency of using high-strength

steel depends on the type of failure mode. Under certain conditions, such as columns with

large slenderness ratios, failure is usually limited by elastic buckling; therefore, the use of

high-strength steel is not always justiﬁed in terms of total cost.

The main disadvantage of CFS with respect to NZEBs is the high thermal conductivity

of steel. Signiﬁcant heat loss (thermal bridging) can be minimized by sheathing, insulation,

and careful detailing. Better thermal performance can be achieved in other ways besides

increasing insulation and sheathing thickness. In reviewing the literature, it was found

that different authors have studied different ways to reduce thermal bridging. Authors

such as Roque et al. [

] have found that direct contact between the steel and the sheathing

reduces the U-value of the structure and that the thermal performance is overestimated by

Buildings 2022,12, 929 6 of 19

about 50% if this is not taken into account. In the work of Soares et al. [

], some techniques

to reduce thermal bridges are presented, such as the simplicity of the façade geometry,

the application of a continuous insulation layer on the outside of the steel framing, the

avoidance of interruptions of the insulation layers, the installation of windows and doors

in contact with the insulation, and the attachment of the studs to the external insulation

layer with fasteners with low thermal conductivity.

Slotted webs of this type of studs can improve the thermal insulation of exterior

walls [

]. Santo et al. [

] concluded that the spacing between steel studs, the thickness of

the steel, and the cross-sectional proﬁle signiﬁcantly affect thermal bridging. Slotted steel

studs are now available in the market to improve thermal performance [

]. Kosny et al. [

]

studied how the spacing between steel studs affects the R-value for different insulation

thicknesses and concluded that the R-value can be increased by 20% by optimizing these

parameters. Santos in [

] also presented some other strategies such as: slotted steel studs,

ﬂange stud indentations, thermal brakes, and thermal break strips. Martins et al. [

]

added that fastening bolts can be used instead of horizontal steel plate connections to

reduce thermal bridging. He also concluded that the best solution is a combination of

several strategies, such as rubber strips (10 mm), slotted steel sections, and a reduction

in bolted connections. On the other hand, Feng et al. [

] concluded that the shape of the

cross-section does not have a decisive effect on the temperature distribution. Dias et al. [

]

concluded that the variation of the bolt cross-section can also contribute to the increase in

the strength, taking into account the inﬂuence of the sheathing limitations.

As can be seen from the literature review, the introduction of CFS into the NZEB system

as the main structural component increases the possibility of potential thermal bridges,

and the main problem arises from the direct connection of CFS to the sheathing. In order to

reduce the thermal bridges and increase the thermal performance, possible conﬁgurations

of CFS should be introduced in LSF by changing the parameters derived from the above-

mentioned previous research. These parameters should be carefully selected because they

affect both thermal and structural performance. For example, increased spacing between

bolts can improve thermal performance but degrade structural performance. In addition,

the static performance of CFS is also affected by other components (sheathing and PUR)

that provide lateral restraint against instability. For this reason, all components must be

analyzed together when solving such a complex problem.

3.2. Sheathing Boards and Finishing Layers

Appropriate boards for LSF are usually selected based on the requirements of ﬁre

resistance, sound insulation, moisture resistance, durability, and economy [

]. As noted

by Soares et al. [

], the most common sheathing panels for low-rise residential buildings

are oriented strand boards (OSBs), structural plywood, and GBs. For non-residential

industrial buildings, steel sheathing is often used due to its high stiffness and strength,

good surface ﬁnish, and high impact resistance. Sheathing boards can be divided into

metallic and non-metallic.

The thickness of steel sheathing is about 1 mm [

] and is offered in various proﬁles

and connected to the rest of the LSF structure by rigid connections such as self-drilling

screws. This, in turn, can also affect the acoustic performance of the entire building [

]. As

for thin steel elements used as load-bearing elements in LSF structures, the steel sheathing

has a very high thermal conductivity. This fact makes them impractical for NZEBs, since

thermal bridges predominate. For comparison, the thermomechanical properties of some

steel variants are shown in Table 2.

Buildings 2022,12, 929 7 of 19

Table 2. Typical mechanical and thermal properties of steel [35,36].

