Double glazing - The science behind the energy ratings

21st March 2016

composite front door and double glazed window Secure, good looking and energy efficient

Craig Smith, Director at Copperfield Windows, discusses the science behind the energy ratings for windows and doors in the UK.

Choosing new energy efficient windows and doors is a big decision. For most of us it’s one of the biggest purchases we make and if we get wrong, it’s there staring us in the face each and every time we come home. Energy efficiency and the desire to cut the cost of heating their home, is the main reason why most of us embark on the window and door replacement journey. More info on our range of A-Rated Windows and Doors can be found at and

Measuring PVC-U windows and doors' energy efficiency?

There are two systems for rating the performance of windows and doors – U-values and Window Energy Ratings (WERs). U-Values assess a product on how effective it is at keeping heat in or how much heat is lost through it, as a single measure of performance.

Window Energy Ratings do the same and use the U-Value (heat loss) but also balance heat loss against solar gain (warmth which passes into the room from the sun) and also throw into the mix additional heat loss through air penetration, giving windows a ranking from ‘A/A++’ to ‘G’ using a ‘traffic-light’ type colour code. The Door Energy Ratings Scheme replicates the same model.

What level of performance do replacement windows need to meet?

To meet building regulations, all new windows must achieve a minimum WER ‘C’ rating or U-value of 1.6 or less. 

How do energy efficient windows work?

Glass is the critical element in energy efficient windows. They work using two sheets of glass in the case of double and three, for triple-glazed units with a gap in between - usually about 16mm - to create an insulating barrier that keeps heat in. Performance is further enhanced using warm-edge spacers, low emissivity gases, for example, argon, xenon or krypton to fill the gap and low-iron glass.

What are the benefits of energy efficient PVC windows and doors?

According to the Energy Saving Trust, fitting new energy efficient double-glazed PVC windows and doors, in a detached three bedroom home, can deliver an annual saving over and above single glazed windows of up to £160 a year in heating bills.

But they also deliver other benefits. New energy efficient and low maintenance PVC-U windows and doors can help to reduce condensation, while high security window and door multi-point locking mechanisms, also make homes far more secure.

What sort of energy rating do I require for my home?

It depends on you, the aspect of your property and how much energy you want to save. Copperfield Windows, for example, supplies a thermally efficient triple-glazed window with a U-value as low as 0.8W/m².K. It’s a great window but with more glass and more complex handling, costs more. Energy efficient double-glazed windows deliver great performancee. We work with our customers to model heat loss and to develop a solution that’s right for their individual requirements and the construct of their property.

For more information, please don’t hesitate to email me on or call a member of our team on 02476 452688.

Request a free brochure here!

Your login details have been used by another user or machine. Login details can only be used once at any one time so you have therefore automatically been logged out. Please contact your sites administrator if you believe this other user or machine has unauthorised access.