What is the best U-Value for windows?

Known as either U-Value or U-Factor, this simple number can say a lot about the energy efficiency, and therefore the quality, of your new windows. A simple search online will tell you that the lower the U-Value, the better the insulation, but there’s more that consumers should be aware of when exploring the market. Here is our expert guide to help you choose the best U-Value for your windows and show you what else you need to check before making any decisions…


Having energy efficient windows means that you and your family can enjoy a warm, comfortable home at any time of the year, no matter how low the temperatures drop. Products designed to have a high-energy performance are well insulated with no air leaks.

By choosing energy efficient windows, you’ll significantly reduce the amount of heat escaping from your home and stop those chilly draughts from making you feel cold during the winter. The cost of heating your property will reduce, which can only be a good thing for the environment, and you’ll also notice a significant improvement in the level of noise coming from outside. It’s a winner all round.

Why are U-values important?

In the realm of energy-efficient construction, U-values play a crucial role. These values quantify the rate at which heat is transferred through building components, such as walls, windows, and roofs. By understanding U-values and their significance, architects, builders, and homeowners can make informed decisions to improve energy efficiency and reduce heat loss or gain. This blog post will delve into the fundamentals of U-values, explain the calculations involved, highlight their importance in energy efficiency, address frequently asked questions, and provide practical insights to enhance the performance of building components.

What is a U-value and why do they matter for windows?

U-value, also known as thermal transmittance, measures the amount of heat transferred through a material or an assembly of materials, such as a window. It is expressed in watts per square meter per degree Celsius (W/m²K). The lower the U-value, the better the insulation and energy efficiency of a building component. U-values are crucial in construction because they help evaluate the thermal performance of walls, roofs, floors, windows, and doors. By understanding the U-values of different elements, designers can optimize building envelopes to minimize heat loss during colder months and prevent heat gain in warmer climates.

What is a good U-value for a window?

U-Values are a measurement of a material’s insulation efficiency. You can check a window’s thermal performance by looking at its U-Value, also referred to as its U-Factor. It is the number that will tell you how much heat and energy a window will let through, and you can gain a better idea of how the glazing will perform. So, do you want a high or a low u-value? The lower the number, the better the energy efficiency is.

Old single-glazed windows can have U-values of around 5.8 W/m2K, and older double-glazed windows can be around 2.8.  U-values are now much lower, and typically a good U-value for replacement windows is anything below 1.6 W/m2K.

Not all windows are made equal and unfortunately, you’re not always going to get the same insulation performance across the board. If you are investing in new windows, you want to be confident that the products you have chosen are going to keep the heat in and keep your energy bills down.

What is the U Value of Westbury Windows and Doors?

Westbury sashes are 68mm deep and therefore can incorporate a much thicker and wider choice of energy efficient and sound reducing glass. These facts enable Westbury to offer independently tested and authorised energy saving products with excellent whole product U-Values.

Westbury Sound & Secure and Sound & Secure Extra Windows have a whole product U-Value of 1.3W/m2K, whilst our triple glazing has a whole product U-Value of just 1.1W/m2K

Take a look at the U-Values of all of our products here.

How do I find out the U value of a window?

Any reputable window company should be able to inform you of their product’s U-Value through the correct performance testing. Unfortunately, this information can still be misleading, as many companies will only choose to promote the U-Value of the window’s centre pane, which is inevitably going to be better than that of the whole product.

The trick is to understand that all windows will experience more heat loss around the edges, as frames and joints are harder to keep insulated. Instead of looking at the centre pane U-Value, which focuses solely on the efficiency of the central glazed unit, you want to check the value of the whole product, which will consider the frame’s thermal performance too. When you know the whole product’s U-Value, you will have an accurate indicator of the window’s energy efficiency. 

What is the minimum U-value for windows UK?

Whether you are embarking on a new build project or renovating your home, it is essential to always adhere to building regulations. When it comes to windows, the regulations cover several areas, with energy efficiency being one of them.

In 2002 Building Regulations changed, Part L refers to the u-values of new windows and doors. For new windows installed in existing properties, the current acceptable u-value is 1.6 W/m²K. From 15th June 2022, this was lowered to 1.4 W/m²K to comply with the enhanced energy efficiency requirements of new build homes. These regulations are due to change again in 2025, as part of The Future Homes Standard.  

If you are looking for windows made to the highest of standards with outstanding performance, then you should look for products that can better these regulations.

Be wary however, windows that have U-Values of just 0.1 or 0.2 under the regulation threshold will not make any difference to your carbon footprint, or energy bills for that matter.

How do you achieve the best U Values for windows?

By following a carefully thought through design process and by constantly developing techniques, Westbury windows are technically superior when it comes to energy performance. We design our timber frames and sashes with deep inside to outside profiles, which improves the windows insulating properties through the wood components. This technique was originally developed in Scandinavia many years ago to protect their homes from the cold weather. They had a standard frame depth of 115 mm – which is the same as we use today.

We have our windows independently tested in accordance with BS 6375 Part 1: Performance of Windows & Doors, Classification and Guidance for Weather Tightness. Our windows have a whole product U-Value of 1.1W/m2K, based on triple glazed, 4mm thick glass with a centre pane value of 0.6W/m2K.

We achieve this by using Accoya®, an engineered timber with very low thermal conductivity, along with specially formulated bonds and strong joints. You can find out more about this brilliantly engineered and highly sustainable timber material here.

What is a good U-value for triple glazed windows?

Westbury windows have a whole product U-Value of 1.1W/m2K, based on triple glazed, 4mm thick glass with a centre pane value of 0.6W/m2K.

How is the U-value calculated for a building component?