Material ρ[kg/m3]E [GPa] ν[–] α[◦C·10−6]λ[W/mK]

Mild steel 7800 206 0.29 13 46

Steel-cold rolled HS 7800 206 0.30 11 46

Stainless steel 7700–7900 196 0.29 11-18 14

Since the high thermal conductivity favors the formation of thermal bridges, it is

essential to provide adequate insulation to minimize this effect. In most cases, steel is

often incorporated in the form of sandwich cladding panels with mineral wool (MW),

polyurethane (PUR), or polyisocyanurate (PIR). The U-value of such panels is highly de-

pendent on the type of insulation and thickness but generally ranges from 0.11 W/(m

K) to

0.56 W/(m

K) [

–

]. This means that the use of sandwich cladding panels can potentially

meet the NZEB requirements, as they comply with the prescribed thermal transmittance

values (U-values). In addition, there are some steel sheathing options that are used as ﬁre

protection layers. Steau et al. [

] reported that two types of ﬁre-resistant composite steel

panels are currently available on the market, containing a ﬁber-reinforced or lightweight

cement core and using steel (perforated and galvanized) as the outer protective layer.

However, these steel sheathing panels do not provide adequate thermal insulation and

require additional insulation to meet NZEB requirements for building elements at the

building envelope.

Other sheathing options are non-metallic and therefore better suited to achieve NZEBs

due to their lower thermal conductivity compared to steel. The main problem with these

types of sheathing boards is that most of them are manufactured in ﬁxed lengths that

rarely exceed 3.5 m [

]. This creates additional problems for the LSF structural system,

as the joints between the panels can lead to additional thermal bridges or provide a path

for moisture, ﬂames in case of ﬁre, dust, and other particles to enter the interior of the

LSF structure.

Table 3provides a comparison of the properties of the most commonly used non-

metallic sheathing boards at room temperature.

Table 3.

Thermophysical properties of commonly used non-metallic sheathing materials for

LSF [38,40,41].

Material/Reference ρ[kg/m3]E [GPa] ν[–] λ[W/mK] Cp [J/kgK]

OSB [36] 600–620 5.500 1/2.200 20.30 0.12 1420–1550

Gypsum [36] 700–1000 2.500 0.27 0.21 880–1000

Magnesium oxide [42] 1000–1100 N.A. 0.35–0.37 0.30–0.60 1400

Magnesium sulphate [43] 1000–1100 N.A. 0.35–0.37 0.30–0.60 ≈1400

Calcium silicate [42]≈800–1100 N.A. N.A. ≈0.25 ≈1000–1100

Perlite boards [42]≈200–280 N.A. N.A. ≈0.10 ≈750–900

Fiber cement boards [43]≈1000–1400 N.A. N.A. ≈0.25–0.30 ≈900–1000

1Longitudinal Axis, 2Transverse Axis

Most non-metallic sheathing boards, with the exception of OSBs and plywood, are

used speciﬁcally as passive ﬁre protection layers. Since OSBs and plywood are wood-based

composites, OSBs are susceptible to ignition [

], and there is a need for an additional

ﬁre protection layer. OSBs are composite multilayer boards made of wood strands with a

speciﬁc shape, thickness, and adhesive strength [

] which are very similar to plywood,

with the only signiﬁcant difference being the price. This has led to OSB panels replacing

plywood in the construction sector [

]. The main use of plywood and OSBs is the outer

sheathing, which is often similar to steel, so they are used as an alternative to steel sheathing.

For this reason, designers nowadays use them as a ﬁnishing layer for walls and ceilings [

and they are often used in low-rise residential buildings [

]. Because OSBs are wood-based

composite laminates, they are susceptible to termite and mold which can lead to swelling

Buildings 2022,12, 929 8 of 19

and disintegration, resulting in loss of structural strength [

]. Basic deﬁnitions of OSBs,

requirements, and classiﬁcation are laid out in the European standard EN 300 [47].

While OSBs are used for external sheathing, GBs are often used as internal sheathing

in LSF structures. Since OSB and GB boards are non-metallic, thermal bridging problems

are easier to overcome because they have much lower thermal conductivity compared

to steel. The boards are also connected to the steel structure with self-tapping screws.

GBs are known for their ﬁre protection properties as a passive ﬁre protection layer [

Standard gypsum boards are composed of approximately 80% gypsum (calcium sulphate

dihydrate), which contains 15–18% chemically bound water and 4–5% free water [

During heating, considerable energy is required to evaporate the free water, which delays

the temperature rise [

] on the other side of the board. In other words, this reaction is

endothermic, meaning that it consumes energy that would otherwise raise the temperature

of the material. When heated, gypsum undergoes two main chemical decomposition

reactions. The ﬁrst reaction leads to the decomposition of gypsum into calcium sulphate

hemihydrate at temperatures between 80 and 120

◦

C, while the second reaction decomposes

the calcium sulphate hemihydrate into calcium sulphate anhydrite at temperatures between

200 and 240

◦

C, which affects the speciﬁc heat change [

]. To further increase the ﬁre

resistance of GBs, various additives such as cellulose ﬁbers and ﬂy ash can be added [

The diagram in Figure 3represents the change in speciﬁc heat values of GBs at different

temperatures compared to other non-metallic sheathing boards, showing the endothermic

reactions mentioned above. The current European standard that regulates the requirements

and classiﬁcation of GBs is EN 520 [51].

