In situ U g -value measurement on three different glazing types

Abstract

This study presents in situ monitoring data of three different glazing systems over a period of one year. An insulated glass unit (IGU), a Vacuum Insulated Glass hybrid unit (VIG-hybrid) and an opaque architectural insulation module (AIM) were monitored under the equivalent environmental condition in this study. Different issues were observed and analyzed. It was found that the U g -value cited by the manufacturers agrees with the U g -values derived from the measured data, to within less than 5 % for the IGU and the VIG-hybrid. The consistency of the U g -value of each glazing types one year after the start of monitoring was validated for similar environmental conditions. Depending on the magnitude of the resistance to heat flow, an increasing U g -value was observed for a higher temperature difference between the inside and outside environments. The effect is much more significant for the glazing type with the largest U g -value (IGU) and less significant for the glazing types with a high thermal resistance (VIG-hybrid, AIM).

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Journal of Physics: Conference Series
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In situ U
g-value measurement on three different
glazing types
To cite this article: F Paschke et al 2021 J. Phys.: Conf. Ser. 2069 012134
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8th International Building Physics Conference (IBPC 2021)
Journal of Physics: Conference Series 2069 (2021) 012134
IOP Publishing
doi:10.1088/1742-6596/2069/1/012134
1
In situ Ug-value measurement on three different
glazing types
F Paschke1, N Bishara1, I Schulz1, C Kocer2, J Schneider1and A
Maier1
1Institute of Structural Mechanics and Design, Franziska-Braun-Straße 3, 64285 Darmstadt
Germany
2School of Physics, The University of Sydney, N.S.W., 2006, Australia
E-mail: paschke@ismd.tu-darmstadt.de
Abstract. This study presents in situ monitoring data of three different glazing systems over
a period of one year. An insulated glass unit (IGU), a Vacuum Insulated Glass hybrid unit
(VIG-hybrid) and an opaque architectural insulation module (AIM) were monitored under the
equivalent environmental condition in this study. Different issues were observed and analyzed. It
was found that the Ug-value cited by the manufacturers agrees with the Ug-values derived from
the measured data, to within less than 5 % for the IGU and the VIG-hybrid. The consistency
of the Ug-value of each glazing types one year after the start of monitoring was validated for
similar environmental conditions. Depending on the magnitude of the resistance to heat flow,
an increasing Ug-value was observed for a higher temperature difference between the inside
and outside environments. The effect is much more significant for the glazing type with the
largest Ug-value (IGU) and less significant for the glazing types with a high thermal resistance
(VIG-hybrid, AIM).
1. Introduction
New regulations introduced by the European governments [1] force the building sector to
minimize energy consumption, e.g. by increasing the resistance to heat flow through the
building facade. A modern facade consists often of glass elements, the glazing systems which
are integrated into the frame elements. The main part of a modern facade can therefore be
represented by the glazing system and may play the most important role in the reduction
of energy loss of a building facade. In this research, three different types of glazing were
monitored under the same measuring conditions over a period of one year to evaluate their
thermal performance and compare the results to the manufacturers’ claims.
In-situ measurements of facade elements have been carried out among others by Ficco et al
[2], Cesaratto et al [3] and Feng et al [4]. They investigated influences on the in situ U-value
measurement for different facade elements under various measuring conditions with diverse heat
flux meters. However, a comparative analysis of different glazing types under the same boundary
conditions have not been performed so far.
The three glazing types investigated in this research are integrated in the north facade of the
ETA-Factory [5, 6] (see figure 1), which is located at the Technical University in Darmstadt,
Campus Lichtwiese. The thermal performance or rather the Ug-value of each glazing was
monitored over a period of one year. The first glazing type under investigation is a double

