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IJSRCE182517 | Received : 21 Nov 2018 | Accepted : 30 Nov 2018 | November-December-2018 [ 2 (5) : 41-49]
International Journal of Scientific Research in Civil Engineering
© 2018 IJSRCE | Volume 2 | Issue 5 | ISSN : 2456-6667
41
Effect of Thermal Bridges on the Heat Balance of Buildings
Malek Jedidi2* , Omranne Benjeddou2
1Higher Institute of Technological Studies of Sfax, Department of Civil Engineering, Sfax, Tunisia
2University of Tunis El Manar, National Engineering School of Tunis, Civil Engineering Laboratory, Tunis,
Tunisia
ABSTRACT
Thermal bridges in the thermal insulation of envelopes occur in all forms of building construction and should
be minimised to reduce local heat losses. This paper presents the effect of thermal bridges on the comfort of the
habitat within a building. Indeed, the thermal bridges increase the heat losses of a building and thus the
consumption of heating. Thermal bridges cause additional heat losses compared to losses through the walls of
the building. These losses can exceed for some 40% of the total heat losses through the envelope. An example of
an energetic effect of a thermal bridge has been presented. For this example, the heat losses on a floor height
and per meter of façade were calculated. The results showed that from 5 cm thickness of insulation, the
difference between the loss curve in the absence of thermal bridge, and the loss curve taking into account the
thermal bridge is almost independent of the thickness of the insulating. Another example was also presented to
demonstrate the effect on thermal bridging when increasing the levels of insulation to a normal corner. The
results showed that for a 50% and 100% increase in the thickness of the insulation, the U-value, linear heat loss
coefficient ψ and surface temperature factor fRsi were considerably decreased.
Keywords : Thermal Bridges, Linear Heat Loss Coefficient , Heat Losses, Insulation, Surface Temperature Factor,
Buildings.
I. INTRODUCTION
Thermal bridges are parts of a building where the
insulation barrier is broken. Ideally, the insulating
complex should be continuous around the heated
space. These weaknesses of the insulation can cause
the condensation of the water vapor and thus the
possible formation of black marks and molds [1-3].
The most common are linear thermal bridges, which
correspond to a junction between two walls (low-wall
exterior floor, intermediate floor-exterior wall, high-
wall exterior floor, balcony-tile, exterior wall-wall,
etc.).. There are also thermal bridges on the outline of
joinery, thresholds of doors and windows, ducts, etc.
The French energy performance of new building
regulation RT 2012 imposes a loss value not to be
exceeded for the thermal bridges between floors and
external walls, but also a limit to the sum of all the
thermal bridges of a building. This measure should
lead to a generalization of insulation from the outside,
which remains the best way to eliminate the majority
of thermal bridges. In cases where only the thermal
insulation from the inside is suitable, it is strongly
recommended to set up thermal bridge breakers and
to provide insulation on floating screed.
Architects and builders have to be clever to avoid
thermal bridging and need to carefully design each
detail using a combination of special products and
ingenuity. They should be able to show you a blown-
up detail drawing showing exactly how they propose
to avoid a thermal bridge for each building junction.
They can also use the Acceptable Details published by
the Department of Environment but you need to be
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Malek Jedidi et al. Int J S Res Civil Engg.
November-December 2018
; 2 (5) : 41-49
42
certain that these are followed exactly on site with no
shortcuts.
Thermal bridges account for 10- 40% of losses. They
drag on the inner surface of the wall a local
temperature drop and create cold areas located in the
house. These areas also cause discomfort for the
occupants because the human body will feel cold if
the walls are cold, even if the air in the room is hot.
Several studies have been conducted to analyze
thermal bridging effects and evaluate energy losses
through the envelope of a test room [4-8]. It was
concluded that the mere internal retrofit was not a
decisive solution to reduce the heat loss from
residential buildings if additional proper attention
was not paid to non-insulated building elements.
II. STUDY OF THERMAL BRIDGES
2.1 Effects of thermal bridges
Thermal bridges have the disadvantage of cooling the
inner surface. This lowering of the inner surface
temperature can cause condensation and mold
problems, causing stains, drips or efflorescence.
