.......Žegarac Leskovar, Premrov, Kitek Kuzman: Energy-Effi cient Renovation Principles...
DRVNA INDUSTRIJA 63 (3) 159-168 (2012) 159
Vesna Žegarac Leskovar
1
,
Miroslav Premrov
1
, Manja Kitek Kuzman
2
Energy-Efficient Renovation
Principles for Prefabricated
Timber-Frame Residential
Buildings
Energetski učinkovita načela obnove
montažnih stambenih zgrada s drvenim
okvirom
Original scientifi c paper • Izvorni znanstveni rad
Received – prispjelo: 30. 8. 2011.
Accepted – prihvaćeno: 6. 9. 2012.
UDK: 630*833.21
doi:10.5552/drind.2012.1127
ABSTRACT • The timber construction along with the use of suitable and correctly oriented glazing surfaces,
whose thermal and strength properties have been considerably improved over the years, represents a great poten-
tial in residential and public building construction. However, necessary renovations of the older structures, which
present quite a large share of residential fund, should not be overlooked. Moreover, those structures should be
adequately energy renovated by the year 2020. Therefore, the key contribution of this paper is the presentation of
the available renovation principles, and namely a combination of the improvement of buildings envelope thermal
properties, usage of a proper type of installation and share of glazing surfaces in the south-oriented façade, ac-
cording to affordable investment input. In order to achieve minimal heating and cooling annual energy demand,
in the current parametric study, different options were carried out with double-layer and triple-pane glazing, in-
stalled in three different types of wall elements, demonstrating the value of optimal glazing surface.
Keywords: timber building, glazing, energy effi ciency, renovation
SAŽETAK • Drvena konstrukcija, uz uporabu odgovarajućih i pravilno orijentiranih staklenih površina čija su
toplinska svojstva i čvrstoća tijekom godina znatno poboljšani, velik su potencijal u gradnji stambenih i javnih
zgrada. Pritom ne smije biti zanemarena ni obnova starijih objekata, koji čine prilično velik udio u stambenom
fondu. Usto ti bi objekti do 2020. godine trebali biti odgovarajuće energetski obnovljeni. Dakle, važan doprinos
ovog članka jest predstavljanje raspoloživih načela obnove kao što su kombinacija poboljšanih toplinskih svojsta-
va fasade zgrade, primjena odgovarajućeg tipa instalacija i udjela staklenih površina na južnoj fasadi, ovisno o
mogućemu investicijskom ulaganju. Kako bi se postigla minimalna godišnju potreba za grijanjem i hlađenjem, u
parametarskoj studiji izvedene su različite mogućnosti s dvoslojnim i troslojnim zastakljenjem, ugrađenima u tri
različita tipa zidnih elemenata, čime se demonstrira vrijednost optimalnih staklenih površina.
Ključne riječi: drvena gradnja, zastakljivanje, energetska učinkovitost, obnova
1
The authors are assistant professor and full professor at the Faculty of Civil Engineering, University of Maribor, Slovenia.
2
The author is
assistant professor at the Department of Wood Science and Technology, Biotechnical Faculty, University of Ljubljana, Slovenia.
1
Autori su docentica i redoviti profesor Građevinskog fakulteta Sveučilišta u Mariboru, Slovenija.
2
Autorica je docentica Biotehničkog fakul-
teta Sveučilišta u Ljubljani, Slovenija.
Žegarac Leskovar, Premrov, Kitek Kuzman: Energy-Effi cient Renovation Principles... .......
160 DRVNA INDUSTRIJA 63 (3) 159-168 (2012)
1 INTRODUCTION
1. UVOD
Timber as a material for load bearing construction
represents a future challenge for residential and public
buildings. Being a natural raw material, timber repre-
sents one of the best choices for energy effi cient con-
struction since it is also a material with good thermal
properties, compared to other construction materials. In
addition, it plays an important role in the reduction of the
CO
2
emissions (Natterer, 2009), it has good mechanical
properties (Vratuša et al., 2011) and ensures a comforta-
ble indoor living climate. Timber construction has better
thermal properties than conventional brick or concrete
construction methods, even with smaller wall thickness.