8th International Building Physics Conference (IBPC 2021)
Journal of Physics: Conference Series 2069 (2021) 012134
IOP Publishing
doi:10.1088/1742-6596/2069/1/012134
2
IGU
VIG
AIM
IGU
VIG
AIM
Figure 1. North facade of the ETA-factory with the glazing systems of interest.
insulated glass unit (IGU). The resistance to heat flow of an IGU is influenced by the cavity
between the two glass panes, the conductivity of the gas in this cavity and the low emittance
surface coatings. The thermal processes which occur in an IGU are direct heat transfer through
the edge seal, convection in the cavity between the glass panes and thermal radiation between
the glass panes. The heat flow perpendicular to the glass surface is assumed to be uniform
over the surface. The second glazing type is a vacuum insulated glass hybrid unit (VIG-hybrid)
which is a combination of an insulated glass unit and a VIG unit. It is a new type of glazing
for the European market but already used on the Asian market. The design as compared to
an IGU differs mostly in one aspect, the resistance to heat flow of a VIG unit is achieved by
evacuating the cavity between two glass panes not by filling the gap with low conductive gas.
At a vacuum pressure at or less than 0.1 Pa the convection and conduction of heat through the
residual gas can be neglected [7]. To maintain the separation of the glass panes under the load
of atmospheric pressure a regular array of support pillars is placed between the two glass panes.
Hence, the main heat flow pathways in a VIG are direct heat transfer through the edge seal
and the pillars, and surface-to-surface radiation between the glass panes. The pillars which are
spread evenly between the two glass panes of the VIG represent point contact thermal bridges [8]
between the glass surfaces. Therefore the surface temperature varies periodically and the heat
flow rate over a pillar is higher than in the regions surrounding the pillars [8]. The third glazing
type is an Architectural Insulation Module (AIM) which is a vacuum panel that is constructed
as a pyrogenic silica technology en wrapped in black fleece layers which makes the panel opaque.
The monitored IGU and the AIM are installed next to each other on the first floor and the
VIG-hybrid on the second floor(see figure 1). Each of the three glazing types has a different
heat flow resistance or rather Ug-value. The specified Ug-values and the construction of each
glazing type are shown in table 1.
Table 1. Constructions and Ug-values of the glazing types.
Glazing type IGU VIG-hybrid AIM
Ug-value 1.10 0.65 0.23
Construction
8 mm
20 mm
6 mm
6 mm
0,76 mm
Outside glass pane
Gas gap filled with argon
Laminated glass with PVB interlayer
5 mm
16 mm
5 mm
5 mm
0,2 mm
Outside glass pane
Gas gap filled with argon
Outside VIG glass pane
Inside VIG glass pane Pillars
8 mm
20 mm
6 mm
6 mm
Outside glass pane
Gas gap filled with
Argon
Laminated glass with PVB interlayer
Vacuum panel Black fleece
0,76 mm
8 mm
20 mm
6 mm
6 mm
0,76 mm
Outside glass pane
Gas gap filled with argon
Laminated glass with PVB interlayer
5 mm
16 mm
5 mm
5 mm
0,2 mm
Outside glass pane
Gas gap filled with argon
Outside VIG glass pane
Inside VIG glass pane Pillars
8 mm
20 mm
6 mm
6 mm
Outside glass pane
Gas gap filled with
Argon
Laminated glass with PVB interlayer
Vacuum panel Black fleece
0,76 mm
8 mm
20 mm
6 mm
6 mm
0,76 mm
Outside glass pane
Gas gap filled with argon
Laminated glass with PVB interlayer
5 mm
16 mm
5 mm
5 mm
0,2 mm
Outside glass pane
Gas gap filled with argon
Outside VIG glass pane
Inside VIG glass pane Pillars
8 mm
20 mm
6 mm
6 mm
Outside glass pane
Gas gap filled with
Argon
Laminated glass with PVB interlayer
Vacuum panel Black fleece
0,76 mm

