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I have an old window which is being partly replaced by new glass making it an 1+2-window. The outer "1-part" is made of old simple glass having a specified U-value of 5,8 W/m2K. The new inner "2-part" is a glass-argon-glass construction havning a specified U-value of 1,0 W/m2K. In between the two parts there is going to be a 41 mm air gap. Putting them together and calculating the totalt U-value I'm getting approx. 0,62 W/m2K. This seems a little low to me and will probably be difficult to achieve in reality. I guess the low value is a result of the air being to good of an insulation component when it is perfectly still and "trapped" in between the old and new glass parts (conduction not accounted for?). How could I handle this instead?

I have an old window which is being partly replaced by new glass making it an 1+2-window. The outer "1-part" is made of old simple glass having a specified U-value of 5,8 W/m2K. The new inner "2-part" is a glass-argon-glass construction havning a specified U-value of 1,0 W/m2K. In between the two parts there is going to be a 41 mm air gap. Putting them together and calculating the totalt U-value I'm getting approx. 0,62 W/m2K. This seems a little low to me and will probably be difficult to achieve in reality. I guess the low value is a result of the air being to good of an insulation component when it is perfectly still and "trapped" in between the old and new glass parts (conduction (convection not accounted for?). How could I handle this instead?