A set of conditions must come together for the molds
to grow: Spores and food are needed, which is not a
problem because spores are ubiquitous and mold feeds
on anything. On the other hand, the local relative
humidity must exceed 80% for a long time. This
superficial humidity depends on the humidity of the
air and the temperature of the surface.
The humidity of the air is controlled by reducing the
sources of humidity and ventilating sufficiently. The
surface temperature of the outer walls is controlled,
for a given climate, by the level of thermal insulation.
In winter, the walls facing the outside have a lower
surface temperature as the insulation is less strong. If
the insulation is weak and the humidity of the indoor
air is relatively high, two types of damage may occur:
- As soon as the internal surface temperature is
equal to or lower than the dew point of the
indoor air, the humidity of the air condenses on
the surface, making it humid. At the extreme,
drips and stains occur.
- If the relative humidity of the air exceeds
approximately 80% near the surface for a long
time, then mold can grow on this surface without
condensation.
This damage occurs when the insulation is too weak
for a given ventilation, or when the ventilation is too
weak for a given insulation.
To estimate the risks associated with condensation
and mold, SIA 180 [9] uses the surface temperature
factor fRsi given by the following equation:
Si e
Rsi ie
f
(1)
Where, (θsi - θe) is the temperature difference
between the inner surface of an envelope element and
the outside temperature (°C), (θi - θe) is the
temperature difference between inside and outside
(°C).
This factor quantifies the level of thermal insulation
at any point of a thermal bridge. If it is equal to 1, the
insulation is perfect, if it is equal to zero, the
insulation is null. For flat and homogeneous surfaces,
the surface temperature factor fRsi is given by the
following equation:
Rsi si
f 1 U.R
(2)
Where Rsi is the surface thermal resistance which
varies from 0,1 to 0,3 m²°C / W according to the
places, U is the surface heat loss coefficient (W/m²°C).
Fig. 1(a) shows a material thermal bridge consisting of
a slab resting on a wall with internal insulation. The
variation of the surface temperature near a thermal
bridge is given in Fig. 1(b). According to Fig. 1(c), we
note that the degree of insulation is excellent in full
wall (fRsi = 0.93), but it is far from sufficient near the
thermal bridge (fRsi = 0.55).
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Malek Jedidi et al. Int J S Res Civil Engg.
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(a)
(b)
(c)
Figure 1: (a) Material thermal bridge consisting of a
slab resting on a wall with internal insulation. (b) The
variation of the surface temperature near a thermal
bridge. (c) Surface temperature factors in the case of
the thermal bridge
To avoid the risk of mold, SIA 180 requires that the
surface moisture (relative humidity of the air layer
near the surface) does not exceed 80% for a prolonged
period. If ventilation is sufficient, this requirement is
fulfilled when:
- the maximum heat transfer coefficients U are
respected for partially solid components and
geometric thermal bridges;
- the surface temperature factor fRsi is greater than
or equal to 0.75 at any location in the building
envelope, particularly at the thermal bridges, with
the exception of windows.
2.2 Types of thermal bridges
There are geometric thermal bridges such as angles
and corners, and thermal material bridges, in which a
heat conductive material passes through the
insulating layer. Thermal bridges are also classified as
linear bridges, which have a certain length, and point
bridges, in which the interruption of the insulating
layer remains local.
Any curvature in the insulating layer or in the wall
constitutes a thermal bridge geometric. The isotherms
must follow the curvature of the wall and the flux
lines, which are perpendicular to them, become
narrower towards the inside of the curvature.
Fig. 2 shows a typical geometric thermal bridge,
consisting of an angle between two walls, the wall
consisting of bricks with mineral wool and an outer
lining of cement blocks. Red corresponds to 20 ° C
and blue to 0 ° C. The hue changes at each degree.
Thin lines are flow lines, plotted every W/m. It can
be seen that the inside and outside temperatures of
the corner are slightly lower than those in the wall. It
is also noted that the flow lines are a little tighter
towards the inside of the corner than in the wall.