Buildings 2022, 12, x FOR PEER REVIEW 8 of 20

OSBs are wood-based composite laminates, they are susceptible to termite and mold

which can lead to swelling and disintegration, resulting in loss of structural strength [46].

Basic definitions of OSBs, requirements, and classification are laid out in the European

standard EN 300 [47].

While OSBs are used for external sheathing, GBs are often used as internal sheathing

in LSF structures. Since OSB and GB boards are non-metallic, thermal bridging problems

are easier to overcome because they have much lower thermal conductivity compared to

steel. The boards are also connected to the steel structure with self-tapping screws. GBs

are known for their fire protection properties as a passive fire protection layer [48]. Stand-

ard gypsum boards are composed of approximately 80% gypsum (calcium sulphate dihy-

drate), which contains 15–18% chemically bound water and 4–5% free water [33]. During

heating, considerable energy is required to evaporate the free water, which delays the

temperature rise [49] on the other side of the board. In other words, this reaction is endo-

thermic, meaning that it consumes energy that would otherwise raise the temperature of

the material. When heated, gypsum undergoes two main chemical decomposition reac-

tions. The first reaction leads to the decomposition of gypsum into calcium sulphate hem-

ihydrate at temperatures between 80 and 120 °C, while the second reaction decomposes

the calcium sulphate hemihydrate into calcium sulphate anhydrite at temperatures be-

tween 200 and 240 °C, which affects the specific heat change [50]. To further increase the

fire resistance of GBs, various additives such as cellulose fibers and fly ash can be added

[41]. The diagram in Figure 3 represents the change in specific heat values of GBs at dif-

ferent temperatures compared to other non-metallic sheathing boards, showing the endo-

thermic reactions mentioned above. The current European standard that regulates the re-

quirements and classification of GBs is EN 520 [51].

Figure 3. Comparison of different specific heat values of the mentioned boards from different re-

searchers [38,41,50].

Other non-metallic, mostly fire-resistant boards include magnesium oxide boards,

magnesium sulphate boards, cement fiber boards, perlite boards, calcium silicate boards,

and boards with phase change materials (PCMs).

Magnesium oxide boards are generally used to improve the acoustic properties, im-

pact resistance, and structural strength of panel systems [52]. The chemical composition

of the board is mainly composed of two elements: magnesium oxide (MgO) and magne-

sium chloride (MgCl2). As with GBs, the boards are mainly used as inner cladding for LSF

elements. Rusthi et al. [52] conducted a study on two types of MgO boards, which differed

only by the percentage of MgO in the board, to evaluate their thermal properties and fire

behavior. In a more recent study, Steau and Mahendran [53] tested numerous sheathing

-2,000

2,000

4,000

6,000

8,000

10,000

12,000

14,000

0 200 400 600 800 1,000 1,200

Specific heat capacity [J/kgK]

Temperature [°C]

Gypsum plasterboard

Calcium silicate board

Magnesium oxide board

Perlite board

Magnesium sulphate board

Fiber cement board

Figure 3.

Comparison of different speciﬁc heat values of the mentioned boards from different

researchers [38,41,50].

Other non-metallic, mostly ﬁre-resistant boards include magnesium oxide boards,

magnesium sulphate boards, cement ﬁber boards, perlite boards, calcium silicate boards,

and boards with phase change materials (PCMs).

Magnesium oxide boards are generally used to improve the acoustic properties, impact

resistance, and structural strength of panel systems [

]. The chemical composition of the

board is mainly composed of two elements: magnesium oxide (MgO) and magnesium

chloride (MgCl

). As with GBs, the boards are mainly used as inner cladding for LSF

elements. Rusthi et al. [

] conducted a study on two types of MgO boards, which differed

only by the percentage of MgO in the board, to evaluate their thermal properties and ﬁre

behavior. In a more recent study, Steau and Mahendran [

] tested numerous sheathing

Buildings 2022,12, 929 9 of 19

boards, including MgO boards. Both studies showed that MgO boards undergo endother-

mic reactions, similar to GBs, due to the signiﬁcant amount of free and bound water. As