Considering the growing importance of energy-effi cient
building methods, timber construction will play an in-
creasingly important role in the future.
Residential buildings represent the biggest share
(47 %) of the existing buildings is Slovenia. More than
half of them are made of brick (56 %), 16 % of concre-
te and mixed construction, and the rest made of mate-
rials including timber are represented to a smaller ex-
tent (Kitek Kuzman et al., 2010). Focusing to the
Slovenian timber construction, current rise has been
noticed, even though the percentage of new timber bu-
ildings in Slovenia is still small regarding the entire
new construction, especially in the public buildings
sector. In 2010 (SORS), the percentage of newly built
pre-fabricated houses, mostly one or two-family, exce-
eded 15 % and the percentage is expected to increase to
20-30 % over the next fi ve years.
The dominating methods of timber construction
in Slovenia include a timber-frame construction, ballo-
on and massive construction. Currently, most Slove-
nian companies offer houses with timber-frame con-
struction. Timber panel construction has had its own
production in Slovenia and Croatia for more than 35
years. The beginnings of pre-fabricated construction
started after the second world war, when the barracks
were put up for the people who had been left without
shelter and those who had migrated from the country-
side. Over the past thirty years, timber in Europe con-
struction has undergone major changes. The most im-
portant changes introduced are the following (Premrov,
2008): transition from on-site construction to factory
prefabrication, transition from elementary measures to
modular building and development from a single-panel
to a macro-panel wall prefabricated panel system. All
of these greatly improve the speed of building.
In timber-frame buildings, the basic vertical load
bearing elements are panel walls consisting of load bea-
ring timber frames and sheathing boards. Depending on
wall dimensions, one can distinguish between single-
panel and macro-panel wall systems. The single-panel
was based on the individual smaller elements in dimen-
sions of 1.30 m (1.25 m) x 2.5 m to 2.65 m (Figure 1a).
The height of the wall elements met the height of the
fl oor and the length of the ceiling elements the span of
the bridged fi eld. The macro-panel system has been de-
veloped from the single-panel system in the last two de-
cades and represents an important milestone in panel
timber frame building. The aim of the system is to pro-
vide whole wall assemblies, including windows and do-
ors, which are totally constructed in a horizontal plane in
a factory from where they are transported to the buil-
ding-site. Prefabricated timber-frame walls, as the main
vertical bearing capacity elements, of typical dimensions
with a width of 1250 mm and a height of 2500–2600
mm, are composed of a timber frame and sheets of bo-
ard-material fi xed by mechanical fasteners, usually sta-
ples, to one or both sides of the timber frame (Figure 1c).
Table 1 Composition of analysed macro-panel (TF 3) and single-panel (TFCL 2, 3) timber-frame wall elements
Tablica 1. Kompozicija analiziranih makropanelnih (TF 3) i jednopanelnih (TFCL 2, 3) zidnih elemenata s drvenim okvirom
TF 3 TFCL 2 TFCL 3 – renovation
material / materijal d, mm material / materijal d, mm material / materijal d, mm
rough coating
hrapava obloga
10 wooden planks
drvene oplate
22 rough coating
hrapava obloga
10
wood fi breboard
ploče vlaknatice
60 / / mineral wool
mineralna vuna
40
//TSS
***
/ open air gaps / bitumen 0.5 gypsum fi breboard
gipsane vlaknatice
15
cellulose fi bre / TF*
celulozna vlakna
360 TSS
***
/ open air gaps / TF
*
20 mineral wool / TF*
mineralna vuna
100
bitumen sheet cardboard / TF
*
TF
*
MW
0.5
mineral wool / TF*
mineralna vuna
80
OSB
**
15 aluminium foil
aluminijska folija
aluminium foil
aluminijska folija
gypsum plasterboard
gipsana fasadna ploča
12.5 particleboard
ploča iverica
13 particleboard
ploča iverica
13
gypsum plasterboard
gipsana fasadna ploča
10 gypsum plasterboard
gipsana fasadna ploča
10
total thickness, mm
ukupna debljina, mm
457.5 total thickness, mm
ukupna debljina, mm
146 total thickness, mm
ukupna debljina, mm
188
U
wall
–value, W/m
2
K
0.102
U
wall
–value, W/m
2
K
0.48
U
wall
–value, W/m
2
K
0.30
*timber frame / *drveni okvir, **oriented strand board / **ploča s orijentiranim iverjem, ***timber sub-structure / *** drvene podstrukture
.......Žegarac Leskovar, Premrov, Kitek Kuzman: Energy-Effi cient Renovation Principles...
