4.2. Calculation Results of Thermal Transmittance of Different IGUs under Different Negative External Air Temperatures
Based on previous research [30
], it was found that the differences in temperature have a small effect on the thermal transmittance of the wooden window frame and the IGU spacer [30
]; therefore, further research focused on the insulating glass units and their influence on heat transfer throughout the window.
The thermal characteristics of the IGU depend on the number of glasses, the distance between the glasses, the Low-E coatings, the filling gases, and their concentration.
For the determination of the thermal characteristics of the abovementioned IGUs, a digital modelling of the double and triple glazing was carried out, changing the spaces between the glass panes from 10 mm to 25 mm (10 mm; 12 mm; 14 mm; 16 mm; 18 mm; 20 mm; 25 mm); the corrected emissivity ε of the Low-E coating varied from 0.03 to 0.09 (0.03, 0.04, 0.05, 0.06, 0.08, 0.09); the argon gas concentration ranged from 95% to 60% (60%; 80%; 90%; 95%).
The internal temperature was always + 20 °C, and the external temperature was changed from 0 °C to −30 °C (0 °C; −5 °C; −10 °C; −15 °C; −20 °C; −25 °C; −30 °C). For comparison, calculations were also made for IGUs without Low-E coatings filled with air.
The results of the calculations of the central part of the thermal transmittance Ug
·K), for double glazing, depending on external air temperature and the distance between the glasses, when the thermal emissivity of glass surfaces was ε = 0.84 (without Low-E coatings) and there was no gas filler (air), are presented in Figure 2
The presented data show that with the decrement of external air temperature, the thermal transmittance decreased at small distances between glasses 10 and 12 mm, and at greater distances between glasses, it changed (mainly increased) slightly up to 2%. The decrement of thermal transmittance with the decrease in external air temperature can be considered a positive phenomenon (at extremely low temperatures, there would be lower heat losses), and small changes in the Ug-value may be ignored as insignificant.
These results confirm why the change in the thermal transmittance of IGUs due to external temperatures has been ignored in the past. However, circumstances are changed when IGUs with Low-E coatings and inert gas fillers began to be used in cold climate zones.
The results of the calculations of the central part of thermal transmittance Ug
·K), of the double-glazed IGU, depending on external air temperature and Low-E coating corrected emissivity value ε, and when the distance between glasses was 16 mm and the argon gas was concentration was 95%, are given in Figure 3
. The obtained calculation results show that the thermal transmittance value increased rapidly with the drop of the external temperature from 0 °C (standard temperature) to −30 °C (the design temperature for calculating the peak heating power of lightweight construction buildings in Lithuania). When using Low-E coating, ε = 0.04, the Ug
of the IGU increased from 1.21 to 1.57 W/(m2
·K), i.e., approximately 30%.
The results of the calculations on the dependence of the Ug
-value of the double-glazed IGUs on external air temperature and distance between the panes, when the Low-E coating’s corrected emissivity value was ε = 0.04 and the argon gas concentration was 95%, are presented in Figure 4
. This graph shows that the distance between the glasses had a different effect on the increment of thermal transmittance as the external air temperature decreased. When there was a small distance between the panes (10 mm), the value of Ug
-value was almost unchangeable, and a small change was also the case with a thickness of 12 mm compared to other instances. The maximum change in the Ug
-value occurred when the distance between the panes was 18 mm, then it was close to a 30% increase (from 1.22 W/(m2
·K) to 1.59 W/(m2
·K)). These results show that when the external air temperature is 0 °C, the lowest Ug
-value is given by the IGU with space between panes of 16 mm (Ug
= 1.21 W/(m2
·K)), and the highest value with a 10 mm distance (Ug
= 1.45 W/(m2
·K)). Accordingly, when the external temperature dropped to −30 °C, the lowest Ug
-value occurred when the distance was 10 mm between the panes (Ug
= 1.48 W/(m2
·K)), and the highest value was when the distance was 25 mm (Ug
= 1.62 W/(m2
The dependencies of the thermal transmittance (Ug
·K)) values of the central part of the double-glazed IGU on the external air temperature and concentration of argon gas fill, when the Low-E coating’s corrected emissivity value was ε = 0.04 and the distance between glasses was 16 mm, are given in Figure 5
. The results show that the changes in the concentration of the argon gas fill did not change the dependence tendency of the thermal transmittance value on the external air temperature.
