The ultimate guide to Window U-Values

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Nick Dardalis

When it comes to choosing windows and doors for your home or project, window U-Values must be considered in your choice of windows or doors, alongside how they look. Of course, styling, security, weather protection and choosing the right provider are important. However, the performance of your windows, particularly in terms of energy efficiency, is a crucial factor. It is window U-Values that are the critical measurement. This guide aims to simplify U-Values, explain to you how they work, why they matter, and why they should influence your choice of aluminium windows and doors.

Why U-Values should influence the windows and doors you buy

Of course, you want the best windows for your project, and it is tempting to focus on how the windows look and the visual difference they can make to the renovation or building project. Window U-Values matter a lot because they directly impact your building’s energy efficiency, comfort, carbon footprint, and energy bills. Materials with lower U-Values provide better insulation, which means less heat loss in the winter and less heat gain in the summer. The windows’ appearance, security, price and confidence in your chosen window company remain important buying decisions. However, window U-Values should not be overlooked.

What is a U-Value?

The U-Value of a window or door represents its thermal transmittance, that is, the rate at which heat transfers through the material. This measurement is crucial for assessing a product’s energy efficiency; the lower the U-Value, the better the insulation and energy efficiency.

Window U-Values and Building Regulations

Under Part L of the Building Regulations 2022 in the UK, replacement windows in existing buildings or new windows in new buildings should have the required window U-Values. This requirement aims to ensure new or replacement windows contribute to reducing energy loss and, consequently, lower carbon emissions and meeting these Building Regulations.

New DwellingsU-Value (Wm2K)Existing DwellingsTypical U-Value (W/m²K)

The difference between U-Value, G-Value, and R-Value

While the U-Value measures how well a material insulates, the G-Value refers to solar gain. It measures how much of the sun’s heat energy can pass through a window – the lower the G-Value, the less solar heat it transmits. On the other hand, the R-Value indicates thermal resistance. The higher the R-Value, the better the thermal performance.

How U-Values are achieved on aluminium windows and doors

The low U-Values with aluminium windows come about as a result of good design and the right materials. First are the thermally insulated aluminium profiles.

Most systems create a thermal break (separating the inside and outside faces of the window) with a suitable material, typically polyamide. The polyamide thermal break creates an effective barrier, preventing the transfer of cold to the inside of the window.

cross section of triple glazed insulated aluminium window samples to explain window u-values

Whilst polyamide is the most popular, other brands of windows use even more ways to enhance the thermal performance of their windows. Origin OW-80 windows come with an upgrade to the aerogel material in addition to the polyamide. Senior Architectural are another company providing windows with very low U-Values, using PURe – an enhanced thermal barrier made from expanded polyurethane.

Schüco and Reynaers are top-tier aluminium window brands, alongside polyamide, enhance their windows with additional seals, gaskets, foams or other insulation materials, again bringing window U-Values down to very low numbers.

Glass is a significant component of any window, and the lower the glass U-Value, the more it contributes to the overall window performance, including the frame. High-specification double or triple glazing, filled with an insulating gas like argon or Krypton, alongside warm-edge spacer bars and glass coatings, also contributes to lower window U-Values. The better the glass performance, the lower the U-Values.

Calculating U-Values for windows and doors, therefore, depends on multiple factors. The window frame make-up and the overall design matter. Window U-Values also change depending on the glass specification chosen.

How window U-Values are calculated

Window U-Values are calculated using simulation software usually, taking three different measurements related to heat transfer, also called U-values:

  1. Uw (w stands for window) – This is the total heat transfer value for the whole window.
  2. Ug (g stands for glazing) – This is the heat transfer value for the window glazing or glass.
  3. Uf (f stands for frame) – This is the heat transfer value for the window frame.

The Uw value, which refers to the entire window, includes the U-values of the glazing (Ug) and the frame (Uf). The size of the window and the heat transfer rate of the glazing also affect this total Uw value.

Very important is how window sizes affect U-Values. Simulations to establish U-Values are typically calculated using a standard window size of 1.23 m x 1.48 m. This is called the CEN standard developed by the European Committee for Standardisation (CEN).

Importantly, as the window size shrinks, the U-value gets worse. Larger windows have better U-values because the heat transfer rate in the window glass is better than in the frame. Therefore, a bigger glass area can provide better thermal insulation.

Window U-Values vs Window Energy Ratings

You will probably come across both window U-Values and Window Energy Ratings in your research for new windows and doors. We’ve explained, in detail, the differences between these two ratings. For this article, U-Values and Window Energy Ratings (WERs) provide insight into a window’s energy efficiency, with the U-Value focusing specifically on heat loss and WERs giving a broader picture of a window’s performance.


Window U-Values are important both for meeting Building Regulations and giving you a more economical, warmer and comfortable home. So it is worth considering window U-Values for your replacement or new build windows. Thanks to technological advancements in aluminium, insulation, glass and design, aluminium windows and doors now offer excellent thermal efficiency, making them an excellent choice.

The Aluminium Trade Supply Website can help you choose the best windows and doors for your project and our unique aluminium information resource helps thousands of homeowners a year find the right doors and windows from the best suppliers. Get in touch for free help and advice.