Akzo Nobel Functional Chemicals GmbH & Co. KG
Liebigstraße 7
D-07973 Greiz
DL-148
Insulating Glass
Energy saving without noble
gas filling?
The combination of Low-E coatings and noble gas filling in today’s Insulating Glass Units (IGU) give
the best overall properties in a window. These windows provide excellent protection against the
environment, and allow day light and ventilation, but it is the additional energy savings and noise
protection benefits that truly set them apart. It is the energy savings in particular that is of
increasing importance; nearly one third of all energy consumption is associated with buildings [1].
Consequently, energy saving has to now be counted among the primary requirements of a window.
The comparison of today’s commercially available glazing types in table 1 [2] demonstrates the
considerable advantages of Low-E, noble gas-filled IGUs. In Europe where heating is the main issue
these advantages can be expressed in terms of oil: low-E, argon-filled IGUs need only around 50 %
oil of air-filled IGUs, and triple pane IGUs just less than 25%. According to various studies (e.g. [3]),
decreasing of the Ug-value by 0,1 W/(m²•K) leads to oil saving of about 1,2 liters per m² per year.
This ratio is also valid in the US [4] – savings of more than 0.12 USD/ft²/y are possible. Consumers
are becoming more aware of this, and R-5 windows are enjoying growing acceptance. It is easy to
predict that consumer demand will go beyond R-5 in the not distant future.
Tab. 1
Single-Pane
Double-Pane
5,60
0.99
1
2,80
0.49
2
1,301
0.23
4.3
60
30
15
air-filled
Double-Pane
Low-E, argon-filled
Triple-Pane
Low-E, argon-filled
Ug-value (SI)
U-value (US)
R-value
Oil
consumption
(h·ft²·°F)/BTU
Ug-value,
U-value
Ug =thermal transmittance value of the glass unit; in general U describes how well a building
element conducts heat; U = 1/R
R-value
Measure of thermal resistance; R is the inverse of U, measured as [W/(m²·K)] or [(hr·ft²·°F)/BTU] in
the US, i.e. US R-values are equivalent to approximately six times SI R-values:
1 (K·m²)/W = 5.678263 (h·ft²·°F)/BTU
W/(m²·K)
BTU/(h·ft²·°F)
l/(m²·y)
0,65
0.11
9.1
7
So what part does the noble gas filling play in this exceptional performance and what is the
consequence of gas loss in a sealed IGU?
1
for comparison: Low-E, air filled IGU: Ug = 1,70 W/(m²·K)=0.3 BTU/(h·ft²·°F)
C:\Documents and Settings\Renzd\Local Settings\Temporary Internet
Files\Content.Outlook\2B3KRKJ3\Energy saving_USversion_finalDL.docx
The improvement of the U-value by argon-filling can be achieved by less than 1 % of the raw
material cost of a window according to [4]. An estimation of the cost of gas filling is made in [5].
The effect of noble gas filling on the Ug-value is shown in figure 1 (data based on [6])—the greater
the level of gas, the greater the benefit. The converse is also true—the more gas that is lost from a
unit, the worse the performance. Should an IGU lose its noble gas entirely the resulting increase in
Ug-value would be around 0.07 BTU/(h·ft²·°F), and oil consumption goes up by 0.1 gallon/ft²/y.
Fig. 1
0,35
Impact of Argon Gas Filling on U-value
U-value [BTU/h·ft²·°F]
0,30
(4/12/4 Low-E)
U = 0.07
0,25
0,20
0,15
0,10
0,05
0,00
>90
80
60
40
20
0
Argon Filling in %
Therefore it is important to minimize gas loss over time. This can be achieved through the diligent
choice of sealing material and careful manufacturing practices.
Almost all of the 7 billion ft² IGUs manufactured world-wide last year are dual sealed: the inner
sealant is based on polyisobutylene (PIB), and the outer or secondary seal has to be an elastic,
rubber-like material which functions as an adhesive, holding the glass unit together and keeping it
tight during the service life. It is clear that the secondary sealant fundamentally determines the
quality and durability of the insulating glass unit.
It is often promoted in advertisement that the UV resistance, weatherability, temperature
resistance, adhesion, mechanical properties and even shrinkage are the most important features of
secondary IG sealants, whereas resistance against permeation of moisture vapor and noble gas are
regarded as insignificant or ignored altogether. This argument is generally based on the belief that
the excellent barrier properties of the primary PIB sealant are sufficient to protect the IGU cavity
against moisture and gas permeation. If this assessment was true, then all IGUs equipped with PIB
primary sealants should show similar test results in gas and moisture penetration, regardless of
which secondary sealant type was applied. Standard tests at various testing institutes performed
with IGU sealed with PIB and the premium secondary sealants - polysulfide, polyurethane, and
silicone - show the opposite:
2
As far back as 1991 it was shown [7] that gas leakage rates2 Li <0.5%/y of argon-filled dual
pane insulated glass units could only be achieved by using polysulfide secondary sealants.
Silicone sealed units show a wide variation from Li<1%/y to Li>10%/yr. Slight deviations in
the IGU manufacturing process could not be compensated by the silicone sealants because
of their high gas permeability. Polysulfide sealants are more tolerant against such
deviations. This is demonstrated by a smaller variation, between <0.5%/y and <2%/y. The
majority of tested polysulfide sealed IGUs had Li<1%/y.