DRVNA INDUSTRIJA 63 (3) 159-168 (2012) 161
Between the timber studs and girders, a thermal insula-
tion material is inserted whose thickness depends on the
type of external wall. Composition of all analysed wall
elements is presented in detail in Table 1.
The fi rst single-panel systems in Slovenia were
used by Marles and Jelovica. In Slovenia and Croatia
there are a few settlements built in the early 70s. For an
illustration, Table 2 gives fi gures of the houses produ-
ced by the Company Marles Houses in the period from
1964 to 1987.
Those fi rst pre-fabricated houses had very good
thermal properties of external envelope. Thermal tran-
smittance of the best panel types was always much lo-
wer than provided by regulations; for example thermal
insulation improved by nearly three times from 1963 to
1972, and after 1992 it was almost four times better
than specifi ed by the current national regulations (Figu-
re 2). Due to the reduction of energy losses in the newly
built residential structures, the fi rst measure introduced
by producers was a gradual reduction of thermal tran-
smittance of external wall elements, resulting in the in-
crease of thickness of the timber-frame wall elements,
thus enabling the installation of thicker thermal insula-
tion. Detailed composition of construction of the older
single-panel external wall elements, as well as the ne-
wer macro-panel system, are explicitly presented in Ta-
ble 1, with the additional graphic presentation in Figure
1. Figure 2 only shows data until the year 1992, when
a) b) c)
Figure 1 a) single-panel system (TFCL2); b) renovated single-panel system (TFCL 3), c) timber-frame wall element with
I-studs (TF 3)
Slika 1. a) jednopanelni sustav (TFCL2); b) obnovljeni jednopanelni sustav (TFCL 3), c) zidni element s drvenim okvirom i
I-stupovima (TF 3)
Table 2 Number of Marles’ pre-fabricated houses from 1964
to 1987 (archive company Marles hiše Maribor).
Tablica 2. Broj Marlesovih montažnih kuća od 1964. do
1987. (arhiva tvrtke Marles hiše, Maribor)
MARLES pre-fabricated
houses (purpose) / MARLES
montažne kuće (namjena)
Produced number
Broj proizvedenih kuća
residential settlements, terraced
houses / stambena naselja,
terasaste kuće
590
schools / škole
90
kindergartens / dječji vrtići
360
health centres / domovi zdravlja
40
individual structures
individualni objekti (1964-1999)
10 000
Figure 2 Thermal transmittance of external wall elements
- U-value comparison of the Marles’ wall with the Slovene
regulations in the period 1963 to 1992
Slika 2. Toplinski prijenos vanjskih zidnih elemenata –
usporedba U-vrijednosti Marlesova zida sa slovenskim
propisima u razdoblju od 1963. do 1992.
External wall slovenian guidelines
vanjski zid – slovenski propis
External wall MARLES
vanjski zid MARLES
Žegarac Leskovar, Premrov, Kitek Kuzman: Energy-Effi cient Renovation Principles... .......
162 DRVNA INDUSTRIJA 63 (3) 159-168 (2012)
the external wall elements met, for the fi rst time, the
requirements of the regulations currently applicable in
Slovenia regarding energy effi cient construction, so that
the thermal transmittance of exterior was, for the fi rst
time, lower than the prescribed limit value of 0.28 W/
m²K, i.e. it has nearly reached the value for light con-
structions, which is 0.20 W/m²K (PURES, 2010). The-
refore, all prefabricated timber framed structures set up
before the year 1992 are considered as a fund needing
energy effi cient renovation by the year 2020. The latter
refers to the wide-ranging package on climate change
adopted by the European Union, the overall 20-20-20
targets, which are binding for buildings, too. Therefore,
the energy performance of the existing buildings has to
be improved through a complex process of energy effi -
cient renovation, and likewise a sustainable new con-
struction of energy-effi cient buildings with the use of
renewables has to be performed.