The results of the calculations of dependencies of the thermal transmittance (Ug
·K)) values of the central part of the triple-pane IGU on external air temperature and Low-E coating corrected emissivity value ε
, when the distance between glass panes was 16 mm and the argon gas fill concentration was 95%, are presented in Figure 6
The graphs from Figure 6
show that the thermal transmittance increases rapidly with the change in the external air temperature from 0 °C (standard temperature) to −30 °C (design temperature for the calculation of peak heating power in Lithuania for lightweight construction buildings). When the Low-E coating’s value was ε = 0.04, the Ug
-value increased from 0.60 to 0.76 W/(m2
·K), i.e., close to 27%. Comparing the results presented in Figure 3
and Figure 6
, we can see that the dependencies of the change in thermal transmittances of double-glazed and triple-glazed IGUs on the external air temperatures are similar.
The results of the calculations on the dependence of Ug
-values of the triple-glazed IGU on external air temperature and distance between the panes when the Low-E coating was ε = 0.04 and the concentration of argon gas fill was 95% are shown in Figure 7
. This graph shows that the distance between the panes influenced differently the increment in the Ug
-value as the external air temperature decreases. When there were small distances between the glass panes (10–14 mm), the Ug
-value varied slightly. If the distance between the glass panes was 10 mm, the Ug
-value even decreased slightly as the external air temperature decreased. The largest change in thermal transmittance value was observed when the distances between the panes were 25 mm, then it increased by approximately 34% (from Ug
= 0.61 W/(m2
·K) to Ug
= 0.82 W/(m2
·K)). This graph also shows that when the external temperature was 0 °C °, the lowest Ug
-value was for the double-glazed IGUs with an inter-pane gap of 18 mm (Ug
= 0.58 W/(m2
·K)), and the highest was with 10 mm gaps (Ug
= 0.82 W/(m2
·K)). Correspondingly, when the external temperature was −30 °C, the lowest value was when the distance between the panes was 12 mm (Ug
= 0.75 W/(m2
·K)), and the highest was when it was 25 mm (Ug
= 0.82 W/(m2
4.3. Simplified Calculation Method of the Thermal Transmittance of IGUs Dependent on External Air Temperature
One of the objectives of this study was to determine the relationships between the thermal transmittance of IGU and the external air temperature. As the manufacturers of windows and IGUs declare the thermal transmittance values at 0 °C external air temperature according to European standards, lower external temperatures are used for design of a peak heating power of buildings (for example, in Lithuania up to −30 °C). There is a need to know the real (design) U-values of windows and IGUs at low external temperatures, because it can be significantly higher than those declared at 0 °C. Therefore, it may be important for designers to have a simple methodology to perform the recalculations from declared values to design using the real U-values of windows. As we know, the insulated glass units make the determinant impact on U-values of windows. Therefore, the simplified recalculation formulas were developed for double-glazed IGUs, filled with argon gas with one Low-E coating depending on the distance between the glass panes, when it is known that the Ug
-value of the double-glazed IGUs are 0 °C (U(0)) and the external air temperature (θe
) (Table 4
); for triple-glazed IGUs filled with argon gas and two Low-E coatings (Table 5
The thermal transmittance of the double-glazed IGU is dependent on the external air temperature and distance between the glass panes. The comparisons of the U-value calculations of the IGUs with the Low-E coating’s value of ε
= 0.06 and a 90% argon gas fill concentration, using standard methods (EN 673 and ISO 15099) and the developed simplified methods are given in Table 6
for double-glazed and in Table 7
for triple-glazed IGUs.
In order to verify the reliability of the composed simplified methods, the differences of Ug-values were found between the values calculated in accordance with the standard methodology and the values calculated according to the developed simplified recalculation formulas.
Calculation data using the standard methodology and simplified formulas for double-glazed IGU with 16 mm cavity, filled with argon gas and one Low-E coating (4-16Ar-4e), are given in Table 8
, and for triple-glazed glass unit with 16mm cavities, filled with argon gas and two Low-E coatings (4e-16Ar-4-16Ar-4e)—in Table 9
The data in Table 8
and Table 9
show that the simplified recalculation formula did not result in a non-compliance of more than 2.4%.
The comparison of U-values of IGU measured in Hot Box and calculated according to simplified methods are given in Table 10