Top Shelf Insulating Glass

Why an energy-harvesting window makes sense

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Over the past 50 years, there have been significant advancements in the field of glazing, particularly when it comes to windows. Today, modern thermal insulation glazing is the standard for protecting indoor spaces from outdoor temperatures. 

U-Value

Thermal insulation glazing is a type of multiple-pane glazing that significantly reduces heat transfer through various properties. The underlying goal is to achieve a low U-value (i.e., the heat transfer coefficient attributable solely to the glass pane), and in addition to the inert gas filling and the multiple-pane configuration, close attention is also paid to the edge seals. Whereas heat used to escape through conductive materials like aluminum, today’s designs focus on avoiding the so-called “warm edge” and using less conductive seals.

Many standard insulating glass units now achieve a U-value of 1.1 to 1.7 W/(m²K), and triple-pane units can even reach as low as 0.5 W/(m²K). For optimal thermal insulation, insulating glass such as GM® ISO TOP is ideal, offering a minimum U-value of 0.4 and available in double or triple glazing depending on the application. 

Function and Quality

Perks of GM® ISO TOP

Environmental awareness and energy costs continue to rise every year. To meet the new needs and expectations that have emerged as a result, insulating glass products such as GM® ISO TOP—our most energy-efficient glass—have been developed. But these offer much more:

  • very good light transmission in double- and triple-pane insulating glass
  • excellent thermal insulation values (Ug values)
  • a high total solar energy transmittance (g-value). This value indicates the percentage of solar energy that passes through the glass into the interior, including secondary heat radiation emitted inward by the glazing due to absorbed solar energy.
  • neutral, bright appearance
  • energy savings and reduced heating costs (you can calculate savings using our free heating cost calculator)
  • high building efficiency
  • excellent color neutrality (Ra value)
  • reduction of distracting glare due to low reflectance
  • CO2 reductions
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The History of Glass Windows

The history of window glazing goes back a long way, but it only became relevant in the late 1970s, when windows were still manufactured with single-pane glass. The problem with single-pane glass is that energy—in this case, heat—can escape from the building through the windows. To address this issue, the first insulated glass units were developed. 

Compared to the original single-pane windows, the use of double-pane glass cut the U-value—that is, the heat transfer coefficient—in half. Until a few years before the 20th century, this insulating glass was still installed uncoated. However, it was soon replaced by coated glass. A wave of changes followed: the sealed spaces between the panes were filled with noble gases instead of air, and soon even triple-pane windows were being used. As a result, the U-value continued to drop steadily, and optimal thermal insulation was achieved. 

FAQs

What U-values can I achieve with GM® ISO TOP and GM® ISO TOP Pure – 2-layer and 3-layer?

Depending on the configuration, GM® ISO TOP / TOP Pure covers a range from approximately Ug 1.1–0.9 W/m²K (double-pane) to Ug 0.8–0.4 W/m²K (triple-pane).

What are the g-value and light transmission of GM® ISO TOP Pure—and what does that mean in terms of energy efficiency?

GM® ISO TOP Pure combines very low U-values with high g-values and tv, so that solar gains actively contribute to heating energy savings.

Which glass configurations are standard—and how do SRC, gas, and configuration affect Ug, g, and element thickness?

Standard structures typically have cell thicknesses ranging from 18 to 40 mm; a larger effective area and the use of noble gases improve the Ug value but increase thickness and weight.

What types of “warm edges” or spacers are available—and how do they affect the U-value?

The UNIGLAS® Thermix Swisspacer significantly reduces the edge Ψ-value and typically improves a window’s U-value by about 0.1 W/m²K while maintaining higher interior surface temperatures.

What are the minimum and maximum permitted sizes and weights for elements—including those made of laminated safety glass (LSG) or tempered safety glass (TSG)?

The weight per square meter is roughly 20–30 kg/m²; the maximum panel sizes are determined on a project-by-project basis based on glass structural analysis, the installation configuration, and the specific installation conditions.

How can GM® ISO TOP / TOP Pure be combined with soundproofing, sun protection, safety glass, etc., without significantly reducing the insulation performance?

GM® ISO TOP / TOP Pure can be combined with PHON, SUN, and SAFE to optimize sound insulation, solar control, or safety, while maintaining a U-value within the range typical of modern insulating glass units.

What test reports, standards, and certificates are available for GM® ISO TOP / TOP Pure?

GM® ISO TOP / TOP Pure bears the CE mark and has been certified in accordance with EN 673, EN 410, EN 12600, and EN 356 for the relevant safety and performance requirements.

How does the level of comfort compare to that of older double-pane windows?

By significantly raising interior surface temperatures, GM® ISO TOP Pure reduces drafts and cold air infiltration and noticeably improves thermal comfort around windows.

Why does condensation form on the outside of very well-insulated glass—and what does this indicate?

Condensation on the outside of highly insulating glass is a normal physical phenomenon that indicates excellent insulation and does not constitute a quality defect.

Replacing the glass vs. replacing the entire window – and what energy savings are realistic?

Simply upgrading to GM® ISO TOP / TOP Pure can already result in significant savings on heating costs, while a complete window replacement further optimizes the frame U-value and the sealing details.