Window & Door Energy Ratings Explained

Published 20 November 2023 by Sternfenster
Categories Updates

The energy efficiency of a home’s windows and doors is extremely important. Large amounts of heat are lost through a property’s windows and doors, and more energy efficient designs can help homes to stay warmer for longer. To help homeowners find the most energy efficient products, many modern windows and doors come with an energy rating label. 

At Sternfenster, we are market-leading fabricators of uPVC and aluminium windows and doors, and our experts are here to share everything you need to know about energy ratings. 

Why Does Energy Efficiency Matter?

Before we get into the details of energy ratings, let’s first take a closer look at energy efficiency. Windows and doors act as a barrier between the inside of a property and the natural elements outside. Energy efficient products are designed to keep homes warmer for less, allowing homeowners to relax in comfort and enjoy reduced heating bills. 

With an energy efficient window and door installation, homes will benefit from reduced heat loss, better weather protection and improved heat regulation. Single glazing is known to offer poor levels of energy efficiency. Heat can easily escape through the single layer of glass, and cold draughts have no trouble making their way inside.

With energy costs as high as they are, replacing outdated windows and doors with energy efficient alternatives is a wise investment for homeowners in 2023. 

high energy efficient windows

What Are Energy Ratings?

Energy ratings are achieved once windows and doors have been assessed by The British Fenestration Rating Council. The BFRC is the most trusted organisation in the UK for assessing the energy efficiency of windows and doors. A rainbow label is given to manufacturers once their products have been tested, allowing potential customers to easily see how energy efficient the window or door is.

It is not a legal requirement to provide an energy rating for window and door products, but the BFRC label is widely recognised as the best way to determine energy efficiency. The BFRC rates windows and doors from A++ to E. A++ products perform the best and offer the highest levels of energy efficiency. To ensure all products are accurately rated, the BFRC also monitors the integrity of the supply chain. 

When windows and doors receive a genuine BFRC rating, they are labelled with a small sticker on the inside of the frame and a visible, larger rainbow sticker too. For maximum levels of energy efficiency, homeowners should invest in windows and doors with an A++ rating. 

Energy Rating Assessment Process

window energy ratings

When assessing the energy efficiency of windows and doors, the BFRC test the following:

  • Solar gain 
  • Thermal heat loss 
  • Air leakage 

Solar gain is a positive attribute of windows and doors, but thermal heat loss and air leakage can negatively impact overall energy efficiency. The BFRC tests windows in each of these areas and uses the results to create an accurate energy rating. 

WER (window energy ratings) and DER (door energy ratings) are determined once the product’s thermal performance has been thoroughly assessed.  The BFRC uses the following measurements to determine energy efficiency: 

  • U-Values – U-Values measure heat transference and demonstrate how easily heat passes through a material. The lower the U-Value, the less heat is lost and the more energy efficient the glass is. The BFRC calculates U-Values to assess windows and doors’ thermal heat loss. U-Values are measured in watts per square metre per Kelvin – W/m2K – and this is calculated by dividing the heat transference rate by the temperature difference on either side of the door or window.
  • G-Values – This measurement is used to calculate how much of the sun’s heat passes through windows and doors. Products with a G-Value of 0 completely block heat from the sun, and products that receive a value of 1 let all of the solar heat through. The best windows and doors often have a G-Value between 0 and 1, as while solar gain is beneficial in the winter, too much can cause homes to overheat during the summer months.
  • L-Values – The L-Value is needed to measure air leakage. This test calculates how airtight windows and doors are and how effective they are at trapping warm air inside while also locking cold air out. L-Values are also measured in  W/m2K, and lower values mean less air leakage. 

When calculating the energy efficiency of windows and doors, the BFRC also take the following into consideration:

  • Property size 
  • Occupancy 
  • Climate data 
  • Window size

The energy rating system allows homeowners to make better decisions when it comes to upgrading their existing windows and doors. Products with better ratings will enable homeowners to save money on energy bills, be more environmentally friendly and keep their property at a comfortable temperature throughout every season. 

Energy Efficient Windows & Doors From Sternfenster

At Sternfenster, we are leading fabricators of energy efficient windows and doors in the UK. We help homeowners to enjoy maximum levels of thermal performance and combat rising energy costs.

Our standard reinforced double glazed units achieve a u-value of 1.2 W/m²K, with a window overall u-value of 1.4 W/m²K. For even higher energy efficient windows and noise insulation, we also offer a range of triple glazed units with a u-value of 0.6 W/m²K (window overall u-value 1.0 W/m²K).

energy efficient windows and doors

We are also proud to supply double and triple glazed units that feature revolutionary high performance low-E glass. This glass has been optimised for Window Energy Ratings (WER) and is the most energy efficient low-E glass available on the market today.

To learn more about our energy-efficient windows and doors, contact our team today at 01522 51 25 25. Alternatively, get in touch with Sternfesnter by filling out our online contact form. 

Use our online tool to receive a bespoke quote for any of the products in our extensive window and door collections. Alternatively, find an approved installer in your local area. 

If you are interested in selling Sternfenster windows and doors, we would be interested in hearing from you. Please send your enquiry to

Latest from Sternfenster


© 2024 Sternfenster Window Systems

Terms & Conditions Privacy Policy

Start Selling Sternfenster

Join Our Network

Outsource Fabrication

Start Your Quote

Find an Installer

I'm a Trade Customer