What are Window U-values, and How Do They Function?

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When shopping for new windows and doors for your home, you will often see specifications referencing U-Values and Window Energy Ratings. These metrics relate to the windows’ and doors’ insulating properties and energy efficiency, which are determined mainly by the glass options.

What Are U-Values and What Are Window Energy Ratings?

The U-Value for windows measures their ability to retain heat. It indicates how fast heat escapes through the window to the outside.

U-Values are expressed in W/m^2 K (Watts per square meter Kelvin). The lower the U-Value number, the better the window performs thermally.

Windows also have Energy Ratings similar to appliance energy labels – from A to G, with A being the most efficient. The Energy Rating provides an easy way to understand a window’s energy-saving properties.

Property MaterialInsulationApproximate
Property RoofWith insulation0.5
Property RoofWithout insulation2.4
Cavity WallWith insulation0.7
Cavity WallWithout insulation 1.7
Typical FloorWith Insulation 0.7
Typical FloorWithout Insulation0.9
Old single-glazed windows None5.8-5.9
Old Double Glazed windows with air space between the glassNone2.8-3.0
Current double-glazed units, argon gas, low-E coating1.2-1.2
Latest triple-glazed units, argon gas, low-E coating0.9-1.0

While some windows list U-Values and others Energy Ratings have become more commonly used. Both measure similar properties – retaining heat and resisting heat loss.

The key things to look for energy efficient windows are:

  • A low U-Value (below 1.2 W/m^2 K for the window overall)
  • A high Energy Rating (A, A+, or B)

This indicates the window will have good thermal insulation to retain the heat inside your home.

How Are U-Values and Window Energy Ratings Calculated?

There is a formula from building regulations to calculate the energy efficiency of windows using two metrics: Window’s U-Value Energy Rating (WER). The procedure estimates how much energy a typical home would use in a year with different window styles and placements.

The formula provides an estimate based on assumptions. A home’s actual energy use with specific windows could be more or less than the calculated estimate. The formula gives a general idea of how different window options compare, but actual performance can vary depending on real-world factors.

What Is The Minimum U-Value And Window Energy Rating For Windows?

The minimum requirement for new or replacement windows in the UK is a Window Energy Rating of C or higher or a U-value of 1.6 W/m2K or less. This indicates the level of insulation and efficiency the window provides.

Older double-glazed windows from before 2006 often used air-filled cavities and basic metal spacers. They typically have higher U-values and lower energy ratings.

Modern windows use low-emissivity coatings, argon or krypton gas fills, and warm edge spacers to achieve lower U-values and higher energy ratings. These technologies substantially improve thermal efficiency.

Standard PVCu and aluminium windows will typically achieve the required U-value and energy rating through the use of:

  • Low-E glass coatings
  • Argon gas fill
  • Warm edge spacers

Obscure patterned or textured glass in windows will use the exact high-performance glass unit specifications, with the internal pane having the pattern.

As a comparison, typical U-values for other building elements are:

  • Roofs: 0.13 to 0.20 W/m2K
  • Walls: 0.18 to 0.25 W/m2K
  • Floors: 0.15 to 0.20 W/m2K

So the minimum requirement of 1.6 W/m2K for windows represents a substantial improvement over older, basic double glazing.

U-Values of Different Glazing Options

Typical I-I-Values On Windows And Window Energy Ratings On Aluminium And Pvcu Windows And Doors

Origin Aluminum Bifolding Doors

Origin aluminium bi-folding doors feature Pilkington KTMS toughened glass as standard. This includes warm edge spacer bars, soft-coat low-e coating, and argon gas filling for improved energy efficiency. Patterned glass is available for areas requiring obscure glazing.


  • Double-glazed Origin bi-fold doors – U-value: 1.6 W/(m2K)
  • Triple-glazed Origin bi-fold doors (with krypton) – U-value: 1.3 W/(m2K)
  • Triple-glazed Origin bi-fold doors (with argon)- U-value: 1.5 W/(m2K)
  • Energy Rating: D

The low U-values ensure this Origin aluminium bi-folding doors provide excellent thermal insulation and energy efficiency for your home. The toughened low-e glass helps retain heat within your property while reducing heat loss.

