Civil Engineering Reference
In-Depth Information
Table 3
Opaque element insulation for case A—thermal bridge influence calculation
Case
Study
Refurbishment action
Calculation
method
EP
H, env
EP
H
EP
gl
Energy class
A1.a
External insulation of the walls
and the floors above the
heated zone
Simplified
74.83
139.76
163.83
E
A1.b
Analytical
95.33
172.40
196.44
E
Percentage gap (%)
21.5
18.9
16.6
B1. a
B0.b ? air-gap insulation
Simplified
75.61
147.04
173.91
G
B1.b
Analytical
97.29
185.62
212.48
G
Percentage gap (%)
22.2
20.8
18.1
B1.c
B0.b ? external insulation
Simplified
70.18
137.59
164.46
F
B1.d
Analytical
79.70
153.81
180.67
G
Percentage gap (%)
11.9
10.5
8.9
In this example, the use of one method or the other does not bring to different
energy classes, but the percentage difference is so wide that in some other cases
could imply different energy label, depending on the considered procedure.
In conclusion, using the simplified method for accounting thermal bridges, it is
possible to obtain an underestimation of their influence, which can be more sig-
nificant as much as the building is insulated, and it could bring to obtain a more
efficient energy class than the real one. Moreover, the analytical calculations allow
both for the design and the realization process, to verify all the possible solutions
for structural linkages and to provide a more accurate energy model of buildings.
Regarding case study B, two different kinds of insulation were tested:
• Gap insulation, filling the cavity wall (8 cm thick) to obtain a thermal trans-
mittance U
wall
= 0.356 W/(m
2
K), whose analyses are case B1.a and B1.b,
(U
wall
= 0.279 W/(m
2
• External
insulation
with
10 cm
of
EPS
K)),
whose
analyses are case B1.c and B1.d.
Energy performance for cases B1.a and B1.c have been assessed using sim-
plified method for thermal bridges, choosing the correction coefficient, corre-
sponding, respectively, to hollow brick wall with gap insulation without reduction
of thermal bridges (b
tr,x
= 10 %) and to external insulation with overhangs and
balconies or not reduced thermal bridges (b
tr,x
= 20 %). In case B1.b and B1.d,
the analytical method has been used in determining the linear thermal transmit-
tance from ISO 14683 and considering the structural linkages shown in Fig.
7
a-f.
Table
3
shows the influence of thermal bridges, calculated with simplified and
analytical methods in both cases: it is apparent that the external insulation guar-
antees a higher energy saving, because the air gap is only 8 cm thick and does not
allow to reach the thermal transmittance U
wall
= 0.279 W/(m
2
K), which is only
possible by applying a 10-cm EPS external layer. In fact, considering performance
indices for B1.a and B1.b, the air-gap insulation does not allow to improve the
energy class of the existing building, even if the value of EP
H, env
is reduced,
respectively, to 19.8 and 34.31 % in comparison with case B0.a. Moreover, the
