Window Energy Ratings
All replacement window installations are subject to Building Regulation requirements These regulations (Part L in England and Wales and Part F in Northern Ireland) have recognised Window Energy Ratings (WER) as an alternative method of compliance. Replacement windows and windows installed in new buildings must have a minimum WER of C or a whole window u-value of no more than 1.6 W/m2K.
But what are Window Energy Ratings and u-values?
Window Energy Ratings (or WERs) were launched in early 2004 by the British Fenestration Rating Council (BFRC), an independent, government recognised organisation dedicated to improving the energy efficiency of window frames, glazing and related components. It is the UK’s national system for rating the energy efficiency of windows.
Window Energy Rating schemes are operated by three Government approved providers in the UK:
- Certass Thermal Rating Register Scheme
- BSI Kitemark Scheme for WER
- British Fenestration Rating Council WER Scheme
How is a window energy rating calculated?
A window’s WER is determined by a formula which takes into account three factors:
Available solar heat gains (window g-value), i.e. how much free heat and light is captured from the sun, minus thermal losses (window u-value) i.e how well heat is retained and radiated back into the room, and air leakage
(L factor) i.e the amount of heat that is lost due to air penetration such as draughts and cold spots.
The resulting numerical value (known as an energy index) is then placed into a band on an A-G scale consistent with other energy performance labels already familiar to the consumer.
Each label will display the following information:
- The rating level – A, B, C, etc.
- The energy index.
- The thermal transmittance / whole window U-value
- The solar factor / g-value
- The effective air leakage / L factor
Simply put, this determines how well a product will:
- Retain heat within the building
- Capture heat and light from the sun
- Keep out draughts and eliminate cold spots
- Resist condensation
Benefits of Window Energy Ratings
Increasing energy costs and growing concerns for climate change are contributing to consumer demand for energy efficient products.
With a WER certification, the consumer can make an informed and responsible choice regarding the environmental impact and energy efficiency of the windows that they purchase.
For the fenestration industry WERs present many benefits:
- Enables manufacturers to select the most cost effective components to improve their energy ratings
- Rates product performance using a standardised and well recognised method
- Positively differentiates products so that they can be marketed accordingly
- Clearly presents the technical advantages of upgrading existing glazing
- Demonstrates compliance with Building Regulations
- Reassures consumers with independent accreditation
- Promotes the use of energy efficient products that may be incentivised by the government and endorsed by third parties like the Energy Saving Trust.