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U-Values for Windows Explained

What is a U-value and how is it calculated?

U-values measure heat transfer to assess how certain building components conduct heat. They are calculated from the rate of energy transfer through a single square metre of the structure divided by the temperature difference between the inside and outside. The resulting value is displayed as watts per square metre per Kelvin.

For a double-glazed window, let’s look at it like this. If the U-value is 1.5, that means that 1.5 watts will be transmitted through every square metre for each degree of temperature difference between the window’s inside and outside.

Confused? Essentially, the higher the U-value, the more heat transfer there will be. A lower U-value means less heat transfer, resulting in a better-insulated structure. For windows, the lower the U-value, the more energy-efficient it is. If you are trying to improve the eco credentials of your home or any other building, you should target windows with low U-values.

What Are the Typical U-values of Building Components?

Each part of a building will have a different U-value. Though this can vary between buildings, there are some typical figures:

  • A solid brick wall commonly has a U-value of 2, while an insulated wall has a 0.18 U-value.
  • Single-glazed windows typically have a U-value of 4.8-5.8, while double-glazed windows typically have a U-value of 1.2-3.7.

The following table shows the typical U-values of more common building components.

Building ComponentTypical U-value in W/m2K
Single Glazing4.8-5.8
Double Glazing1.2-3.7 (depends on type)
Triple Glazing<1
Solid Brick Wall2
Insulated Wall0.18
Cavity Wall (no insulation)1.35
Solid Wood Door3
Composite Door1.1-1.6 (depends on glazing)

How Do I Find Out the U-value of a Window?

Reputable window companies should be transparent about the U-values of their products, which they will have obtained via performance testing. You should be diligent in checking this, however, as some companies will only promote the U-value of their centre pane, which is always lower than that of the full product (more on this later).

If a company doesn’t readily display the U-values of their windows, this should be an immediate red flag. Contact companies and ask them about the energy performance of their windows if you want to learn more. This can be very helpful when you are comparing products.

Window U-values In More Detail

When it comes to windows, the two U-values you need to understand are as follows:

  • Ug (which measures just the centre pane of glass).
  • Uw (measuring the entire window, including the glass and frame).

The Ug value will always be lower than the Uw value. This is because the entire window includes the frame, seals and spacer bar. The rate of heat transfer for the frame and spacer bar is typically considerably higher than for the glass on its own.

Another important consideration is the smaller windows tend to be less energy-efficient, meaning a higher U-value. This is because the glazing is usually the most heat-efficient component of the window, so more glass in the frame means a lower heat loss ratio for the overall window.

The main factors that influence a window’s U-value are as follows:

  • The type of glass in its design.
  • The distance between the panes.
  • The gas in between the panes.
  • The thermal performance of the spacer bar.
  • How many panes of glass are used.
  • Materials used in the frame’s construction.

What is Considered a Good U-Value?

You should look to the Building Regulations as the guideline for what makes a good U-value. If it meets the building regulations (currently 1.4 for existing dwellings) or is lower, you have a good U-value on your hands.

European window manufacturers place a greater emphasis on U-values and often produce windows with the lowest on the market. You may find products with a 0.80 U-value in some cases.

Be mindful that it is predicted that the U-value requirement for windows in new homes will be reduced before the Future Homes Standard becomes law in 2025. The new prescribed U-value may be as low as 1.0 W/m2L.

Why Do U-values Matter?

Consumers were provided with the industry standard Window Energy Rating (WER) to make it easier to compare the energy efficiency of different window products. These rainbow stickers provide a simple rating system similar to the stickers you see on large electrical products.

The calculation in WER includes the G-value to establish its rating. This measures the amount of solar gain captured through glass, but many window products make the G-value less efficient. What’s more, in cold rooms with no direct sunlight, it’s more important to reduce heat loss than to capture solar gain, while rooms subject to direct sunlight will benefit from reduced solar gain to prevent overheating in summer.

It’s better to look at the U-value and G-value separately to get an accurate idea for your needs. In the UK, the more prominent need is to reduce heat loss, so U-value is arguably a more critical value to consider.

Houses built with low U-value components enjoy:

  • Less heat loss from rooms.
  • Lower energy consumption.
  • Prevention of surface mould growth thanks to higher indoor surface temperatures.
  • Healthy buildings with good indoor thermal climates.

What Are the Building Regulations for U-Values from 2023?

The Building Regulations 2023 edition was updated to cover limiting heat losses and gains. Approved Document L relates to standards for fabric performance of building elements. They came into effect in June 2022, providing the following minimum U-value requirements:

Thermal ComponentCurrent U-value Requirement
Roof0.15
Wall0.18
Floor0.18
Window1.4
Door1.4

What Can I Do to Get the Best Possible U-Values from My Windows?

To enjoy maximum energy-efficiency from your windows, consider the following strategies:

  • Install modern double glazing: Windows with multiple layers that have argon in between reduce heat transfer substantially.
  • Look for enhanced frame designs: Thermally broken frames with insulating materials, or frames built from materials with low thermal conductivity, often have lower U-values.
  • Air leakage: Keep on top of the sealing and weatherstripping of your windows. When seals start to fail, air will infiltrate, and this can reduce the energy-efficiency of your windows.
  • Window orientation: The positioning of windows in relation to the sun is important. If you live in a colder region, you may want to maximise solar heat gain when possible, but constant sunshine can cause excessive heat gain in summer, so shading devices can be helpful.

Why choose Eco Thermal Design for your new windows?

Eco Thermal Design has transformed countless homes across Barnsley, Sheffield and other areas of South Yorkshire. We are experts when it comes to windows and can help you find the right windows for your home. We manufacturer our windows in our factory in Barnsley, and only work with quality brands such as KömmerlingSchucoSheerline and Origin. We also offer Residence Collection windows, perfectly suited for period properties.

Our expert team can help you decide on the right window for your home, and our team of fitters can complete their work to the highest standards, causing you minimal disruption along the way. We’re proud of the quality of our products, work and people, which is echoed by our customers in our Which Trusted Trader reviews.

If you would like to speak to one of our window experts or get a quote, you can contact us here or give us a call on 01226 764841.

What Our Customers Say

“Quality products are only as good as the team that fit them and in our case both the product and the fitters resulted in a first class and professional installation. Respectful of our home and tidied up at the end of each day….Thank You”
Les Finucane

“Excellent service from Eco Thermal again. Would recommend to anyone. Also good to have a local firm that you can trust.”
Ross Duguid

“Very happy with the whole process and of course the finished installation.”
Michael Senior

You can also read our reviews on Which Trusted Trader where we are rated five stars.