Glazing Technologies

How to Improve the Thermal Performance of a Window

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As the thermal insulation of the entire building envelope becomes more important, incremental improvements in thermal performances also become more important.

The science of building envelope design develops all the time and with it, the materials and products used as part of this envelope must also do so.

Emphasis is often placed on the glazing to provide improved performances, with the phrase “I want to improve the thermal performance of the windows” often heard amongst the specification office. But if you want to make the best gains in window insulation, you need to know how Uw values and thermal performances are put together, and what makes them improve.

Learn More: What is a U Value?


Where Does Energy Escape Through a Window?

The first step in improving the performance of your window design is understanding where heat and energy can escape.

Two thirds of the energy lost through a window is lost due to radiated heat through the glazing. The rest of the thermal energy loss is through the frame and spacer bar via conduction.

The energy lost through the glass unit of a window is referred to as the Ug value. The energy lost through the frame is the Uf value and the energy transferred through the spacer is the Psi value.

uw of a window breakdown graphic


How Can I Improve the Thermal Performance of a Window?

As the most energy travels through a window through the glazing, the best gains in thermal performance can be made by improving the thermal performance of the glass unit itself.

If you look back at the development of insulated glass units over time, you can see the importance that the Ug value (thermal performance of the glass) has on the typical Uw value expected of the window.

The best thermal performance improvement for a window can be achieved by improving the Ug value.

If you have reached the maximum possible performance for the glass specification, your next material to improve is the frame.

How Can I Improve the Thermal Performance of a Metal Framed Window?

When using a metal frame it must be thermally broken to ensure that no ‘cold bridging’ occurs across the frame. The technology behind the thermal breaks within metal window frames would be its own entire technical article but what is important to understand is what makes a good thermal break.

Learn more: What is a thermal break and how does it work in Metal Windows and Doors?

A good thermal break will be in line with the gas cavity of the glass unit. This creates a continuous thermal break through the window.

a graphic showing the thermal break of a metal window lining up with the thermal break of the glass

Systems can offer a higher level of overall performance by using multi chamber thermal breaks. This works in the same way that triple glazing does, integrating more air chambers within the insulation for improved performance.

a multi chamber thermal break diagram

What is the Expected Thermal Performance of a Window?

The expected thermal performance of a window will depend on the framing material and opening size. Exact Uw values can be provided for all window installations so that you know from an early stage what performance the window you have chosen will achieve.

Very often, specifiers will have a target performance that glaziers like IQ will have to achieve. We do this through a mixture of improving the glass performance and ensuring the frame performance is the best option for the installation.

a table showing different Uw values of windows

All modern double-glazed units will use a low e coating with an argon gas filling offering Uw values from 1.5 W/m2K to 1.2 W/m2K when used within a thermally broken metal frame.

If you want to achieve Uw values of less than 1.2 W/m2K then you will need to upgrade the glazing specification to triple glazing.

Well-engineered thermally broken metal systems will offer Uw values from 0.8 W/m2K with a triple glazed unit.

If you are looking for a Uw value of less than 0.8 W/m2K that is when you will need to look at improving the performance of the frame for additional gains. Timber is a natural insulator and is a great framing option for window designs that require a thermal performance of better than 0.8 W/m2K.

Uw values from 0.7-0.6W/m2K are very possible with timber systems that integrate triple glazing.

For Uw values of 0.5 W/m2K Passivhaus certified frames are often the best option. These frames use timber as the base material (with its natural insulation characteristics) plus an additional layer of insulation.

a diagram showing the frame build up of a passiv haus certified frame


How Can I Get the Window Thermal Performance I Need?

If you are looking to achieve a specific performance value with your window designs, contact the team at IQ with your design intent and performance requirements. Our expert team will be able to advise you on what will be possible for your design and performance needs.

Want to read more?

Technical Article: Thermal Performance in Modern House Design - All You Need to Know