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 in reality, 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 coming out of 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.

What are U-Values?

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. U-Values are a measurement of a material’s insulation efficiency, and the lower the number, the better the energy efficiency is.

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.

The centre pane vs. the whole product

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 take into account 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.  

Building regulations

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

In 2018, Part L building regulations for standard replacement doors and windows specified that a product should have a U-Value no higher than 1.6 for windows, and 1.8 for doors. 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.

Results that speak for themselves

By following a carefully thought through design process and by constantly developing our techniques, our 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.