8th International Building Physics Conference (IBPC 2021)
Journal of Physics: Conference Series 2069 (2021) 012134
IOP Publishing
doi:10.1088/1742-6596/2069/1/012134
3
2. Method
The Ug-value describes the inverse of heat flow resistance in the center region of a glazing. The
Ug-value is equivalent to the thermal transmittance Λ which is defined in ISO 9869 [9] as
Λ = Pn
j=1 qj
Pn
j=1(Tij −Tej )(1)
where q is the heat flux density and Tiand Teare the inside and outside environmental
temperatures respectively. But equation 1 delivers only the actual Ug-value for the defined
condition given in ISO 9869, which will be described and discussed in detail in chapter 3. Most
important is that the heat flow over the analyzed time period is constant. Otherwise, if the heat
flow varies too much or even changes direction during the period of interest, it is not possible to
determine the actual Ug-value accurately. As the average method was used for the evaluation
of the Ug-value the temperature difference between the inside and outside environment should
be higher than 3 K to ensure the accuracy of the evaluated data [10].
2.1. Monitoring concept
Each glazing type was equipped with an U-value measurement kit. The measurement
instruments use one air temperature sensor on each side of the glazing and one heat flux meter
which is installed at the internal surface of the glazing, as can be seen in figure 2. The heat flux
meter was applied on a 0,75 mm thick tape which was glued on the glass surface. The tape is
part of the U-value measurement kit and is therefore tuned to the sensor. The properties of the
U-value kit are shown in table 2.
Figure 2. Sensor arrangement
(exemplary for VIG).
Heat flux sensor
Pillar separation = 20 mm
Heat flux sensor length = 30 mm
VIG pillars
Figure 3. Heat flux sensor
placement on VIG-hybrid surface.
The measurement frequency was set to 10 minutes, which provides reasonable accuracy and
small effects like environmental conditions or user behavior can be tracked. The Ug-value was
calculated based on the temperature and heat flux measurement, using the average method
according to ISO 9869. The Ug-value describes the thermal conductance through the center of a
glazing from the hot to the cold environment. Therefore, the heat flux and temperature sensors
are attached at a distance of at least 25 cm to the frame edge, where the edge conduction has
no influence on the measurement [11]. Furthermore, the heat flux sensor should be attached in
an area with a constant heat flow over the surface, avoiding thermal bridges next to the sensors.
For the IGU and the AIM the heat flow in the center region of the glazing is considered to
be constant. But, as it is shown in figure 2, the pillars of the VIG, which are thermal bridge
contacts may influence the measurement. The pillars have a separation distance of 20 mm from
each other which is smaller than the size of the heat flux sensor (30 mm x 30 mm x 3.3 mm).
The heat flow meter is placed in the center of 4 pillars, as one can see from figure 3. As the
Ug-value with units of W m−2K−1is investigated in this study, the size of the glazing system
has no influence on the results, presented in section 3.2.

8th International Building Physics Conference (IBPC 2021)
Journal of Physics: Conference Series 2069 (2021) 012134
IOP Publishing
doi:10.1088/1742-6596/2069/1/012134
4
Table 2. Properties of the measurement instrument. [12][13]
Properties Range
Heat flux measuring -300 W m−2to +300 W m−2
Heat flux resolution <0,22 W m−2
Heat flux sensitivity >7µV
Temperature sensors -20
°
C to 65
°
C
Temperature accuracy ±0.5
°
C
Minimal temperature difference ±5 K
2.2. Heat flux density measurement
With the two temperature sensors and the heat flux sensor positioned, the Ug-value of the three
glazing systems was evaluated and monitored from November 2018 until November 2019. The
heat flux progression over the year for the three glazing types is shown in figures 4 to 6. As long
as the inside room temperature Tiis higher than the outside environmental temperature Te,
the heat flux density is directed from the room-side to the outside environment and is therefore
measured as a positive value. This is the case for most of the time as it can be seen in figures 4
to 6. Figure 4 shows the measured heat flux for the IGU. The two linear progressions in figure
4 are due to a sensor failure. From November to March the heat flux is mostly positive due to
the cold outside and warm inside temperatures. Some high negatives heat flux values occur due
to specific effects which will be discussed in section 3.1. From May to August 2019 the heat flux
value becomes negative more often due to the hot outside temperatures. As of October 2019,
the heat flux follows a similar course as at the beginning of the measurement.
-40
-30
-20
-10
0
10
20
30
40
30.11.2018
20.12.2018
09.01.2019
29.01.2019
18.02.2019
10.03.2019
30.03.2019
19.04.2019
09.05.2019
29.05.2019
18.06.2019
08.07.2019
28.07.2019
17.08.2019
06.09.2019
26.09.2019
16.10.2019
05.11.2019
25.11.2019
15.12.2019
Heat flux density [W m-2]
Date
Figure 4. Heat flux density over one year for the IGU.
The measurement data of the heat flux for the VIG-hybrid (figure 5) gives similar results. It
is significant that the negative peaks of the heat flux values are highest in magnitude for the
VIG-hybrid.
-100
-80
-60
-40
-20
0
20
40
60
08.11.2018
29.11.2018
20.12.2018
10.01.2019
31.01.2019
21.02.2019
14.03.2019
04.04.2019
25.04.2019
16.05.2019
06.06.2019
27.06.2019
18.07.2019
08.08.2019
29.08.2019
19.09.2019
10.10.2019
31.10.2019
21.11.2019
Heat flux density [W m-2]
Date
Figure 5. Heat flux density over one year for the VIG-hybrid.
The heat flux data of the AIM, which shows the lowest Ug-value, fluctuates much more
compared to the other glazing types. However, the tendency of the heat flux curve over the year

