Energy saving without noble gas filling?

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)

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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