Andersen Window’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 important, it was the start of an industry that in the last 70 years has incrementally improved the thermal performance of windows several times over. Multi-pane 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 coatings and applying them selectively, glassmakers 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 ultra thin 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 are…