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Community and Q&A

Window IGU thickness debate

67Shelby | Posted in Energy Efficiency and Durability on

I have been in the window industry for about 16 years and i have heard many different takes on the difference between a 3/4″ IGU and a 7/8″ IGU. Are there any significant differences between the two related to energy efficiency such as the rate of convection between the panes? I would think a larger air space would allow for a more efficient unit but i would like some more input professionally. I also know that the type of spacer used makes a difference as well but for comparing apples to apples, lets say that we are comparing two units with the same specs, just with different thicknesses.

Thanks for any advice and help.

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  1. GBA Editor
    Martin Holladay | | #1

    There are three issues here:

    1. Glass thickness. (Thicker glass is stronger than thinner glass.)

    2. The optimum gap between the window panes of an IGU.

    3. The different methods used to calculate U-factor in North America and Europe.

    The most common glass thicknesses for residential windows are 3 mm (for smaller windows) and 4 mm (for larger windows).

    If you want to know the optimum gap thickness between panes -- assuming that you want the lowest possible U-factor, as calculated in North America -- the answer is simple: "The optimal thickness of the space between panes of argon-filled glazing units is 1/2 inch. Increasing or decreasing the thickness of this space degrades performance. For krypton, the optimal space is thinner — only 3/8 inch — so krypton, a much more expensive gas than argon, is usually reserved for applications where the total glazing unit thickness must be minimized." (This information comes from one of my articles, All About Glazing Options.)

    In Europe, the U-factor calculation favors bigger gaps. To learn more about this issue, see Presumptive European Superiority Syndrome.

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