shown in Figure 3, the ﬁrst two peaks occurred at about 180–230

◦

C, the third and fourth

peaks occurred at 400–475

◦

C, and the last peak occurred at about 520

◦

C [

]. According to

Rusthi et al. [

], the ﬁrst two peaks were due to dehydration of magnesium oxychloride

and evaporation of water, the third and fourth were due to hydrolysis and pyrolysis, and

the last peak was due to the release of chemically bound water. Although the boards

have an advantage over GB and OSB panels in terms of acoustic properties, the authors

concluded that the main disadvantage of this type of boards is that they have a very high

mass loss at elevated temperatures, about 15% higher compared to GBs. These results were

later conﬁrmed by Steau et al. [

] and Martins et al. [

]. Therefore, under the high thermal

load, cracks occur in the board and the integrity of the whole assembly is compromised.

Another disadvantage of this type of sheathing is the chemical degradation of the material

when exposed to higher humidity. Aiken et al. [

] and Rode et al. [

] investigated this

effect and concluded that the board is not suitable for exterior façades or other climates

where the board is exposed to higher moisture content. Upon contact, the board forms

saline water on the surface, which can lead to corrosion and signiﬁcant damage. As a fairly

new ﬁre protective board, to date, there is no harmonized European standard deﬁning the

terms, requirements, and tests for this type of board.

A variant of MgO boards is magnesium sulphate boards which have a similar chemical

composition to magnesium oxide boards. They are composed of magnesium oxide (about

55%), sulphate (about 25%), and glass ﬁbers (about 18%) [

]. Replacement of MgCl2 in

basic magnesium oxide boards with MgSO

eliminates the corrosive effect of chlorides [

Gnanachelvam et al. [

] studied the thermal properties of this type of board at elevated

temperatures and found that the ﬁre behavior of magnesium sulphate boards was slightly

better than that of magnesium oxide boards, but the same rapid cracking and mass loss

occurred. Since magnesium oxide and magnesium sulphate boards are similar, their

chemical decomposition at elevated temperatures is identical; the process is shown in

Figure 4. As with magnesium oxide boards, to date, there is no harmonized European

standard deﬁning the terms, requirements, and tests for this type of board.

Buildings 2022, 12, x FOR PEER REVIEW 9 of 20

boards, including MgO boards. Both studies showed that MgO boards undergo endother-

mic reactions, similar to GBs, due to the significant amount of free and bound water. As

shown in Figure 3, the first two peaks occurred at about 180–230 °C, the third and fourth

peaks occurred at 400–475 °C, and the last peak occurred at about 520 °C [53]. According

to Rusthi et al. [52,54], the first two peaks were due to dehydration of magnesium oxychlo-

ride and evaporation of water, the third and fourth were due to hydrolysis and pyrolysis,

and the last peak was due to the release of chemically bound water. Although the boards

have an advantage over GB and OSB panels in terms of acoustic properties, the authors

concluded that the main disadvantage of this type of boards is that they have a very high

mass loss at elevated temperatures, about 15% higher compared to GBs. These results

were later confirmed by Steau et al. [38] and Martins et al. [55]. Therefore, under the high

thermal load, cracks occur in the board and the integrity of the whole assembly is com-

promised. Another disadvantage of this type of sheathing is the chemical degradation of

the material when exposed to higher humidity. Aiken et al. [56] and Rode et al. [57] inves-

tigated this effect and concluded that the board is not suitable for exterior façades or other

climates where the board is exposed to higher moisture content. Upon contact, the board

forms saline water on the surface, which can lead to corrosion and significant damage. As

a fairly new fire protective board, to date, there is no harmonized European standard de-

fining the terms, requirements, and tests for this type of board.

A variant of MgO boards is magnesium sulphate boards which have a similar chem-

ical composition to magnesium oxide boards. They are composed of magnesium oxide

(about 55%), sulphate (about 25%), and glass fibers (about 18%) [41]. Replacement of

MgCl2 in basic magnesium oxide boards with MgSO4 eliminates the corrosive effect of

chlorides [41]. Gnanachelvam et al. [41] studied the thermal properties of this type of

board at elevated temperatures and found that the fire behavior of magnesium sulphate

boards was slightly better than that of magnesium oxide boards, but the same rapid crack-

ing and mass loss occurred. Since magnesium oxide and magnesium sulphate boards are

similar, their chemical decomposition at elevated temperatures is identical; the process is

shown in Figure 4. As with magnesium oxide boards, to date, there is no harmonized

European standard defining the terms, requirements, and tests for this type of board.

(a)

(b)

(c)

Figure 4. (a) Cold-framed construction; (b) hybrid-framed construction; (c) warm-framed construc-

tion [7].

