Understanding U-Value Ratings: How to Get ‘A-Rated’ Glazing for Your Windows and Doors

As the cost of living soars and energy prices continue to rise, glazing manufacturers in the UK need to demonstrate that their glass materials comply with energy efficiency requirements. It’s becoming increasingly important that buildings help to reduce carbon emissions, whilst also decreasing the cost of rising energy bills, and glass u values are an integral aspect of this.


25 Nov 2022


Simon McAuliffe

Part L of the Building Regulations and the Approved Documents L1A (new dwellings) and L1B (existing dwellings) requires that all glass installations and products must have their U-value declared, and must achieve a value of 1.6W/m²K or less when installed in residential buildings. These regulations are controlled by various local authorities and government-approved agencies, and the rating system shows the level of insulation performance to expect from doors and windows. 

But what exactly does this mean – for example, is a higher U-value better? Are U-values and energy ratings the same thing? This article will explain everything you need to know about understanding U-value ratings and how to apply this technical knowledge to your glazing solutions to ensure that you’re compliant with legal requirements and regulations.- 1.6W/m2k means you are compliant, but we all have a responsibility to do more when we can.

What are U-Values?

U-values are found on all building components that offer insulation against cold, including windows, doors, bricks, and insulation itself. Many of these elements have to comply with thermal standards that are expressed as a maximum U value (W/m²K); this describes how much thermal energy in Watts [W] travels through 1 square metre [m2] of a building component at a temperature difference on either side of 1 Kelvin [K] (=1°C). The values generally range from 0.25 to 1.5.

The value is a scientific measure of how well these building materials work as a thermal insulator. High U-values mean more heat flow, so the lower the figure, the better the insulation. This is about temperature transmission through a material, keeping heat in during winter and heat out during summer – essentially the lower the U-value, the greater the performance in terms of how much energy is required to maintain a comfortable temperature within a property. 

Whilst the U-value indicates the insulation levels, it does not necessarily provide the energy efficiency of the material. For example, a window with a high-energy performance rating may also have a high U-value, as other aspects of the window or the door may assist the overall efficiency, such as the coating used, or the gap between the glass panes. The U-value simply measures the rate of heat loss per m2 of surface, telling you how well the glass and frame insulates – not how efficient the installation is. That is where energy ratings come in.

What are Window Energy Ratings (WER)?

Energy ratings were introduced by the British Ratings Fenestration Council (BRFC –  The premier UK authority for independently verified ratings of energy efficient windows and energy efficient doors) to provide an overall rating that’s better understood and recognised by consumers. 

Energy ratings not only take into account the U-value, but several other factors too, including the solar gain (G-value) and air leakage of a product. The whole window (the frame and the glass) is assessed on its efficiency at maintaining temperature – thus giving a clearer and more accurate reading.

Most window manufacturers show the energy efficiency of their products using an energy rating performance scale from A++(most efficient) to G (least efficient). ‘C’ is the minimum to satisfy Building Regulations for windows and ‘E’ for doors (except in exceptional circumstances). The rainbow scale used is the same as the one used to show overall energy efficiency in everything from hoovers to houses.

What the law says:

To meet the UK’s commitments to lower domestic energy consumption and reduced carbon emissions, legislation requires all replacement windows to meet energy efficiency standards.


These require new build or replacement windows to satisfy at least one of the following criteria:

  • To have a whole window ‘U’ value of 1.6 W/m²K or lower.
  • To have a minimum window energy rating (WER) of ‘C’.
  • To use sealed units with a ‘U’ value of better than 1.2 W/m²K with argon gas fill and warm edge spacer bars.


At the moment there is no energy rating scheme for doors, but all doors with more than 50% of their total area glazed must have a U-value of 1.6 W/m²K or lower in residential buildings and 1.8W/m2k for commercial buildings. 

What are the benefits of U-Values?

Understanding the U-values of any glazing solution is important in knowing how energy efficient it is. With the high cost of gas and electricity, consumers are demanding more energy efficient products, as a low U-value means a more comfortable indoor temperature, fewer draughts and cold spots, along with cheaper heating bills. Perhaps most importantly a lower U-Value also means lower carbon emissions. 

Architects and specifiers need the thermal transmittance values of windows, in order to calculate the SAP or SBEM ratings of the buildings into which the windows are to be fitted, and in order to establish thermal efficiency. You should be very concerned if a glazing manufacturer cannot provide U-values for their products. You also need to be very careful if you are  given the “centre-pane” U-value, which relates only to the glazing and not the frame as well.

Whilst it can often be tempting to choose a lower grade, cheaper glass material, this can lead to higher costs in the future – not only in bills for your clients, but also if the time comes when the glazing needs to be replaced because it has not provided an efficient enough solution in the first instance. New build properties, in particular, are increasingly subject to minimum energy ratings, so future-proofing now by providing the most efficient solution at the design-and-build stage is imperative.

And with energy bills on the increase, putting pressure on families heating their homes, A+ rated double or triple glazing provides the ideal solution to a warmer, welcoming and more comforting home.Whilst every glazing solution Cantifix sells already achieves a U-value of 1.8W/m²K or less, we also offer the option to upgrade further, with many of our projects achieving far lower U-values than the minimum requirement for the given scenario. Cantifix offers thermal upgrade and triple glazed options, which will both improve the thermal efficiency of your glass, increase the overall energy rating and improve the U-value. For more information, contact us here.