8th International Building Physics Conference (IBPC 2021)
Journal of Physics: Conference Series 2069 (2021) 012134
IOP Publishing
doi:10.1088/1742-6596/2069/1/012134
5
is similar to the heat flux curves of the IGU and the VIG-hybrid. Due to a sensor failure, the
heat flux data is available only until September 2019. Since all three glazing types are installed
-60
-50
-40
-30
-20
-10
0
10
20
30
40
07.11.2018
27.11.2018
17.12.2018
06.01.2019
26.01.2019
15.02.2019
07.03.2019
27.03.2019
16.04.2019
06.05.2019
26.05.2019
15.06.2019
05.07.2019
25.07.2019
14.08.2019
03.09.2019
23.09.2019
Heat flux density [W m-2]
Date
Figure 6. Heat flux density over one year for the AIM.
on the same facade, the environmental conditions for all three glazing types were similar, as one
can see from table 3 below.
Table 3. Averages of the measured data point for each glazing type.
Glazing type Heat flux TiTe
[W m−2] [
°
C] [
°
C]
IGU 11.35 21.75 10.17
VIG-hybrid 1.37 21.51 11.41
AIM 1.08 21.71 10.86
3. Results and discussion
From figure 4 to 6 one can see, that frequency of fluctuation of the heat flux course increases
with the magnitude in resistance to heat flow. The IGU has the lowest resistance to heat flow
and the smallest relative change of the heat flux. Even though the resistance of the AIM is
about triple in magnitude of the VIG-hybrid, the average measured heat flux density on the
VIG-hybrid is only 26 % higher than that compared to the AIM due to the high variation of
the heat flux measurement of the VIG-hybrid. This could be attributed to the small pillars in
the VIG which act as small thermal bridges [8] and could lead to quick changes in the direction
of the measured heat flux.
3.1. Specific effects
Since the facade is facing north, it can be assumed that the influence of direct solar radiation on
the measurement results is small. Especially during the winter period (December to February)
sun-light exposure does not influence the sensor output.
However, the measurement is not only influenced by the external environmental conditions,
but also by the behavior of the room users. One specific effect could be identified clearly. The
manual ventilation processes during the winter period by opening and closing the window have
a great influence on the measured values. If one opens a window, the cold air floats over the
heat flux sensor and creates a colder environment at the internal surface of the sensor. The
glazing is still heated up and therefore the heat flux is directed from the glazing towards the
inside of the room and the heat flux density sensor delivers a negative value. In figure 7, the
heat flux density for the IGU is shown during a typical working week. From Monday to Friday,
one can identify recurring peaks (marked with red numbers) of the measured heat flux values.
They all occur during the day while the office is occupied. Over the weekend when the office is

8th International Building Physics Conference (IBPC 2021)
Journal of Physics: Conference Series 2069 (2021) 012134
IOP Publishing
doi:10.1088/1742-6596/2069/1/012134
6
empty these effects cannot be observed. Furthermore, the effect could not be observed during
winter break (22nd of December 2018 to 7th of January) when the offices were empty. This effect
explains the high negative measured heat flux values over the year for all glazing types.
Mo 17/12/18; 8:12
-29,49
Mo17/12/18;14:52…
Tue18/12/18;8:22;
-10,14
Tue18/12/18;11:02;
-3,78
Tue18/12/18;15:32;
-17,56
Wed19/12/18;14:12;
10,01
Thu20/12/18;8:22;
-14,38
Thu20/12/18;14:52;
7,36
-40
-30
-20
-10
0
10
20
30
0:02
4:02
8:02
12:02
16:02
20:02
0:02
4:02
8:02
12:02
16:02
20:02
0:02
4:02
8:02
12:02
16:02
20:02
0:02
4:02
8:02
12:02
16:02
20:02
0:02
4:02
8:02
12:02
16:02
20:02
0:02
4:02
8:02
12:02
16:02
20:02
0:02
4:02
8:02
12:02
16:02
20:02
Heat flux density [W m-2]
Time
Figure 7. Heat flux density over one week for the IGU showing the influence of opening a window.
During the summer period, the heat flux density varies greatly, as can be seen in figure 8.
The periodic change of the heat flux density during the day and night cycle, with respect to the
inside and outside temperatures, Tiand Teis representative for the summer period. At night,
when Ti>Te, the heat flux is positive. As soon as the outside temperature Teexceeds Ti, the
heat flux direction changes and the heat flux becomes negative. The heat flux density gradient
increases with the convergence of Tiand Te. The peaks due to manual ventilation can not be
identified as simply for the summer period as they were for the winter period.
0
5
10
15
20
25
30
35
40
45
-20
-15
-10
-5
0
5
10
0:08
3:28
6:48
10:08
13:28
16:48
20:08
23:28
2:48
6:08
9:28
12:48
16:08
19:28
22:48
2:08
5:28
8:48
12:08
15:28
18:48
22:08
1:28
4:48
8:08
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14:48
18:08
21:28
0:48
4:08
7:28
10:48
14:08
17:28
20:48
0:08
3:28
6:48
10:08
13:28
16:48
20:08
23:28
2:48
6:08
9:28
12:48
16:08
19:28
22:48
Temperature [°C]
Heat flux density [W m-2]
Time
Heat flux density
Ti
Te
Figure 8. Heat flux density during one week in the summer period for the IGU.
These effects show why it is important to evaluate suitable time periods with constant
environmental conditions which leads to a constant heat flux density. In compliance with the
standard ISO 9869, the Ug-value could be evaluated for each glazing type and is presented in
the next section.
3.2. Ug-value analyses
The primary objective of this study is to confirm that the Ug-value as given by the
manufacturers as a glazing specification is consistent with the measured in situ data under
varying environmental conditions. The number of time periods which could be analyzed
depended on the consistency of the measured heat flux density. Therefore, it was possible
to evaluate more time periods for the IGU and the VIG-hybrid than for the AIM. In figure 9,
all the measured and analyzed Ug-values in accordance to ISO 9869 and calculated from eq. 1
for the three glazing types are presented.