Figure 4.

(

) Cold-framed construction; (

) hybrid-framed construction; (

) warm-framed construction [

Commercially available calcium silicate boards can be divided into three categories:

low density (200–500 kg/m

), medium density (500–1000 kg/m

), and high density

(1000–1800 kg/m

) [

]. They are generally stiffer than GBs, which sometimes makes

Buildings 2022,12, 929 10 of 19

them more attractive than GBs. Their main material is calcium silicate, and at high temper-

atures, the speciﬁc heat values reach similar peaks to GBs due to the water evaporation

process, as shown in Figure 4.

In addition, perlite boards are also a sheathing alternative made from volcanic glass.

Perlite boards are lightweight, easy to handle, and offer low cost, high strength, and chemi-

cal stability [

]. Modern sheathing boards also include PCM plasterboards which consist

of microencapsulated PCM, gypsum core, and small quantities of additives [

]. PCMs

absorb or lose considerable energy and undergo a phase transition, melting during the

day and solidifying at night, helping to maintain indoor thermal comfort [

]. Although

PCM plasterboards are expected to have a higher heat absorption capacity at elevated

temperatures due to their higher speciﬁc heat capacity, their ﬁre performance might be

worse compared to insulation materials due to their higher mass loss and thermal conduc-

tivity [

]. In a study by Gnanachelvam et al. [

], it was also demonstrated that the PCM

gypsum board has a very high mass loss at elevated temperatures due to dehydration and

evaporation PCM.

Since the market offers different sheathing options, it is important to evaluate the best

option depending on the requirements needed and the building environment in which the

board will be installed.

3.3. Thermal Insulation Materials

When considering NZEBs, insulation materials are the most important component

of the LSF building system because they provide the required thermal properties and

thermal comfort. Based on the position of insulation in LSF structures, they are divided

into warm-framed, cold-framed, and hybrid-framed [7] structures, as shown in Figure 4.

For all three types, different types of thermal insulation materials can be used, which

are described in detail in the rest of the text. In warm-framed LSF constructions, it is

common to use ETICS for the outer face layer [

]. Commonly used materials for ETICS are

expanded polystyrene (EPS), and since the materials are quite similar, extruded polystyrene

(XPS) [7].

The thermophysical properties of the different insulation materials used in LSF panels

are shown in Table 4. As can be clearly seen from the table, the thermal conductivity of

polymers is signiﬁcantly lower compared to the other materials listed.

Table 4. Thermophysical properties of different insulation materials [58,59].

Material ρ[kg/m3]λ[W/mK] Cp [J/kgK]

MW 12–200 0.030–0.040 0.8–1.0

EPS 10–80 0.032–0.045 1.25

XPS 15–85 0.025–0.040 1.45–1.7

PUR 30–160 0.022–0.035 1.3–1.45

PIR 28–40 0.020–0.035 1.4–1.5

Cellulose ﬁbers 30–70 0.038–0.040 1.3–1.6

Vacuum-insulated panels 180–210 0.008 0.8

Gas-insulated panels N.A. 0.040–0.046 N.A.

Aerogels 100–150 0.013–0.021 1.0

MW is often used when higher ﬁre resistance of the entire building system is required

and is the most commonly used insulation material for LSF constructions. The market

share of MW in Europe is about 60% [

]. This proves the wide range of applications

of this material. Researchers have shown that the thermal insulation performance of

mineral wool for construction purposes is not affected by high temperatures [

], since it

is an inorganic material containing a small amount of organic components (binders). In

addition to the high ﬁre resistance, the sound insulation of the material is also a remarkable

advantage [

]. Wool acts as an absorber due to the ﬂexible, porous structure of the material.

The thermal conductivity, the most important factor for thermal insulation materials, is

= 0.035 W/mK [

]. This thermal conductivity can meet the requirements of energy

Buildings 2022,12, 929 11 of 19

efﬁcient construction. The biggest problem for this material arises when the wool is used in

places where there is a risk of condensation and greater moisture exposure. When water

enters the system, MW absorbs the moisture, which increases the thermal conductivity of

the material, reducing its insulating properties. This in turn requires additional membranes

and precautions, which are discussed later. The basic requirements for mineral wool are

deﬁned in the European standard EN 13,162 [61].