Calculating the U-value involves considering the thermal conductivity of individual materials and their respective thicknesses within a building component. The formula for U-value calculation is as follows:

U = 1 / (R1 + R2 + R3 + … + Rn)

In this equation, R1, R2, R3, and so on represent the thermal resistances of the different layers or materials present in the component. The thermal resistance (R-value) of a material is the reciprocal of its thermal conductivity (k), multiplied by its thickness (d). Mathematically, this can be expressed as:

R = d / k

By summing up the thermal resistances of all layers, the U-value for the component can be determined.

What is the difference between U-value and R-value?

While U-value and R-value are related, they represent different aspects of thermal performance. U-value quantifies heat transfer through a material or assembly, focusing on the rate of heat flow per unit area. It measures the inverse of the total thermal resistance (R-value) of a building component, considering both conductive and convective heat transfer.

On the other hand, R-value represents the thermal resistance of a specific material, indicating its ability to impede heat flow. Higher R-values imply better insulation properties and lower rates of heat transfer. R-values are commonly used to evaluate the effectiveness of insulation materials, such as fiberglass, cellulose, or foam boards.

How do U-values affect the energy efficiency of a building?

U-values directly impact the energy efficiency of a building. Lower U-values signify reduced heat loss during colder months and decreased heat gain during hotter periods. By investing in building components with low U-values, such as well-insulated walls, roofs, windows, and doors, the amount of energy required for heating or cooling can be significantly reduced. This leads to lower energy consumption, decreased utility bills, and a reduced carbon footprint.

Moreover, buildings with low U-values provide better thermal comfort for occupants. They minimize cold spots, drafts, and condensation issues, creating a more comfortable indoor environment.

The recommended U-values vary depending on climate, building type, and regional building codes. However, some general guidelines exist to help achieve energy-efficient buildings. Here are a few examples:

Walls: In colder climates, U-values around 0.15 W/m²K or lower are desirable for external walls. For internal walls, U-values around 0.3 W/m²K or lower are recommended.

Roofs: U-values for roofs should aim for around 0.1 W/m²K or lower, especially in colder regions.

Windows: For windows, lower U-values indicate better insulation. Double-glazed windows typically have U-values between 1.1 and 1.6 W/m²K, while highly efficient triple-glazed windows can achieve U-values of 0.8 W/m²K or lower.

Doors: Well-insulated doors usually have U-values between 1.2 and 1.8 W/m²K, with lower values indicating higher efficiency.

These values serve as a starting point and can be adapted based on local requirements and energy performance targets.

How can I improve the U-value of my windows or doors?

To improve the U-value of windows and doors, several strategies can be employed:

Install double or triple glazing: Multiple layers of glass with insulating gas in-between reduce heat transfer. High-performance glazing, low-emissivity coatings, and warm edge spacers can further enhance energy efficiency.

Enhance frame design: Opt for thermally broken frames that feature insulating materials or utilize materials with lower thermal conductivity, such as timber.

Address air leakage: Proper sealing and weatherstripping techniques help minimize air infiltration, which can significantly impact the overall U-value of windows and doors.

Consider window orientation: Proper placement of windows and shading devices can reduce solar heat gain in hot climates or maximize solar heat gain in colder regions.

Are there any regulations or standards for U-values in building construction?

Building regulations and standards set by local authorities or national bodies often dictate the minimum U-values for various building components. These regulations aim to improve energy efficiency, reduce carbon emissions, and enhance occupant comfort. The specific standards and permissible U-values may vary across countries or regions.

It is crucial to consult the relevant building codes or consult with professionals to ensure compliance with local regulations and to achieve energy-efficient designs.

How does insulation impact the U-value of a building?

Insulation plays a pivotal role in improving U-values and overall energy efficiency. It reduces heat transfer through building components, thus decreasing heat loss or gain. By effectively insulating walls, roofs, floors, and other elements, the U-values can be significantly improved.

Insulation materials with high thermal resistance, such as fiberglass, cellulose, mineral wool, or foam boards, are commonly used. The thickness and installation quality of insulation are vital factors that impact U-values. Proper insulation techniques, such as eliminating thermal bridging and ensuring complete coverage, help optimize the thermal performance of the building envelope.

How do U-values relate to heat loss and heat gain in a building?

U-values directly influence the amount of heat that is lost or gained in a building. Higher U-values indicate greater heat transfer, resulting in more significant heat loss during cold periods or higher heat gain in warmer conditions. Conversely, lower U-values signify reduced heat transfer, leading to minimized heat loss or gain.

By focusing on lowering U-values, architects and builders can create more energy-efficient structures. Well-insulated walls, roofs, windows, and doors, with low U-values, effectively reduce the flow of heat between the indoor and outdoor environments. This helps maintain a comfortable temperature inside, reduces reliance on heating or cooling systems, and ultimately lowers energy consumption and costs.

What are the units of measurement for U-values and R-values?

U-values are expressed in watts per square meter per degree Celsius (W/m²K). This unit represents the amount of heat (measured in watts) that flows through one square meter of a material or assembly when the temperature difference between its surfaces is one degree Celsius.

R-values, on the other hand, are expressed in square meter Kelvin per watt (m²K/W). This unit indicates the thermal resistance of a material, representing the ability of the material to impede heat flow.

Understanding U-values is essential for achieving energy-efficient buildings. By comprehending the calculations, significance, and recommended values, architects, builders, and homeowners can make informed decisions to improve the thermal performance of building components. Through proper insulation, window selection, and adherence to building regulations, it is possible to minimize heat loss or gain, reduce energy consumption, enhance occupant comfort, and contribute to a more sustainable future.