Gas fill and double glazing unit (DGU) thermal performance

The thermal performance of a double glazing unit (DGU) is dependent on the gas fill type and gas gap thickness. This article aims to provide a detailed explanation of the relationship between gas fill and DGU thermal performance.

• Typical DGU gas fills
  • DGU system in example calculations
• Gas fill and DGU winter condition U-value
• Gas fill and DGU summer condition U-value
• Gas fill and DGU SHGC
• Gas fill and DGU thermal performance: summary

More information

• Argon concentration and DGU thermal performance

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Typical DGU gas fills

In Singapore, 12 mm thick air space is the most common type of gas fill for DGUs. In some special cases, argon gas is used for better thermal performance. The gap thicknesses also vary from 6 mm to 20 mm. Other gas types (e.g. krypton and xenon) or gap thicknesses are rarely used.

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DGU system in example calculations

In this article, the DGU system presented in an earlier article on the DGU testing procedures is used in the example calculations.

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Gas fill and DGU winter condition U-value

Shown below is the variation of winter condition U-value (of the example DGU) with gas type and gap thickness.

If the gap thickness is too small (e.g. 3 mm), the winter condition U-value is high. This is because, for thin gas layers, the main heat transfer mode is conduction, and a thin gas layer conducts more heat.

If the gap thickness is too large (e.g. 36 mm), the winter condition U-value is high again. This is because, for thick gas layers, convection overtakes conduction as the main heat transfer mode and a thick gas layer allows stronger gas flow and more convective heat transfer.

There is an optimum gap thickness, with the lowest winter condition U-value. The optimum gap thicknesses are 12 mm for both air and argon in the chart above (3 mm resolution is used in our examples. The optimum gap thickness is slightly different if another resolution is used).

It is apparent that argon-filled DGUs are with lower winter condition U-value than air-filled DGUs. This is mainly due to two reasons:

• Smaller thermal conductivity of argon: at 10 °C, the thermal conductivity of argon is 0.01684 W/(m⋅K), which is significantly smaller than that of air [0.2496 W/(m⋅K)]. Smaller thermal conductivity reduces the conductive heat transfer effectively.
• Greater density of argon: at 10 °C, the density of argon is 1.699 kg/m3, which is significantly higher than that of air [1.232 kg/m3]. Larger density reduces the gas flow rate due to natural convection and reduces the convective heat transfer effectively.

Other insert gas types (e.g. krypton and xenon) are with smaller thermal conductivity and greater density than argon. Therefore, they are more effective in reducing winter condition U-value. Of course, the costs are also higher, and they are rarely used in Singapore.

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Gas fill and DGU summer condition U-value

Shown below is the variation of summer condition U-value (of the example DGU) with gas type and gap thickness.

The overall pattern and the underlining heat transfer mechanisms are like the winter condition U-value scenario discussed above. The main difference is that the optimum gas thicknesses are different. They are 24 mm for both air and argon in the chart above.

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Gas fill and DGU SHGC

Shown below is the variation of SHGC (of the example DGU) with gas type and gap thickness.

The overall pattern and the underlining heat transfer mechanisms are like the winter condition and summer condition U-value scenarios discussed above. In the chart above, the optimum gap thickness is 18 mm for air and 15 mm for argon.

As explained in what is SHGC, SHGC consists of two components:

• Primary solar heat gain: the solar heat in its original solar radiation. Gas fills do not affect this component, as the gases are fully transparent, and they do not affect DGU optical performances.
• Secondary solar heat gain: the solar heat absorbed by a glass and further transmitted to a room. For this component, the heat transfer mechanism is like that in U-value heat transfer. Gas fills affect the secondary solar heat gain component.

Additionally, both summer condition U-value and SHGC are in terms of the summer environmental conditions. It seems that 12 mm is not the optimum gap thickness in the summer environmental conditions and a larger gap thickness is more appropriate.

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Gas fill and DGU thermal performance: summary

There are 3 heat transfer paths in a DGU gas gap:

• Conduction
• Convection
• Radiation

Listed in the table below is the influence of gas fill to each heat transfer path:

Gas fills affect not only DGU U-value, but also SHGC. The optimum gap thickness is different in the winter environmental conditions and in the summer environmental conditions.

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If you are interested in more in-depth information, please continue reading this article. If your concerned questions are not explained, please feel free to leave a comment at the bottom of this page. This article will be reviewed and updated regularly.

More information

Argon concentration and DGU thermal performance

For argon gas filled DGUs, argon gas naturally leaks out from gas gaps to external air (refer to this article on the reasons for argon gas leakage).

Shown below are the variations of U-value (both winter and summer U-values) and SHGC on argon concentration. The same example DGU system used above is used and the gap thickness is 12 mm.

The variations are linear. The initial argon concentration of a new DGU glass is typically 95%. Assuming a leakage rate of 1% per year, the argon concentration is 80% after 15 years. The thermal performance degradation is still considered small and acceptable (as described in this article, EN 1279 demands that only 1 % of gas is allowed to leak in 1 year).

In the NFRC simulation manual, the following maximum argon concentration is recommended based on the gas filling technique:

It is possible to use our online glass U-value calculator to understand the impact of argon concentration on glass U-value.

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Last update: 29/05/2022

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