EPS and XPS for ETICS in LSF systems are polymeric materials similar in structure and

properties. They are both classiﬁed as highly combustible, closed-cell foams without signif-

icant acoustic properties [

]. The lower acoustic properties are due to the rigid structure

of the material compared to MW. EPS and other petrochemical-based foams account for

27% of the European market [

]. The thermal conductivity is slightly lower and therefore,

the thermal insulation is better than MW and ranges from 0.032 to 0.045 W/mK [

The notable difference between EPS and XPS materials is that EPS has higher moisture

absorption compared to XPS [

]. The basic terms and requirements for EPS and XPS are

deﬁned in the European standards EN 13,163 [62] and EN 13,164 [63], respectively.

When considering sandwich cladding panels used for LSF systems, PUR and PIR have

had a great inﬂuence on the use of the sandwich concept, as they are generally less costly,

solid, easy to bond at the macroscopic level, and provide high thermal insulation (thermal

conductivity around

= 0.025 W/mK [

]) [

]. For this reason, these two types of insula-

tion materials have seen a strong upsurge in the construction sector and in LSF building

systems. In general, PUR foams have higher thermal resistance (R-value) compared to

other commercially available insulation products, thus PUR results in thinner building

elements with lower height while increasing space utilization, maximizing efﬁciency, and

reducing operating costs [

]. In recent decades, PUR in particular has been used with other

materials to obtain composites with low density, high toughness and ductility, high impact

resistance, efﬁcient sound insulation, and excellent mechanical properties [

]. They are

produced in many variations and in densities ranging from 30–500 kg/m

[

], allowing

different mechanical, dynamic, thermal, etc. properties to be obtained [

]. On the other

hand, ﬁre behavior is one of the main problems of PUR. Although the overall ﬁre resistance

can be improved by adding additives, there are signiﬁcant concerns about the gases formed

during the combustion process of the material. PIR foam is formed by a similar chemical

reaction as PUR, and the two materials are similar but differ signiﬁcantly in ﬁre resistance.

Of all the foams, PIR has the best ﬁre resistance [

]. The thermal conductivity of the

material is also slightly lower, ranging from 0.020–0.035 W/mK, depending on the density

of the material [

]. The European standard for rigid foam boards PUR and PIR is EN

13,165 [66].

Cellulose ﬁbers for thermal and sound insulation purposes are produced in a mill from

recycled paper, wood ﬁbers, and some chemical composites to improve their vermin, ﬁre,

and rot resistance [

]. Although panels and mats can be produced, it is more commonly

marketed as a loose material that is blown into wall cavities [

]. Due to its elasticity and

porosity, the material is excellent for sound absorption in ﬂoating ﬂoors. The thermal

conductivity of the material ranges from 0.038–0.040 W/mK [

]. Water, i.e., moisture,

has a negative inﬂuence on the thermal properties of the material.

Several innovative materials have also been invented to reduce the thermal conductiv-

ity of insulating materials and increase the overall thermal resistance of building compo-

nents. These materials are vacuum-insulated panels, gas-ﬁlled panels, and aerogels. Their

properties, durability, and overall safety are still being researched and developed [

Materials such as cork could be used for LSF ﬂoor systems, while sheep wool could be

used as cavity insulation. Materials made from cork grains are also characterized by good

acoustic properties in terms of impact sound insulation, airborne sound insulation, and

sound absorption, are marketed in the form of boards, strips, loose material, or plaster

additives, and can be easily recycled [

]. The thermal conductivity of these materials

ranges from 0.037–0.050 W/mK [

]. Sheep wool is an organic material mixed with

polyester or ﬁxed on a polypropylene grid. Like cork, the material is usually marketed in

Buildings 2022,12, 929 12 of 19

rolls and its elasticity allows it to be used as a resilient material in ﬂoating ﬂoors [

]. Before

use in construction, sheep wool must be treated with ﬂame retardants, moth repellents, and

parasiticides, and the material is easily recycled and reused [

]. The thermal conductivity

of the material ranges from 0.035–0.04 W/mK [58,59].

As can be seen, insulation materials for LSF building systems have different properties,

each of which has its advantages and disadvantages. Among conventional insulation

materials, polymeric materials such as PUR and PIR provide the best insulation properties

at room temperature due to the signiﬁcantly lower thermal conductivity of the material.

Of the modern insulation materials, i.e., vacuum-insulated panels, gas-ﬁlled panels, and

aerogels, aerogels appear to have the best insulation properties at ambient temperatures,

but more research is certainly needed to properly evaluate these materials.

When new insulation materials are developed, their use and compatibility with LSF

structures should be further researched, as some issues such as high ﬂammability of poly-

meric materials and low permeability of MW limit their application in different building

environments, thus affecting the usability of such systems.

