What is the U Value Of Aluminium Windows?

Carmakers have for decades offered different trims and options for their models to cater for their customers’ varying needs and budgets. The base model may provide basic performance that suits most requirements, while the fully equipped top-of-the-range offers some extra pizzazz for those who demand maximum performance.

These days the same is true of aluminium window systems: manufacturers now offer various products in their portfolio to meet the budgets and performance requirements of customers and their houses. There are several factors that will influence the cost of your glazing project – but the “U-Value” determines budget more than anything else.

That’s because it’s primarily a measure of protection from the weather – and here in the UK how protected you are from the cold and wind is arguably more important to your windows purchase than anything else.

So What Exactly Does the U-Value Measure?

Bear with me here – I’m about to get technical, but I’ll switch back to English as soon as I can.

Heat is lost (although sometimes gained) through the building as a whole, and heat flow (also cited as heat transfer or thermal transmittance) happens, to varying degrees, in any building.

Heat flow may be quantified, and subsequently conveyed, as “U-Value” (or thermal transmittance coefficient) being the heat flow through one square metre of a building when the temperature on either side of the building differs by one degree Celsius.

In other words, the U-Value measures how much heat is lost by passing through building material – whether that’s floors, walls, roofs or windows. The lower the number, the less heat is lost – which effectively means the material has better insulation. Better insulation keeps the cold out, keeps the warm in, and reduces your energy bills.

How Low Can You Go?

So now you want to aim for as low a U-Value as possible to keep out the Great British Weather. U-Values range considerably according to budget and quality of the product, but even our entry-level ECO window system has an excellent U-Value of just 1.6W/m2k – so it comfortably passes current Document L Building Requirements.

At the other end of the spectrum we offer industry leading high thermal performance windows in our CS104 system. With a U-Value as low as 0.77W/m2k this product is a fantastic choice for PassivHaus construction.

The U-Value and Cost Depends on The Window’s Construction

How is there such a difference in U-Value between these two systems and why does one cost more than the other? To explain this you just need to examine them closely side-by-side.

Aluminium Windows U Values
Eco Window
Aluminium Windows U Value
CS104 Window














You can instantly see that the CS104 has many more cavities in both the outer frame and vent section (Eco has 2 cavities and CS104 has 6 cavities). Note also that the CS104’s innermost cavities are filled with insulation material allowing it to further reduce the heat lost through the window.

As well as having better thermal performance the CS104 has superior air, wind and water performance. This is partly to do with the size and rigidity of the actual profiles as well as the far more advanced rubber gaskets between outer frame and vent frame.

So the price difference between the low U-Value and the higher U-Value is largely due to the better design and better supporting materials of the high performance window system.

Double or Triple Glazing?

One last difference you may have noticed is that the CS104 has triple glazing while the Eco Window has standard double-glazing. This is a little misleading as the Eco Window system can also incorporate triple glazing. But as Eco is a slimmer (not as strong) system it has limitations as to how much weight it can carry. Therefore triple glazing can only be used in certain size Eco windows, while CS104 can accommodate triple glazing at almost limitless sizes.

I hope this brief introduction has helped you to understand what U-Values are, the impact they have on your project, and the reasons for difference in price and ratings between different window systems.

If you have any questions please leave a comment below and we’ll be happy to answer them.

Or call us on 0203 00 22 088 for further information on this subject or any of our aluminium window systems.


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



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