Geometric thermal bridges do not generally have
significant effects, especially on heat losses, because
the insulating layer is not interrupted, it is only
deformed. However, when conditions are critical,
lowering the temperature to the inner surface may be
sufficient to promote mold growth. Material thermal
bridges can be found anywhere where the insulating
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Malek Jedidi et al. Int J S Res Civil Engg.
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; 2 (5) : 41-49
44
layer is interrupted or traversed by a more conductive
material.
Figure 2: Geometric thermal bridge: angle of a
building. On the left, in plan, on the right, isotherms
(colored zones) and flux lines.
In the example of the material thermal bridge
consisting of a slab resting on a wall with internal
insulation (Fig. 1), red corresponds to 20 ° C and blue
to 0 ° C. The hue changes at each degree. Thin lines
are flow lines, plotted every W / m. It is very clear
that the flux lines are strongly concentrated across the
bridge, like a river in a gorge, and that the isotherms
deviate, as the water level drops near a dike break.
There is a clear cooling and concentration of the heat
flux lines near the thermal bridge. Material thermal
bridges often have more serious consequences than
geometric bridges.
Fig. 3 (a) shows an example of heat losses which can
be modeled by an additional heat leak located along a
horizontal line inserted into a wall. It is a linear
thermal bridge, which can be assigned a coefficient of
linear loss (in W/m°C) and a length.
Fig. 3 (b) shows an example of a metal fixing bar
crossing a wall can be modeled by localized additional
point loss; it is a point thermal bridge to which a
coefficient of loss (in W /°C) is attributed.
(a)
(b)
Figure 3: Examples of thermal bridges. (a): Linear
thermal bridge; (b): Point thermal bridge
2.3 How to avoid thermal bridges?
Some thermal bridges, such as door and window
frames, balcony brackets and fittings between shell
elements are unavoidable. They should therefore be
designed to reduce their effects. Here are some
general principles that can be applied together or
separately.
A design that places the insulation on the outside of
the load-bearing structure often makes it possible to
avoid most thermal bridges. It is as well the double
wall (the interior wall being carrier) as the outer
insulation plastered or barded. Buildings with
homogeneous walls made of light materials (solid
wood, autoclaved aerated concrete or porous bricks)
can also be considered as such if the slabs are made of
similar materials or, if they are more conductive
(concrete) they do not completely cross the walls, but
stop in the middle.
External insulation has many other advantages:
- Increased inner thermal inertia, thus improving
summer comfort and better use of passive solar
gains in winter.
- Stabilization of the temperature of the structure,
thus slower aging of this one.
- Decrease, and in most cases total elimination of
the risks of condensation in the building
elements.
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Malek Jedidi et al. Int J S Res Civil Engg.
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; 2 (5) : 41-49
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To prevent a thermal bridge from being inevitable
causes damage, it is advisable to take measures that
will increase its internal surface temperature. This
amounts to dividing it, heating it, or lengthening it.
These operations will often increase energy
consumption but reduce the risk of condensation or
mold
2.4 Detection of thermal bridges
In the plane and section of construction details, a
material thermal bridge appears as an interruption of
the insulation layer. It is therefore easy to detect, and
should be corrected or treated appropriately before
building. plans and detail cuts represent only a section
of the envelope element, and it is possible that a
thermal bridge, including a point bridge, exists
outside this section. The more complicated the
construction, the higher the probability of finding
thermal bridges.
On an existing building, the thermal bridge is
detected primarily by its effects: appearance of mold,
condensation, cold or hot areas. It can also be
detected using thermography [10-12]: it is an image of
the external surface temperature. As this temperature
is even higher than the external surface is better
heated, especially by thermal bridges, it is also, to a
certain extent, an image of thermal bridges.
Fig. 4 shows the result of the diagnosis of the face of a
building using an infrared thermal camera. We notice
the presence of thermal bridges. They are located at
the junction of partitions. Most likely, the reason is
the lack of insulation of the thermal connection
panels on the building.
