A guide to the thermal performance of architectural glazing 

Architectural glazing is essential for a building’s thermal performance and energy efficiency. Understanding the thermal characteristics of windows is crucial for architects, builders, and homeowners alike as the need for sustainable and energy-efficient solutions rises. In this detailed guide, we’ll explore the variables affecting architectural glazing’s thermal efficiency and offer useful guidance to support your decision-making process.  

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What is a U value for glazing? 

When speaking about U values in glazing, there are various terms that you will hear architects and builders refer to. Each U value indicates the glazing thermal performance values for the different components of the system. For example, a Uf value tells you the thermal performance of the framing around the glazing. A Uw value tells you the performance of the system as a whole. A Ug value will tell you how quickly heat transfers through the glass, IQ products typically have a Ug of 1.0-1.1. For triple glazing the Ug if 0.4-0.5. 

Another term commonly used is the G factor/value of the glass. This indicates the glazing thermal performance regarding solar radiation. If glass has a rating of G1.0, this will mean that 100% of the solar radiation is passing through. Therefore, if the glass scores a G0.2 then only 20% of the solar radiation will pass through. 

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How is the glazing thermal performance measured?

The thermal performance for glazing is measured using a metric called U value (as discussed above), this measurement is then expressed in units of watts per square meter per kelvin (W/m2K).   

This expression is determined by calculating all the U values as well as taking in to account the size, thickness, configuration and frame type of the glass. The testing process will be done within a laboratory where you can control the environment that the glass is placed in. This allows for highly accurate testing and also pushes the glass to its limits before it would melt, distort or shatter.  

What is a good U-value for glazing thermal performance?

A good U-value for glazing can depend on a variety of factors which include, climate, building type and the goals of the building in terms of what thermal performance they need. A lower U-value indicates better thermal insulation and a reduction in heat loss. While this is usually the main goal, specific U-value targets can vary based on local building codes, energy efficiency standards and project requirements.  

In colder climates a lower U-value is desirable to minimise heat loss and improve the energy efficiency. In these cases, a value below 1.0 W/m2K is considered good.  

In milder climates, the majority of glazing needs to adapt to the changing weather throughout the year. This allows a balance between insulation and solar heat gain. U-values of 1.2-1.8 W/m2K are considered good.  

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Glazing thermal performance of Triple Glazing 

Triple glazing has the most benefits that will increase the thermal performance of a system. Triple glazing is always the better option opposed to double glazing thanks to the extra pane of glass and gas cavity which is becoming more standard. Triple glazing is great for colder climates as it will retain more heat.  

On the other hand, triple glazing is also good for hot climates as the triple glazing can keep the exterior heat from penetrating into the interior to ensure the space will not overheat.  

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