What is a window energy rating?
A window energy rating (WER) is a scheme used in the UK to assess a window’s efficiency; it shows how well a window product performs from an energy saving and energy efficiency point of view. These ratings – which range from A++ to G – help people to decide on which window or door they would like to have installed in their home.
What does the window energy rating look at?
When deciding on a window’s energy rating, a number of factors are considered, including:
- Thermal efficiency – This is how well the window insulates and prevents heat loss during the colder months; a lower U-value is better, shown in units of Btu/hr·ft²·°F (W/m²·K).
- Air leakage – This looks at how well the window is sealed to prevent drafts. A lower L-value indicates a tighter seal.
- Solar heat gain coefficient (SHGC) – How much heat from the sun is allowed to enter. This is important for balancing heat gain in colder conditions, and is expressed in decimal values.
- Visible transmittance – How much natural light your window allows to filter through. The higher the number, the more natural light allowed through.
How is the performance rating calculated?
Using factors such as the above, a number of figures will be determined, which can then be used in the following formula:
Energy Index = 218.6 x g window – 68.5 x (U window + Air Leakage Factor)
Ways to make your heritage building more energy-efficient
There are two routes you can take to make your heritage building more energy-efficient: passive design strategies and integrating modern technologies. You may choose to take one route, rather than the other, or you could combine both principles
The different window energy rating organisations
Here in the UK, there are actually four energy rating organisations which provide rating and labelling for window manufacturers and installers.
- The British Fenestration Rating Council (BFRC) – An independent and non-profit organisation that sets the standards for window energy performance rating. Ratings from BFRC are shown on a label with an A-G scale – quick and easy for consumers to understand!
- Kitemark – An independent, third-party provider whose ratings are based on U-value, air leakage and solar heat gain coefficient (SHGC) measurements. Their ratings are given to companies who comply with their testing procedures and standards.
- Certass Thermal Rating Register – Backed by the Department of Energy and Climate Change (DECC), the aim of this organisation is to make it easy to compare different energy ratings and products, so they can make a fully informed purchase decision.
- Assure – A comprehensive service that ensures all products comply with legal requirements and meet industry standards.
Window legalities
UK building regulations state that all new replacement windows must achieve a minimum WER of B or a U-value of no more than 1.2 W/m²K. This ensures a baseline level of energy efficiency for new installations.
How to find a WER label
Reputable window manufacturers should provide a label displaying the WER for their products. Look for this label when shopping for windows – it’s your key to understanding the window’s efficiency. It should be displayed prominently and be easy for you to locate and interpret.
What’s the right WER value for my home?
The most ideal WER value can ultimately depend on your location. If your local weather patterns tend to err on the colder side, you’ll want to prioritise high insulation (a low U-value) in order to retain as much heat as possible. Look for WER ratings of A or A+, and double-glazed windows with insulated frames.
If the weather tends to be warmer, look for a balance of solar heat gain (G-value) with insulation. For areas of your home that are south-facing during the summer, consider installing windows with slightly higher U-values, and strategically add overhangs for shade.
And while WER is a great starting point, you should also consider the following when choosing new windows:
- Frame materials: Opt for double-glazed windows with insulated frames (like wood or composite) for superior efficiency compared to single-glazed windows with metal frames.
- Window placement: South-facing windows can benefit from solar gain in winter, while north-facing windows might not require high insulation.
While window efficiency may not be the most interesting topic to get your head around, it’s important for homeowners to try and glean at least a basic understanding. This will allow you to make informed choices so you can create a more comfortable and energy-efficient home.