3.4. Wind and Air Tightness Membranes

The airtightness of buildings, i.e., the resistance of the building envelope to air leakage

to the inside or outside, is a crucial aspect of building energy efﬁciency [

]. In addition, air

leakage can allow pollutants and other particles to enter the building, affecting occupant

safety. No building is 100% airtight because inﬁltration is an uncontrollable process that

occurs through walls, windows, and all types of cracks and breaks in the building envelope.

Air leakage in LSF systems is a problem that must be properly addressed. The interior of an

LSF structure is often used as a pathway for utility installations, creating holes and various

pathways for contaminants and other harmful particles within the building.

The inﬁltration process is caused by the difference in air pressure between the inside

and outside of an envelope element. Wind further enhances inﬁltration, and tall buildings

have a chimney effect that draws air in at the bottom of the building and pushes it out at

the top [67].

To reduce air inﬁltration and interstitial condensation, two membrane layers should be

used along the LSF building exterior envelope [

]. The ﬁrst barrier should be placed on the

warm side of the insulation layer and is commonly known as a vapor barrier. It prevents the

moisture in the air from entering the interior of the LSF elements and prevents condensation

on the colder elements (usually steel). The other membrane, i.e., the wind barrier, should

be placed on the cold side and should be vapor-permeable to allow moisture to escape

from the LSF elements [

]. The most common air leaks found in ﬁeld measurements in

the literature are at the junctions between the exterior wall and ceiling or ﬂoor, exterior

wall and window or door, and exterior wall and penetrations in the barrier layers [68]. To

improve overall airtightness, sealing tapes should also be used in combination with the

above-mentioned membranes.

4. Discussion of General Requirements for Development of LSF Systems

Thermal performance of LSF structures is mostly inﬂuenced by the choice of conﬁgu-

ration, being cold-framed, warm-framed or hybrid-framed, the components that create the

whole assembly, and how well thermal bridges are reduced as they decrease the energy

performance. There is no clear and easy path to deciding what component should be used

because it ultimately inﬂuences other performance criteria of the assembly, for example,

the ﬁre resistance, acoustic performance, or mechanical resistance.

When considering the ﬁre resistance of the system, it is one of the key aspects and

requirements that must be met for any building element. CFS as main load-bearing system

requires additional protection against high temperatures because of instabilities that are

related to thickness. Additionally, the inﬂuence of other components on the structural

response is not negligible. As shown earlier, OSBs and plywood are a good alternative

when it comes to the load-bearing capability of the sheathing, but they have a reduced

Buildings 2022,12, 929 13 of 19

ﬁre protection which must be further protected by implementing a suitable ﬁre protective

board. Furthermore, some boards have shown great thermal performance on ambient

temperatures but signiﬁcantly reduced properties at elevated temperatures. This also goes

for polymer insulation materials. By choosing a polymer insulation, thermal properties

would theoretically be increased as they generally have lower thermal conductivity, but

at the same time, ﬁre performance of the assembly is decreased due to high ﬂammability

of polymers.

Further, the cost associated with the assembly process of the system is also connected

with the system design as some components cost more due to the cost of raw materials

and manufacturing. A potential solution to this is solving the thermal-structural response

of system introducing the interaction between components. This may lead to a potential

reduction in steel that has a signiﬁcant inﬂuence on the overall cost.

As stated earlier, by widening the distance between the steel studs, a 20% increase

in the thermal properties can be achieved in LSF structures. On the other hand, by in-

creasing the distance of mechanical properties, the structural stability of the assembly may

become questionable.

5. Proposal of New Light Steel Framed Construction Panel

Considering the advantages and disadvantages of the materials described in the

previous sections, the goal of modern LSF systems for NZEBs is to achieve better thermal

comfort while reducing thermal bridging within the system. Research has shown that

thermal bridging is mainly due to the proﬁle of CFS and its high thermal conductivity,

and that the best way to reduce the inﬂuence of steel on the thermal bridging effect is to

physically remove the steel from the outer sheathing [1]. At the same time, it is important

that the overall structural performance of the system is not compromised. Therefore, within

the scientiﬁc project “Composite lightweight panel with integrated load-bearing structure

(KLIK-PANEL)”, an LSF composite panel is proposed as it is conceptually shown in Figure 5.

The main objective of the project was to develop a walling panel which satisﬁes all the

important aspects required for NZEBs with adequate structural performance properties by

combining all the components into a cohesive and composite structural assembly.

Buildings 2022, 12, x FOR PEER REVIEW 14 of 20

Figure 5. Proposed concept of a LSF composite panel.

