06 Sep 2021

A Homeowner’s Guide to U-values

At first glance, U-values seem very complicated to understand but they don’t need to be. Peter Daniel, Product Innovation Director at The Rooflight Company, explains how to interpret these values to ensure you find the right product for your energy-efficient home.


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U-values are a measure of thermal efficiency or, more simply put, how well a window stops heat from passing through it. The slower or more challenging it is for heat to pass through the window, the more energy efficient it is. The more energy efficient it is, the lower the U-value.

Windows with a low U-value reduce the flow of heat throughout your home, stopping heat leaving in the winter and ensuring that your home doesn’t get too warm in the summer. This will save you money on fuel costs in the long run. The higher the U-value, the more heat is being transferred, leading to higher costs when heating and cooling your home.

As simple as that sounds, it can still get confusing. So, here’s how to choose the most efficient windows for your home:

Check the requirements

As covered in Part L of the Building Regulations, current legislation requires a whole-unit value of 2.0W/m2K for newly-built homes and 1.6W/m2K for existing homes. For properties other than houses, the requirements are 2.2W/m2K for new buildings and 1.8W/m2K for existing buildings.

Take the Uw-value into account

While many companies report on the performance of the pane, which is the most efficient part of the window, heat loss will be higher around the edges of the window because joints and frames are harder to insulate. Therefore, any report of the U-value that doesn’t take this into account could lead to you choosing a less efficient option for your home. This will increase your household bills as it will take more to heat your home as the temperature drops. The Rooflight Company reports on the whole window unit (Uw), rather than just the glazing. This figure measures the overall efficiency of the window, including the value of the frame, the glass and the glass edge. These figures will change depending on the size and shape of the window you’re installing, meaning you will need to check the value of the different windows you’re choosing for your home. Ensuring you have the lowest Uw-values throughout will ensure that your home is as efficient as possible.

Consider the orientation of your windows

You need to know whether you’ll be installing vertically, as with a window, or horizontally as with a flat-roof rooflight, or somewhere in between. That’s because the regulations force manufacturers to report the U-values of all glazed products vertically. But rooflights are usually oriented horizontally, with the sun directly beaming down onto them at a more aggressive angle. To account for this, a good rule of thumb is to add 0.5W/m2K (for double-glazed windows) to establish their real-world performance. Our neo Advance, for example, may have a U-value as low as 1.06W/m2K, which equates to 1.56W/m2K when horizontal, which is still well below the regulations set out in Part L.

Look out for G-values as well as U-values

It is important to recognise the difference between G-value and U-value when choosing windows for your property. The G-value of a window simply tells you how well the glass transmits heat from the sun, rather than how energy efficient they are. Like with U-values, the lower the G-value, the better as the unit transmitting less solar heat. It is useful to have a low G-value to ensure that the amount of natural light entering your home remains high, but that solar glare is factored in to manage overheating. Combining low U-values with this ensures maximum energy efficiency as heat from the sun will be blocked using glazing, as well as the other elements of the window that ensure energy efficiency.

Improve thermal efficiency of your window units, if you can

Our patented Thermoliner and thermal shield are product specific. The Thermoliner is fitted within our steel rooflight range, and our neo Advance ranges use a thermal shield. This treatment ensures that the heat stays inside your home, while condensation and mould stay out. These windows can, therefore, be much lighter in terms of weight, as their low U-values mean you can choose a double-glazed, rather than triple-glazed, option.

Despite thousands of pages of regulations being published, all you really need to know are the U-value requirements. Remember that the lower the U-value of the window, the better the insulation and, therefore, the efficiency. This means you will save money on your bills in the future. Trusted companies will always provide these values with their products, allowing you to find the best window for your home.

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