Thermal bridges should therefore be avoided, but this
is not always possible and in this case, it must be
taken into account in the thermal balance of the
building.
Figure 4: Thermography of the face of a construction
using an infrared thermal camera.
III. THEORETICAL AND EXPERIMENTAL
STUDY OF THERMAL BRIDGES
3.1 Calculation of heat losses due to thermal bridges
The thermal losses (in Watt) of a flat wall without
thermal bridge are calculated by multiplying the area
of this wall by its coefficient U and by the
temperature difference between inside and outside.
The thermal losses is given by:
ie
A.U
(3)
Where, A is the surface of the wall (m²), U is the
surface heat loss coefficient (W/m²°C), θi -θe is the
temperature difference between inside and outside
(°C).
In order to take into account thermal bridges in the
calculation of heat losses, a linear heat loss coefficient
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Malek Jedidi et al. Int J S Res Civil Engg.
November-December 2018
; 2 (5) : 41-49
46
ψ (W/m°C) has been attributed to thermal bridges of
this type which, multiplied by the length of the
thermal bridge ( for example the perimeter of the
slab), is added to the losses of the walls as indicated by
the following equation:
ie
A.U l.
(4)
Table 1 gives the maximum value for the linear
coefficient of heat transfer ψ of thermal bridges
according to the SIA 380/1 standard [13].
Table 1 Limit values for thermal bridges
Linear heat loss coefficient ψ
Limit values
(W/m°C)
Type 1: projecting part such as
balconies, eaves
0.30
Type 2: interruption of thermal
insulation by walls, slabs or ceilings
0.20
Type 3: interruption of the insulating
envelope towards the horizontal or
vertical edges
0.20
Type 5: window sill against wall
0.10
Thermal transmittance coefficient χ
Limit
values
(W/°C)
Point elements crossing the thermal
insulation
0.30
Similarly, a thermal transmittance coefficient χ is
attributed to the local thermal bridges constituted by
fasteners or bar-shaped elements passing through the
insulating layer (Table 1). If all the elements of the
envelope are taken into account, the coefficient of
transmission losses of this envelope is calculated by
the following equation:
n n n
T i i k k j
i 1 k 1 j 1
H A U l
(5)
This HT coefficient is the power required to
compensate for transmission losses through the
enclosure for a difference of 1 degree between the
inside and the outside.
3.2 Example of energetic effect of a thermal bridge
We are interested in the example shown in Fig. 1. A
classic case of thermal bridge results from the interior
insulation technique. The slabs, or even the walls of
the slit, pass through the insulating layer to cling to
the outer bearing wall.
The heat losses on a floor height and per meter of
façade were calculated. The results of these
calculations are illustrated in Fig. 5. The dotted line
indicates these losses in the absence of thermal bridge,
and the continuous line gives the losses taking into
account the thermal bridge. It is noted that, from a
thickness of 5 cm of insulation, the difference
between these two curves is almost independent of
the insulation thickness, namely about 10 W/m.
Figure 5: Losses for a floor height, with or without
the thermal bridge shown in Fig. 1.
0
20
40
60
80
100
120
140
0 5 10 15 20
Thermal losses [W/m]
Thickness of insulation [cm]
With thermal bridge
Without thermal bridge
0
20
40
60
80
100
120
0 5 10 15 20
Additional losses [%]
Thickness of insulation [cm]
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The relative importance of additional heat losses
resulting from thermal bridging increases
significantly with insulation thickness, as shown in
Fig. 5 on the right. They go from a few percent if
there is no insulation to 60% for 10 cm and almost
100% with 20 cm of insulation. Thus, at the
thicknesses of insulation currently installed, losses
through the thermal bridge consisting of a slab
passing through the interior insulation are
comparable to those of the solid wall or, in other
words, and taking into account thermal bridges, 20
cm of interior insulation is no more effective than 10
cm of external insulation, which does not have a
thermal bridge of this type.
