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Getting The Measure Of U-Values And WER Ratings

8 June 2016

u values explainedDon’t be left out in the cold with these important figures

Whether you’re a homeowner, tenant or window installer, it’s important to understand the ratings systems that tell you the level of insulation performance to expect from your doors and windows.

If you’re in the first two groups, it’s a quick way to find out just how efficient your household insulation will be (don’t forget, up to 25% of the heat within most homes escapes through the windows) and if you’ll need to wrap up warm in winter.

And if you’re in the trade, you should be able to easily use the systems to ensure the windows and doors you install are both compliant and offer your customers the insulation properties they expect without leaving things to chance.

Next time you see a U-Value or WER rating, use the following information to make sure you’re just as comfortable with them as you would be in the cosiest home.


You’ll find U-Values on all building components designed to offer insulation against cold. Things like windows, doors, bricks and of course, the insulation itself. The U-Value is a scientific measure of how heat flows through the element and it works like this: High U-Values mean more heat flows, so the lower the figure the better (in a cold climate, anyway). This means that the component will let minimum heat through and keep it inside where it’s most welcome. When things get warmer outside, they’ll keep the heat out too, so the U-Value works both ways.

Here are some typical examples:
  • Cavity walls have a U-value of around 1.6 W/m²K
  • Solid brick walls have a U-value of around 2.0 W/m²K
  • Double glazed windows have a total U-value of between 0.7 and 2.8 W/m²K 

How U-Value is calculated

The unit for U-values is W/m²K. This describes how much thermal energy in Watts [W] travels through 1 square metre [m2] of a building component at a temperature difference on either side of 1 Kelvin [K] (=1°C).

Measuring U-Value

On the drawing board or blueprint, estimated U-Value calculations can be made using the conductivity figures of building materials and calculating the thermal transmittance of a particular area, such as the profile and glazed unit on a door or window. More accurate post-construction U-Values can be measured using a heat flux meter. This involves fixing a thermopile sensor to a test area in order to monitor heat flow from inside to outside, usually over a period of around two weeks.

What’s it to U?

Simply put, for homeowners a low U-Value means a more comfortable indoor temperature. Energy-efficient buildings also lose less heat and suffer from fewer draughts and cold spots. But as well as being more comfortable, a lower U-Value also means paying less for heating and lowering carbon emissions. So it’s good news for the environment as well as your bank balance.

Window Energy Ratings (WER)

The Window Energy Rating scheme is a scale designed by the British Fenestration Rating Council (BFRC) - the premier UK authority for independently verified ratings of energy efficient windows and energy efficient doors. It measures the thermal performance of windows and offers in an at-a-glance way to tell you exactly how energy efficient your windows are so you can choose the most suitable for your home. The scale was developed to provide a rating system specifically for windows as it takes into account not just U-Values, but also solar heat gain (G Value) and the air leakage rate of all the components of the fitted window.

The scale runs from A++ to G, with A++ being awarded to the most energy efficient windows. 'C' is the minimum to satisfy Building Regulations for windows and 'E' for doors (except in exceptional circumstances). You’ll probably recognise the same rainbow scale as the one used to show overall energy efficiency in everything from hoovers to houses.

Who provides rates WER ratings?

WER ratings are awarded by the BFRC. During the rating inspection, they calculate and verify the energy efficiency level in a process that’s independent from any manufacturing or installing company. Manufacturers of BFRC rated product and BFRC Authorised Installers are then audited to ensure that their energy efficient windows and doors continue to achieve the stated rating.

What the law says

To meet the UK’s commitments to lower domestic energy consumption and reduced carbon emissions, the latest legislation requires all replacement windows to meet new energy efficiency standards.

These require new build or replacement windows to satisfy at least one of the following criteria:
  • To have a whole window ‘U’ value of 1.6 W/m²K or lower.
  • To have a minimum window energy rating (WER) of ‘C’.
  • To use sealed units with a ‘U’ value of better than 1.2 W/m²K with argon gas fill and warm edge spacer bars.

At the moment there is no energy rating scheme for doors, but all doors with more than 50% of their total area glazed must have a U-value of 1.8 W/m²K or lower. 

Energy Performance Certificates

Government legislation requires that Energy Performance Certificates (EPCs) are produced whenever a property is built, sold or rented. An EPC contains information about a property’s energy use and typical energy costs and recommendations about how to reduce energy use and save money.

Windows and doors play a major role in a home’s insulation, so look for lower U-Values and better WER ratings and your EPC will demonstrate better energy-efficiency, making it even more attractive to potential purchasers or tenants.

All Eurocell products are designed to deliver the highest levels of energy efficiency in their class. Innovations like advanced thermal inserts and heat retaining multi-chambered profiles help cut heat loss to a minimum, allowing us to offer flagship products such as our Modus and Eurologik systems with the industry’s highest A++ rating.

To learn more about how our products can help you install more energy efficient windows and doors, call 0300 333 6525 or fill in our quick contact form here.

Your comments

by Eurocell Online Team on August 11, 2017 at 16:51
Hi There are many elements that form a U-value. Eurocell is the frame extruder so we provide the profile (frame) to fabricators to manufacture the window, they source the glass and hardware for the final window to the customer’s specifications to achieve the required WER and U-value, as per an example below taken from our Eurologik brochure. Your installer will need to contact the fabricator to request this information and we can provide you with a similar specification once we have this information, if the fabricator cannot supply this to your installer. Hopefully this answers your question but if you need any further assistance please contact us using the contact form above.
by Matthew on August 2, 2017 at 9:24
Hello. I have had a quote from a window fitter who is proposing to fit Eurocell casement windows, sliding patio doors and two external doors. Can you please confirm the A rating for these and also can you advise whether these items are ARGON filled. Many thanks Matthew

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