8th International Building Physics Conference (IBPC 2021)
Journal of Physics: Conference Series 2069 (2021) 012134
IOP Publishing
doi:10.1088/1742-6596/2069/1/012134
7
Figure 9. Measured Ug-values for the three different glazing types.
For the IGU and the VIG-hybrid, most of the evaluated Ug-values are within 5 % of the
Ug-value specified by the manufacturer. Only a few measurements of the AIM could be
evaluated, therefore the accordance between the measured Ug-value and the Ug-value stated
by the manufacturer is not well and has to be investigated in future work. The variation of the
evaluated Ug-values is higher for the IGU than for the VIG-hybrid. This could be attributed to
the temperature-dependent convective heat transfer in the IGU, which has a greater influence
on the Ug-value of the IGU. Another influence on the evaluated Ug-value due to convection
is shown in figure 10. The Ug-value rises with an increase of the temperature difference ∆T
between the inside and outside environment since a rise in temperature difference promotes an
increase in the convective heat transfer.
Figure 10. Correlation between temperature difference and Ug-value.
The measured average Ug-value of the IGU and the VIG-hybrid are in good agreement with
the manufacturer’s information, as it can be seen from table 4. The measured data for the AIM
differs 27.8% from the manufacturer’s specification. Discrepancies over 20 % could be attributed,
among other effects to the heat flow lines which were not straight and perpendicular to the glass
element [9]. This might be the case as the frame is a significant thermal bridge to a glazing type
with such a low Ug-value.
Table 4. Comparison of measurement and manufacturer’s information.
Glazing type Manufacturer’s information Average measured Ug-value Deviation
[W m−2K−1] [W m−2K−1] absolute rel.[%]
IGU 1.10 [14] 1.12 +0.02 +1.8
VIG-hybrid 0.65 [15] 0.63 -0.02 -3.2
AIM 0.23 [14] 0.18 -0.05 -27.8

8th International Building Physics Conference (IBPC 2021)
Journal of Physics: Conference Series 2069 (2021) 012134
IOP Publishing
doi:10.1088/1742-6596/2069/1/012134
8
4. Conclusion
In situ monitoring of three glazing types with the same environmental and structural boundaries
was carried out over a period of one year. Depending on the magnitude of the resistance to heat
flow, it turned out that the data collection in compliance with ISO 9869 for high insulating
glazing types is more difficult than for glazing types with a low resistance to heat flow. During
the monitoring period two major effects influenced the measurements significantly: The manual
ventilation effect. And the change of the heat flux direction and amplitude. The heat flux
measurement for a VIG-hybrid unit delivers accurate values despite the small thermal bridges
(pillars) if the measurement boundary conditions and the data evaluation are complied within
the ISO 9869 standard. The analyzed Ug-value for all three glazing types is consistent over the
monitoring period of one year. Further investigations will include the assessment of the solar
radiation in the office rooms and the thermal transmittance through the frame.
Acknowledgment
Special thanks go to David Bewersdorff for his support in developing and implementing the
monitoring concept at the ETA-factory.
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