3.3 Example of Calculation of equivalent U-value
Fig. 6 presents a wall at one of the intermediate floors
. It has a wall corner, partition wall, and balcony. The
wall is insulated and its U-value without thermal
bridges is 0,65W/m²°C. The U-value of the window is
3.00W/m²°C. Linear heat loss coefficient are given in
Table2.
Table 2 Linear heat loss coefficient [14].
Description
ψ
(W/m°C)
Window perimeter
0.15
Window perimeter if the frame is in the
plane of the thermal insulation
0.00
Outer corner of homogeneous wall
0.10
Outer corner of wall with external
insulation
0.15
External wall with internal insulation
0.00
Joint of homogeneous external wall and
internal wall
0.06
Joint of external wall with external
insulation and internal wall
0.03
Joint of homogeneous external wall and
floor slab with insulated strip
0.15
Joint of external wall with external
insulation and floor slab
0.03
Parapet wall, cornice
0.20
Balconies
0.25
Figure 6: Description and dimensions of the insulated
wall
The total surface of the wall
A 5.00x2.65 13.25m²
The total surface of the window:
win
A 2x 0.75x1.50 1.50x2.40 5.85m²
The surface of the brick wall:
wall win
A A A 13.25 5.85 7.40m²
Table 3 gives the linear heat loss coefficient of the
different elements multiplied by the length of the
thermal bridge.
Table 3 Calculation of the linear heat losses
Element
linear heat loss
coefficient
ψ (W/m°C)
length of
the
thermal
bridge l(m)
∑Ψ. l
(W/°C)
Window
0.15
9.30
1.395
Corner
0.15
2.65
0.398
Balcony
0.25
6.00
1.500
Ring
beam
0.03
4.00
0.120
Partition
0.03
2.65
0.080
Total
3.493
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The coefficient of transmission losses is calculated by
equation 5 :
n n n
T i i k k j
i 1 k 1 j 1
H A U l
n
T wall wall win win j
j1
H U x A U x A .l 0
T
H 0.65x7.40 3.00x5.85 3.493
T
H 25.850W / C
3.4 Effect on thermal bridging when increasing the
levels of insulation
In order to demonstrate the effect on thermal
bridging when increasing the levels of insulation to a
normal corner, we chose a wall based on blocks
(thermal conductivity equal to 0.11W/m°C) and an
insulation (thermal conductivity equal to
0.020W/m°C) with variable thickness (50 mm, 75mm
and 100 mm) [15].
Table 4 Calculation of U-value and ψ value for
different insulation thickness
Insulation
thickness
(mm)
Increase
U-value
(W/m²°C)
ψ value
(W/m°C)
fRsi
50
-
0.27
0.052
0.957
75
50%
0.21
0.043
0.967
100
100%
0.17
0.038
0.973
According to the results presented in Table 4, it is
noted that for a 50% and 100% increase in the
thickness of the insulation, the value of U decreases
by 22% and 37% respectively. Regarding the values of
linear heat loss coefficient, there is also a decrease of
16% for an insulation thickness of 75mm, and a
decrease of 27% for an insulation thickness of
100mm.
we also note a condensation risk reduction since the
values of the surface temperature factor fRsi have been
decreased by 1% and 2% respectively.
IV. CONCLUSION
Through this work, it has been shown that the
method of construction chosen should make it
possible to avoid as much as possible the thermal
bridges which must always be taken into account in
the calculation of the thermal transmittance.
Under normal conditions, the maximum values of the
thermal transmittance of the building elements of the
heated rooms make it possible to satisfy the
requirements of thermal comfort and absence of
superficial condensation. In addition, the absence of
condensation on the thermal bridges must be ensured.
The following conclusions can be drawn:
- The presence of thermal bridges allows heat loss
and cooling of interior surfaces.
- Thermal bridges increase the risk of mold
especially when the relative humidity of the air
layer near the surface exceed 80% for a prolonged
period.
- To avoid thermal bridging, place the insulation on
the outside, the supporting structure being inside
the insulating layer.
If necessary, the thermal bridges must be treated so as
to increase their inner surface temperature, even if
they lose energy.
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