2 ENERGY-EFFICIENT BUILDINGS
2. ENERGETSKI UČINKOVITE ZGRADE
Researching energy effi ciency of buildings is not
a matter of the last decade only, since the fi rst intensive
studies related to energy and buildings were already
carried out in the seventies and eighties of the last cen-
tury. Many studies focusing on the research of specifi c
parameters infl uencing energy performance of buil-
dings, such as Johnson et al. (1984) and Steadman et
al. (1987) have been performed since then. Previous
research fi ndings indicate that the process of defi ning
the optimal model of a building is very complex. The
most important parameters infl uencing energy-perfor-
mance of buildings are listed below:
location of the building and climate data for the -
specifi c location,
orientation of the building, -
properties of installed materials, such as timber, -
glass, insulation, boards, etc.,
building design (shape factor, length-to-width ratio, -
window-to-wall area ratio, building envelope pro-
perties, windows properties),
selection of active technical systems. -
According to the Slovene legislative framework,
particularly the Energy Act, the system of energy per-
formance certifi cation is defi ned in Rules on the metho-
dology of construction and issuance of building energy
certifi cates (2009). On the basis of these rules, the clas-
sifi cation of energy-effi cient houses was carried out, as
listed in Table 3.
Table 3 clearly shows that energy effi cient struc-
tures can be constructed only by an adequate combina-
tion of external envelope effi cient insulation and high
quality glazing installation. Respecting climate change
conditions and the subsequent European directions rela-
ted to energy performance of buildings, the building
industry must construct a nearly zero energy house by
2020. Searching for the optimal model of an energy-
effi cient house has, therefore, become an issue of major
importance. Similar concept of optimal solution will
consequently have to be introduced into the fi eld of re-
novation of numerous older buildings, which are far
from achieving standards of energy effi cient buildings.
Therefore, our analysis is directed into the fi eld of pre-
fabricated timber-frame construction, which will try to
fi nd an optimal renovation solution as combination of
additional layers of insulation on the external wall ele-
ments and double-layer or triple-pane quality glazing.
Table 3 Classifi cation of energy-effi cient houses on the basis of “Rules on the methodology of construction and issuance of
building energy certifi cates”
Tablica 3. Klasifi kacija energetski učinkovitih kuća na temelju Pravilnika o metodologiji gradnje i izdavanja energetskih cer-
tifi kata za zgrade.
Degree / Classifi cation in
accordance with the rules
Klasifi kacija u skladu s
pravilima
Generally used classifi ca-
tion in practice
Općenito primjenjivana
klasifi kacija u praksi
Q
h
* (kWh/m
2
a)
Variation of execution / Varijanta izvedbe
(according to Praznik and Kovič, 2010)
Class C / klasa C
minimal requirements for
low-energy house
minimalni zahtjevi za
nisko-energetsku kuću
35 – 50 (60) classical prefabricated construction, conventional
heating system, contemporary windows (doors),
no central ventilation system / klasična konstru-
kcija, konvencionalni sustav grijanja, suvremeni
prozori, bez središnjega ventilacijskog sustava
Class B2 / klasa B2
low-energy house
niskoenergetska kuća
25 – 35 thermally improved building envelope
toplinski poboljšana fasada zgrade
Class B1 / klasa B1
better low-energy house
bolja niskoenergetska kuća
15 – 25 thermally improved building envelope + HRV**
+ HP*** / toplinski poboljšana fasada zgrade +
HRV** + HP***
Class A2 / klasa A2
passive house
pasivna kuća
10 – 15 additionally thermally improved building
envelope + HRV + HP / dodatno toplinski
poboljšana fasada zgrade + HRV + HP
Class A1 / klasa A1
1-litre house ≤ 10 additionally thermally improved building
envelope + HRV +HP + improved U-value of
windows (doors) / dodatno toplinski poboljšana
fasada zgrade + HRV + HP +poboljšana
U-vrijednost prozora (vrata)
* specifi c annual heating demand / specifi čna godišnja potreba, **heat recovery ventilation / povrat energije, ***heat pump / toplinska pumpa
.......Žegarac Leskovar, Premrov, Kitek Kuzman: Energy-Effi cient Renovation Principles...
