U-Values and WERs explained

Knowing the thermal performance of any windows and doors you are installing in your home is massively important. Whether it’s for a refurbishment, renovation, extension or new build project, they all must comply at the very least with Building Regulations. But for bigger energy savings, you need to know what gets measured, and what it means to you.


There are two ways to measure the thermal performance of windows: U-values and Window Energy Ratings (WERs).

When we are looking at refurbishment, we look only at the thermal performance of the glazing, and both measurements can be used. Building Regulations demand we meet certain energy performance levels – which are currently either a U-value of 1.4 W/m²K, or a minimum of ‘A’ in Window Energy Ratings.

For new build and larger extensions which are covered under Building Regulations or planning consents for the complete project, it’s not quite as straightforward. The thermal efficiency of the total building is calculated, which includes the glazing. The U-values play a part in these calculations, but to comply with Building Regulations there’s an overall calculation including the walls, floor and roof, although the glazing element must hit a minimum target.

So, what are U-values and WERs, and how are they different?

U-values for glazing

Let’s start with U-values. These measure the ability of an element to transmit heat from a warm space to a cold space in a building, and vice versa. The lower the U-value, the better insulated the building element. This means the lower the U-value of a window, the better, as this indicates it will work efficiently to maintain indoor temperatures.

U-values are used for glazing, walls, floors and roofs. The U-value of the glazing is always going to be higher than the other parts of the building, so the glazing – especially where wide span glazing is being included – has a big impact on the overall calculation.

The measurement has the formula W/m²K – watts per square metre per Kelvin – and the lower the value, the better the performance. So, a window with a 0.8 W/m²K is more thermally efficient and will give bigger energy savings than a window with the U-value of 1.4 W/m²K.

The U-value takes into consideration every part of the window – including the frame, any reinforcing materials, the sealed unit including the spacer bar, glass type and cavity gas.

Window Energy Ratings (WERs)

Window Energy Ratings are far more recognisable than U-values, as they are based on the same rainbow scale you will have seen on electrical appliances. They work in the same way too, with A++ being the highest rating, and G the lowest. For windows you need a minimum of a B rating to comply with Building Regulations.

WERs don’t just take into consideration the U-values, they also look at solar gain and air leakage.

Solar gain, sometimes called the g-value, is the amount of solar heat that passes through the glass to increase the inside temperature.

Air leakage is the amount of air that escapes through the frame, gaskets and joints: a well manufactured window means less air leakage. It’s important because we know that draughts and cold spots are one of the key indicators for turning up the dial on your heating thermostat, so reducing air leakage adds to feeling more comfortable in your home.

The difference between U-values and WERs

U-values and WERs are both there to give an indication of the energy performance of windows. And even though they use different measurements, U-values and WERs are interchangeable when it comes to refurbishment projects. We think WERs are much easier to compare from one window to another.

But there are times it can become confusing, especially around the use of solar control glass. Choosing solar control glass might be necessary, especially for large areas of glazing in south facing rooms, as it would keep your home from overheating. It could also mean you are marked down for WERs, but may offer a better U-value.

U-values are a must when it comes to a new build or extension that is part of a building project, as they feed into the total energy performance calculation.