Similar tests of argon permeation of double sealed IGUs (submitted by various European
IGU manufacturers) performed between 2008 and 2010 at ift Rosenheim [8] provide
analogous results (Tab. 2):
Tab. 2
Secondary sealant type
Polysulfide
Polyurethane
Silicone
Number of tests
121
93
111
% of failed tests
criterion Li 1,0 %/y (EN 1279-3)
22
27
34
Statements about IGUs applied in low-energy-houses confirm that gas leakage rates
between 0.2 and 0.6 %/y could be easily achieved with PIB/polysulfide sealed IGUs whereas
it was difficult to meet Li 1.0 %/y with PIB/silicones [9].
Tests according to EN 1279-2 and 3 on IGUs sealed with PIB and silicone or polysulfide
secondary sealants, respectively, were performed by AkzoNobel in cooperation with IG
sealant and IGU manufacturers in 2010 in order to investigate the influence of sealant
thickness and PIB consumption (among other parameters) on both noble gas (gas leakage
rate Li) and moisture permeation (average moisture penetration index3 Iav). The results
published in [10] confirm the well known truth: the thicker the sealant layer and the more
PIB that is applied, the lower the moisture penetration and argon permeation; but there
are differences between the silicone and polysulfide sealed IGUs:
o moisture penetration of silicone sealed IGUs varies significantly (2%<Iav<14%)
dependent on the IGU features whereas Iav of polysulfide sealed IGUs was only
between 4.5 and 7.5 %. All units passed the test, but results show that IGUs with
silicone sealants are more sensitive to sealant thickness and PIB application than
those sealed with polysulfides.
o the gas leakage rate Li before and after the EN 1279-3 test of the polysulfide sealed
IGUs was nearly the same and independent of the sealant thickness and applied PIB
whereas the silicone sealed units vary considerably (Tab. 3):
2
Li is the gas leakage rate defined by EN 1279-3 as the proportion expressed as a percentage by volume of gas i leaking
from a gas filled unit per year
3
Iav is the moisture penetration index defined by EN 1279-2 as the amount of drying capacity consumed after
standardized ageing conditions
3
Tab. 3
average
standard deviation ±
PIB/Polysulfide secondary sealant
Li (initial)
Li (EN 1279-3)
PIB/Silicone secondary sealant
Li (initial)
Li (EN 1279-3)
[%/y]
[%/y]
[%/y]
[%/y]
0.55
0.07
0.67
0.02
0.57
0.47
1.22
0.90
Obviously, there is a considerable impact of the secondary insulating glass sealant type on gas and
also on humidity tightness. Permeation properties are material properties and are related to the
polymer type and the sealant formulation. Consequently, it is not arbitrary which secondary
sealant type is chosen. Data received of membrane testing according to EN 1279-4 performed by
independent institutes confirm the different moisture and gas permeability of the premium
secondary IG sealants (Tab. 4).
Tab. 4
Moisture Vapor Transmission Rate
Source
[g/(m²·day)]
[6]
Polysulfide
Polyurethane
Silicone
7 -9
1–4
15 – 20
Permeation of Argon
[11]
[10-3g/(m²·h)]
[6]
[8]
[12]
3–6
2–4
15 – 20
5–8
30 – 50
500 – 1000
4.5
36
888
4.3 – 6.8
40 – 75
700 – 800
In case the primary seal fails, the secondary sealant is left as the only barrier and therefore it
should possess the best permeation properties. In regards to gas tightness the right choice of
secondary sealant is essential because the molecule diameter of argon is in the range of rugosity
(unevenness) of the glass surface 4. Since the PIB adhesion to glass is physical, not chemical,
diffusion is possible along the interface between the glass and PIB.
Polysulfide sealant’s excellent barrier properties against gas loss and humidity penetration should
make it the first choice for noble gas filled IGUs in order to provide maximum tightness. Maximum
tightness also translates to maximum safety because in contrast to moisture penetration into an
IGU cavity which is visible, loss of gas from the cavity is invisible and can only be non-destructively
measured with expensive specialized instruments by experts. The window owner’s sole way to
recognize gas loss on his own is through increasing energy bills. Unfortunately it can be difficult for
the owner to reliably attribute increasing energy bills to IGU gas loss as there can be many other
causes of increasing bills, such as weather and energy cost variability.
In conclusion, the question posed by this article’s headline can be answered with “No”: energy
saving without noble gas filling is ineffective and does not correspond to the state-of-the-art of
insulating glass. Polysulfide secondary sealants ensure that noble gas remains in the cavity and
therefore they contribute essentially to the longevity of insulating glass units.
4
the tin-side of the float glass surface has an unevenness of 1.2 to2 nano meters; the Argon diameter is 0.34 nano
meters
4
Literature
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
EU Commission 2006; Glaswelt, 07(2011), p. 13
W. Feist; Passivhaustagung, Oct. 31, 2006
Announcement of BF-Bundesverband Flachglas; July 22, 2009
Jackson, R.; Window & Door, Oct./Nov. 2010, p. 65
Almasy, J.; USGlass, vol. 38, May 2003, p.32
E. Mognato; GPD Proceedings, 2005, p. 235
Feldmeier, F., Schmid, J.; IfT-Sript 01/91, p. 16
Lieb, K; ift Rosenheim, unpublished data 2002-2010
energiesparhaus.at; powered by tripple.net
Lange, D; 09/2012, Chin. Glass Ass. Proceedings (Guangzhou)
Holler, G.; Mehrscheibenisolierglas, Expert Verlag 1995, p. 68-99
Wittwer, W., Lange, D.; unpublished report, 2009
D. Lange
August 20, 2012
5