What Are Thermally Broken Aluminium Windows?

Aluminium is in most ways the perfect material for constructing windows and doors: it is incredibly strong, light, weather proof and needs very little maintenance to provide years of reliable use. However, aluminium is also a very good conductor of heat, which means that on its own it provides very poor insulation against the winter weather and provides little protection from the summer sun.

Despite sounding like an unfortunate manufacturing defect, “thermally broken” aluminium windows and doors immediately solve this insulation problem by using a “thermal break” (sometimes known as a “thermal barrier”). This is a piece of material that doesn’t conduct heat at all, used to separate two materials or structures that do conduct heat. It acts as a barrier between the two to prevent heat transferring between them. In the case of aluminium windows, its main benefit is to prevent heat transferring from the inner frame to the outer frame – and to prevent the cold moving in from the outer to the inner.

So thermally broken aluminium windows and doors use a barrier in this way to improve insulation and energy efficiency. These days, the thermal barrier conducts up to 1,000 times less heat than aluminium, so it provides a tremendous reduction in heat loss and increases thermal performance. It also has the side effect of making your windows more soundproof, by dampening vibrations between the outer and inner frame.


A brief history of thermally broken aluminium windows

Until only about 5 years ago this is how most aluminium window frame sections were produced:

Aluminium would be extruded to form the entire section of the frame in a single piece, and because aluminium is an excellent conductor its thermal performance was poor – there was no insulation to prevent heat escaping to the outer frame of the window.

To combat this, these full frame sections would then have to go through a process called “Cut and Pour”: The frame section would be cut in half and then the two halves were joined back together with a polymer called polyurethane between them to create the thermal break.

Polyurethane is a very useful and durable material – it’s still used to make everything from flexible foam seating to roller coaster wheels! – but it is no match for the strength and expansion properties of aluminium. So only very narrow barriers of polyurethane could be used, which meant that the actual improvement in insulation wasn’t as good as it could have been. The thermal break also caused the frame to lose its stiffness and rigidity – and the polyurethane had a tendency to shrink as it aged, causing leaks in the frame.

Also, different colours inside and outside further complicated the “Cut and Pour” process, which meant that if you wanted your windows and doors to have dual colours things got very expensive and time consuming.


The modern process

Now, thanks to improvements in aluminium window design and manufacturing – as well as advances in polymer technology – the process to thermally break windows and doors is far more effective:

The aluminium frame sections are now manufactured in two halves, designed to fit together perfectly in the final product.

The material used for the thermal break is now a high strength polyamide that has very similar strength and expansion properties to aluminium.

The polyamide thermal break is now mechanically joined to the frame sections to maintain the frame’s stiffness and rigidity – and the width of the break can now be much wider which means that the heat insulation and thermal performance is several times greater than before.

Each half of the frame can now be painted separately and then easily joined either side of the thermal break, so dual colour options are much easier and more cost-effective.


I hope this summary of the benefits and process of thermally broken aluminium windows has been useful (and not too technical!) – if you have any questions on anything raised in this article please leave a comment below or call us on 0203 00 22 088.


This article is part of our Where Do I Start? series. To read the next article, please click here.



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