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Product Guide

The Road Ahead for Insulating Glass

Glazing is still the weak link in the thermal envelope, but new techniques promise better performance in the future

A worker at Cardinal Glass prepares to move a completed insulated glass unit on the production line. IGUs are made to order for a variety of customers. Photo courtesy Cardinal Glass Industries.

Andersen Windows’s 1952 introduction of the Welded Insulated Glass panel was a big deal, at least for homeowners in the northern tier of the country. Now, consumers could buy an assembly that married two sheets of glass and an insulating layer of air in a single glazed product. For untold numbers of homeowners, Andersen’s commercial launch meant an end to the drudgery of storm windows.

More importantly, it was the start of an industry that in the last 70 years has incrementally improved the thermal performance of windows several times over. Multipane insulated glass units (IGUs) combine metallic coatings, inert gas fill, and insulating spacers in assemblies that make houses more comfortable and lower heating and cooling costs. By tweaking the characteristics of low-emissivity (low-e) coatings and applying them selectively, glass makers can customize IGUs for specific needs and climates.

Even with the best coatings and insulating gas fills, glass makers are fighting an uphill battle. The best glazing makes a poor insulator when compared to the exterior walls of a high-performance house. Walls in a Pretty Good House would be rated at roughly R-40, for example, while high-quality triple-glazing might have a U-factor of 0.15, the equivalent of just R-6.6. The 2018 version of the International Energy Conservation Code calls for a minimum U-factor of 0.32 (roughly R-3) for windows in even the coldest parts of the country.

At the same time, work continues on new technologies that could make better windows more widely available. They include vacuum units, triple-pane designs with an ultrathin center pane, and suspended film units with as many as eight internal layers and a center-of-glass insulating potential of more than R-19.

The thermal performance of glazing and complete window units are two different things. IGUs may come with very low center-of-glass U-factors (high R-values), but when they…

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  1. John Prospect | | #1

    Very interesting. Thank-you, Scott.

    It is good to have an update on where all these different technologies have gotten up to now.

  2. User avater
    Nathan Kipnis, FAIA | | #2

    Can someone explain to me why the glass industry uses 'U' values and not 'R' values? I have never had this explained to me that made any sense.

    1. PioneerBuilders | | #3

      From what I know, U factor is actually more of an industry standard and can be used with many different assemblies. R value is the inverse of the U factor, and that number (the R value) is easier for consumers to understand. I can't vouch for what I'm about to say, but it's my understanding that R value is driven my sales/marketing.

    2. User avater GBA Editor
      Martin Holladay | | #4

      The response from Pioneer Builders is accurate. Scientists have used U-factors for decades. R-value was invented to help marketers of insulation -- since marketers assume that consumers are confused by the idea of "a lower number is better."

      For more information, see "All About U-Factor."

      1. User avater
        Nathan Kipnis, FAIA | | #5

        I hear what you are saying, but it's like someone would say 'I'm 0.01470" tall, inverted' Consumer speak seems to make a lot more sense to me, as you can understand the order of magnitude way better than the minutiae of an inverted number.

        1. Expert Member
          Malcolm Taylor | | #6

          Hang around the contractor sales desk at a lumberyard for a hour or so and you will hear at least one confused customer trying to buy thicker brad nails or flashing by asking for a higher gauge.

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