PVCU Windows

These multipurpose plastic windows from the inventory offer outstanding insulation all year round. The multi-chamber profiles result from a specialized extrusion process that segments the frames into insulated sections. This compartmentalized structure reduces heat transfer, keeping interiors cool in summer and warm in winter.

You’ll also find features like bubble gaskets and fin-type glazing seals that form an airtight barrier between the frames and glass panes. Together with high-performance double-glazed units, our standard casement windows deliver U-values as low as 1.2 W/m2K and qualify for an A+ window energy rating.

For even greater insulation, consider the EnergyPlus series. The 6-chamber technology facilitates U-values as low as 0.8 W/m2K when paired with the right double or triple-glazed windows.

The 9-chamber Residence models push the envelope further with class-leading insulation metrics. Homeowners benefit from U-values as low as 0.8 W/m2K and window energy ratings as high as A++.

Aluminum Windows

Origin aluminium windows can achieve various energy efficiency levels depending on the glass specifications.

Standard double glazing with 28mm cavity width and warm edge spacer bars provides a U-Value of around 1.7 W/(m2K). This would be around Energy Rating B on the BFRC scale.

Upgrading to double glazing with aerogel-filled cavities can improve this to around 1.5 W/(m2K), achieving around Energy Rating A+ on the BFRC scale.

Origin windows are available with 44mm triple glazing for even better insulation. This can achieve a U-Value as low as 1.2 W/(m2K), reaching around Energy Rating A+ at the top end.

The highest-performance Origin windows utilize 44mm triple glazing with two 16mm cavities filled with argon gas and warm edge spacers. Combined with aerogel-filled insulating glass units, this can achieve a U-Value as low as 1.0 W/(m2K). This would place the windows at the top of the BFRC scale at Energy Rating A+21.


This guide provides approximate U-Values and window energy ratings based on company data and specifications. However, the actual ratings for a specific window can vary due to the glass specification, window configuration, and hardware. Please contact us for the most accurate window ratings for your specific needs. We can provide a detailed assessment based on your requirements and property.

About the author

Glen Ramsey, who presently holds the positions of customer support and content manager, is a cherished member of the Sash Window Refurbish team. He is in a unique position to provide advice on all questions pertaining to the renovation of historic windows because he has a wealth of experience in all elements of window renovation, dealing with clients regularly and assisting them in choosing the best window-related course of action. He oversees the creation of information for all of our platforms in his capacity as content manager, including web content and other materials. Glen has witnessed the whole gamut of Sash Window Refurbish's window restoration activities, from our work on significant listed Georgian homes to a wide variety of smaller-scale private residential projects.


My technician Andrew replaced all my sash windows that had rotten sills. Accessing the rotten sills was not an easy task. He explained to me what needed to be done in order to fix the problem. He is a great person and I’m happy that he was the one who did the job. He is reliable, tidy, friendly and very professional. I highly recommend him.



I hired Andrew and Alex a while back to repair my damaged sash windows. They were was very considerate and very patient as he did his job. A few days ago, I had another problem with one of my sash lock keys and I called him again to come and fix the problem. I’m very impressed that Liam responded quickly and he not only fixed the problem but he also gave me a spare key. What a great after sales service!

Alan W.


I’m very impressed with the new windows that Andrew installed and also the reconditioning job that he did on other sash windows. The quality of his work is by far more superior than what we got when we hired another company a few years back. We will certainly hire him again if any window work arises

Kelly (Sleaford)


Our decorator referred us to Andrew because some of the work needed a window expert. Andrew together with our decorator did a fantastic job. The work was done efficiently and to the highest standards. Andrew is understanding, committed and very professional. I recommend him without any hesitation.

Steaven Reader


Alex did a great job in fixing windows in our terraced house. The windows were broken, frames damaged and the wood was also rotten. Alex's job was high quality and he did it pretty fast. I was also very impressed that he was considerate of my son’s asthma condition and maintained cleanliness. Top work.



Andrew just recently replaced three sash windows in my Edwardian house. He also repaired and replaced some sills and draught proofed three windows. In addition to that, he also did some carpentry work. What I like the most about Liam is that he did a clean job. He is very professional and always updated me about the timescales and costs. He also did his job with minimal disruptions. I highly recommend him.

Cris M.