A U-value measures how quickly heat passes through a window or door, expressed in watts per square metre per Kelvin (W/m²K). The lower the number, the better it insulates. Replacement windows and doors in England must now achieve 1.4 W/m²K or better.
Energy prices are rising. As a homeowner, you want to save as much money on your energy bills as possible while doing your bit for the environment.
Prioritising how energy efficient your windows are can help you achieve both. And you'll be complying with building regulations in the process! This inevitably means conversations around the term U-value – the measure of a window or door’s thermal efficiency.
In the current economic climate, this is becoming an increasingly important consideration for homeowners across the UK, as a poorly insulated home will mean higher heating bills. Energy-efficient windows help keep your home at desirable temperatures by keeping the warm air in and the cold air out.
In this article, we'll unpack what u-values mean and how they're calculated, so you know how to determine the optimal U-value of your windows.
If you are ready to get thermal efficient aluminium fixtures, make sure to check out the aluminium doors we offer.
What Is a U-Value for Windows?
A U-value is a measurement of thermal efficiency. It's an indication of heat loss and the rate at which it is lost. Simply put, it's a determination of how quickly heat passes through a window or door. The lower the U-value, the more effective the insulation.
Windows and doors with a low U-value are ideal for houses and offices, as they keep warm air in during the winter months, keeping energy bills low.
The U-value is noted with a number that is followed by the letters W/m²K. What does this mean? The U-value is calculated as being the watts per square metre per Kelvin. If this sounds a bit complicated, don't stress. There are a number of online U-value calculators that can crunch the numbers for you.
How Do U-Values Relate to Window Energy Ratings?
A U-value only measures heat loss, while a Window Energy Rating (WER) combines heat loss with solar gain (g-value) and air leakage (L-factor) into a single band from A++ to G. Building Regulations accept either a compliant U-value or a compliant WER band.
The U-value on a window is similar to the energy rating you see on home appliances, rated from A++ (the most efficient) down to G (the least efficient). Some window performance figures are expressed as U-values and others as Window Energy Ratings (WER). U-values have become the more commonly used industry term.
What Is the Difference Between a U-Value and a Window Energy Rating?
While the U-value tells you how well a window prevents heat loss, energy rating takes into account other factors like the window's G-value and L-factor to give it an overall efficiency score.
G-value is a determination of how much heat will pass through a window, whereas the L-factor measures how airtight and draught-proof the window is. The higher the energy rating of a window, the more effective it will be.
How Is a Window U-Value Calculated?
A window's whole U-value combines the frame (Uf), the glass (Ug) and the glass-edge spacer (Psi-g), weighted by their areas. It is calculated to BS EN ISO 10077 or measured by testing, and expressed in W/m²K.
A formula is used to calculate the U-value (a measure of how well a building's insulation works) and window energy rating (a measure of how much heat is lost through the windows).
This calculation estimates the energy balance of a typical UK house in a year, considering different window styles and configurations.
There are three key factors to consider when calculating a window’s U-value:
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Uf Value: The thermal performance of the frame
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Ug Value: The thermal performance of glass panels
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PSIG: The thermal performance of the glass edge
Combining these factors, the window’s opening mechanism and design, will give you the total U-value for your window.
What Is a Good U-Value for Windows?
Anything at or below 1.4 W/m²K meets current Building Regulations for replacement windows in England. A good double glazed window achieves around 1.2–1.4 W/m²K, high-performance double glazing around 1.0–1.2 W/m²K, and triple glazing 0.8 W/m²K or lower. Passive House projects need 0.8 W/m²K or better.
What Is the Minimum U-Value for Windows in the UK?
When shopping for windows, you need to think about more than just the aesthetics or colour of the window. You also need to consider the efficiency of the new windows.
In England, replacement windows (whether aluminium, PVCu or timber) must achieve a U-value of 1.4 W/m²K or better, or a Window Energy Rating of band B, under Part L of the Building Regulations (2021 edition). Windows in new-build homes have a limiting U-value of 1.6 W/m²K.
What Is the U-Value of Single Glazed Windows?
Single-glazed windows are not very well insulated. They have U-values of around 4.8–5.8 W/m²K.
What Is the U-Value of Double Glazed Windows?
Double glazing is fast becoming the go-to window for replacements or new windows because it's easier to source and more efficient than single-glazing units. Modern A-rated double glazed windows typically achieve a whole-window U-value of around 1.2–1.4 W/m²K.
What Is the U-Value of Triple Glazed Windows?
Triple glazing is made up of three panes of glass with two insulating gas-filled cavities. An extra pane of glass gives you energy efficient windows that improve thermal efficiency and minimise noise. The result is a whole-window U-value of around 0.8 W/m²K or lower, among the best performance available.
Check out our post on how to stop water leaking from aluminium windows to make sure your home stays as warm as possible during winter.
What Are FINEO Windows?
FINEO is a brand of ultra-thin vacuum insulated glazing that offers U-values comparable to triple glazing in a unit slim enough to fit many heritage frames designed for single glazing. See whether you can retrofit double glazing into existing aluminium windows.
FINEO the new kid on the block. It is ultra-thin double glazing with better U-values than regular double glazing. Although, up to four times thinner than traditional triple-glazed windows, they perform just as well.
The frames of heritage-style windows are typically constructed with single glazing, which means that standard double-glazing glass won't fit into the existing window frame. FINEO offers an environmentally friendly solution to spruce up heritage properties.
What Do UK Building Regulations Say About Window and Door U-Values?
Under Part L of the Building Regulations (2021 edition, in force since June 2022), replacement windows in existing homes in England must achieve a U-value of 1.4 W/m²K or better, or a Window Energy Rating of band B. Replacement external doors must also achieve 1.4 W/m²K, while windows in new-build homes must not exceed 1.6 W/m²K. Scotland and Wales set their own, similar standards, so check with your local building control if you're outside England.
For more about building regulations and installing doors and windows, head to our blog for aluminium doors tips and advice.
Window U-Value FAQs
Does the window frame affect U-value?
A window frame’s material can affect its energy efficiency. You should consider the difference in energy efficiency between metal, wood, and uPVC. Timber and uPVC insulate naturally, while aluminium relies on a polyamide thermal break. Modern thermally broken aluminium frames comfortably meet current regulations, but older aluminium frames without one lose heat quickly.
What is a 'Passive House'?
Passive Houses are extremely energy efficient and environmentally friendly. These buildings leverage passive elements like sunshine and shading in place of central heating and air conditioning to keep the property warm and cool as required.
To meet Passive House standards, a new building must be super-tight and super-insulated. To pass the strict Passive House guidelines, a window’s U-value must be no higher than 0.80W/m²K.
Is a lower U-value better?
Yes. A lower U-value means less heat escapes through the window or door, so your home stays warmer and your heating bills are lower. A window rated 1.2 W/m²K loses less heat than one rated 1.6 W/m²K.
What U-value do aluminium windows have?
Modern thermally broken aluminium windows typically achieve around 1.2–1.4 W/m²K with double glazing and below 1.0 W/m²K with triple glazing, with some triple glazed systems reaching below 0.9 W/m²K. Always ask for the whole-window figure, not the centre-pane glass value. See how much aluminium windows cost.
What U-value do bifold and patio doors need?
Replacement external doors in England must achieve 1.4 W/m²K or better. When comparing aluminium bifold doors or sliding patio doors, check the whole-door U-value, which depends on the thermal break and the glazing specification.
How can I improve the U-value of my existing windows?
Upgrading to better glazing, adding secondary glazing, and sealing gaps around the frame all help. Our guides on insulating aluminium window frames and draught proofing aluminium windows explain the DIY options.