When considering thermal insulation materials, it was found that polymer foam pro-

vides significantly better insulation performance at ambient temperatures compared to

other materials. Consequently, the use of polymer foam in LSF structures would reduce

the U-value of the building element, which in turn would provide better thermal protec-

tion for the building. In addition, since polymer foam has better water permeability than

MW, airtight membranes would not be required. In addition, the adhesive property of the

foam inside the cavity provides bonding between components and support for the struc-

ture as a whole. By considering the positive effects of foam and sheathing components,

the optimal design of CFS structural components can be achieved.

A brief analysis of the thermal transmittance (U-value) of four different configura-

tions of LSF wall panels was performed as a possible technical solution for the construc-

tion of NZEBs. For analysis shown in Table 5, U-values of the thermal envelope were cal-

culated using the HRN EN ISO 6946 [69] standard for calculating thermal transmittance

for building components and building elements. The influence of thermal bridges by the

steel studs was not considered in the analysis.

Figure 5. Proposed concept of a LSF composite panel.

Buildings 2022,12, 929 14 of 19

When considering thermal insulation materials, it was found that polymer foam

provides signiﬁcantly better insulation performance at ambient temperatures compared to

other materials. Consequently, the use of polymer foam in LSF structures would reduce the

U-value of the building element, which in turn would provide better thermal protection

for the building. In addition, since polymer foam has better water permeability than MW,

airtight membranes would not be required. In addition, the adhesive property of the foam

inside the cavity provides bonding between components and support for the structure as a

whole. By considering the positive effects of foam and sheathing components, the optimal

design of CFS structural components can be achieved.

A brief analysis of the thermal transmittance (U-value) of four different conﬁgurations

of LSF wall panels was performed as a possible technical solution for the construction of

NZEBs. For analysis shown in Table 5, U-values of the thermal envelope were calculated

using the HRN EN ISO 6946 [

] standard for calculating thermal transmittance for building

components and building elements. The inﬂuence of thermal bridges by the steel studs

was not considered in the analysis.

Table 5. Comparison of different LSF wall panel conﬁgurations.

Conﬁguration A B C D

Section

Buildings 2022, 12, x FOR PEER REVIEW 15 of 20

Table 5. Comparison of different LSF wall panel configurations.

Configuration

Section

Wall composition

(from interior to exte-

rior side)

Gypsum fibreboard 25

Air layer 40 mm

MW 120 mm

Gypsum fibreboard 25

MW 60 mm

Gypsum fibreboard 25 mm

Air layer 100 mm

MW 60 mm

Gypsum fibreboard 25 mm

MW 160 mm

Gypsum fibreboard 25 mm

PUR 160 mm

Gypsum fibreboard 25 mm

U-value [W/(m2K)]

0.294

0.293

0.183

0.117

The same sheathing material was used for all configurations (gypsum fibreboard

with a thickness of 25 mm). The thermal conductivity of the air layer was calculated ac-

cording to HRN EN ISO 6946 [69], taking into account the effects of convection on heat

transfer.

Configurations A and B were taken as the most commonly used configurations in the

past, with a total of 12 cm MW, while configuration C is similar to configuration A with

16 cm MW. As expected, the proposed concept of the panel with 16 cm PUR (configuration

D) had the lowest U-value (0.117 W/(m2K)). This U-value would meet both the EU require-

ments for a decentralized NZEB and the requirements for a passive house, with a rela-

tively low wall thickness compared to possible other configurations with the same U-val-

ues. The achieved result is of course due to the PUR insulation and its lower thermal con-

ductivity (0.025 W/(mK)) compared to the thermal conductivity of MW (0.04 W/(mK)). In

the development of the panel system with an integrated structure, it is necessary that the

panel is thermally adjustable, that is, regardless of structural and other aspects, it is pos-

sible to adjust the thickness of the thermal insulation which directly affects the U-value of

the panel itself. It is also necessary to design the panel in such a way as to minimize the

thermal bridges created by the supporting steel structure of the panel and also by the

connections of the panel with other structural and non-structural elements of the building.

In addition, it is necessary to define the panel joints (extensions, corners, angles, collisions,

etc.) so that the thermal bridges can be minimized and sealed according to the require-

ments for maximum allowable air permeability and fire resistance. At the same time, fur-

ther development of the panel would allow the integration of technical systems such as

electricity, water supply, sewerage, mechanical ventilation, heating and cooling systems,

etc. Since the fire behavior of PUR foam is of great importance, a suitable non-combustible

sheathing material with sufficient thickness must be used to achieve the best possible fire

protection. As mentioned before, in the field of energy efficiency, technical requirements

regarding the rational use of energy and thermal protection of the building element must

be met during the design and construction of new NZEBs.