DRVNA INDUSTRIJA 63 (3) 159-168 (2012) 163
3 NUMERICAL STUDY
3. NUMERIČKA STUDIJA
This chapter presents a numerical case study of a
two-storey house and its parametric analysis of the im-
pact of the glazing-to-wall area ratio on energy de-
mand. The infl uence of south oriented glazing area size
on heating and cooling energy demand is analysed in
the case-study of a single-family house, carried out
with three different types of external wall elements:
a) a new macro-panel timber-frame wall element (TF
3), which satisfi es the requirements of a passive hou-
se design, of a total thickness of 456.5 mm and U
wall
value of U = 0.102 W/m
2
K (Table 1, Figure 1c),
b) an old classical single-panel timber-frame wall ele-
ment (TFCL 2) of a total thickness of 146 mm and
U
wall
value of U = 0.480 W/m
2
K (Table 1, Figure
1a).
c) a renewed timber-frame single-panel wall element
(TFCL 3) of a total thickness of 195 mm and a U
wall
value of 0.30 W/m
2
K, which is developed from the
TFCL 2 old system by inserting an additional insu-
lation in the external side of timber frame (Table 1,
Figure 1b) – case of renovation.
3.1 Simulation model
3.1. Simulacijski model
Description of the base case study model
The external horizontal dimensions are 11.66 m x
8.54 m for the ground fl oor and 11.66 m x 9.79 m for
the upper fl oor (Figure 3). The total heated fl oor area is
168.40 m
2
and the total heated volume is 437.80 m
3
.
Climate and orientation
The house is located in Ljubljana with its longer
side, the large glazed area, facing south. The city of
Ljubljana is located at an altitude of 298 metres, latitu-
de of 46°03’ and longitude of 14°31’ east. According to
data from http://www.geodetska-uprava.si/DHTML_
HMZ/wm_ppp.htm the considered average annual ex-
ternal temperature is 9.8 °C. The average duration of
solar radiation is 1712 hours annually.
Construction
Exterior walls are constructed in three different
variations, as presented in Table 1. For all analysed
wall elements, the timber characteristics are of the
same class - C22 according to EN 338. The U-values of
other exterior construction elements are in all cases
0.135 W/m
2
K for the fl oor slab, 0.135 W/m
2
K for the
fl at roof and 0.130 W/m
2
K for the south-oriented
overhang construction above the ground fl oor area.
Glazing
Two types of glazing were separately considered
in the analysis:
a) a window glazing (Unitop 0.51 – 52 UNIGLAS)
with three layers of glass, two low-emissive coa-
tings and Krypton in the cavities for a normal con-
fi guration of 4E-12-4-12-E4. The glazing confi gu-
ration with a g-value of 52 % and U
g
= 0.51 W/m
2
K
assures a high level of heat insulation and light
transmission, Gustavsen et al. (2007). The window
frame U-value is U
f
= 0.73 W/m
2
K, while the fra-
me width is 0.114 m.
b) a window glazing with two layers of glass, one low-
emissive coating and Argon in the cavity for a nor-
mal confi guration of 4-16-E4, with a g-value of
60 % and U
g
= 1.2 W/m
2
K. The window frame
U-value is U
f
= 1.11 W/m
2
K, while the frame width
is 0.116 m.
The glazing-to-wall area ratio (AGAW) of the
base case in the south-oriented façade is 27.6 %, while
the AGAW values of the rest of the cardinal directions
are 8.9 % in the north-oriented, 10.5 % in the east-
oriented and 8.5 % in the west-oriented façades.
Figure 3 Floor plans of the base-case study model
Slika 3. Tlocrt osnovnog modela proučavanja