What is g-value? image

What is g-value?

Henrik Müller
Written by Henrik Müller27.10.2023

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When speaking about windows, there are many terms to know about. One of the less familiar ones is g-value. In short, the g-value is used to indicate how much solar heat or solar energy is coming in through your window. The higher the number, the more solar energy is coming in. The g-value therefore provides information about how much heat your window adds to your home. This can be beneficial if you want a home that is more energy efficient.

When measuring the g-value of a window, you can either measure the value from the window itself or the glass pane. If you want to indicate the g-value for a pane, you use the term 'gg', whereas you use 'gw' for the window.

You will notice when measuring the g-value that the gw value of the window is always lower than the gg value of the pane. This is because the heat gain from the sun does not penetrate through the frames and sash of a window, but significantly more through the glass pane. The difference between the two is normal, so there's no need to worry if the gw value is lower than the gg value.


How to calculate the g-value

When specifying the g-value, you need to use either decimal numbers or percentages. The g-value will always be somewhere between 0 and 1 or 0% and 100%. The solar heat itself will always be 1 or 100%, while the value of the window itself will vary. If you have a window that has a g-value of 0.63, 63% of the sun's energy will penetrate through the window.

You can calculate the g-value by adding the direct transfer (DT) of the solar energy to the secondary radiation (SR). The direct transfer refers to the solar energy that enters directly through the window, while the secondary radiation is the amount of solar energy that the window itself has absorbed, which also provides more heat to the room.


What is a good g-value for glass panes and windows?

Some parts of the UK can get very cold, especially during winter. Your home can therefore benefit from windows with a high g-value, as they allow you to utilise the sun's natural warmth during the winter months. A high g-value is especially important for south-facing windows, as it allows as much solar energy as possible to enter through the window. If, on the other hand, you find your home too warm, especially when the temperature rises in the summer, it's more beneficial to choose a window with a lower g-value. The same is true if you live in a warmer place or will be staying in your summer or holiday home.

This means, that it can be difficult to judge what a good g-value is, as you need to choose a window according to your needs and preferences.

What is the reason for choosing a high g-value window?

When buying windows, it's important to find the balance between bringing natural light into your home and controlling the heat that is added through sunlight. It can quickly become unbearably cold in your home if there isn't enough heat coming in or the heat is just leaking out. You can quickly find yourself spending a lot of money on heating your home, so if you are doing so, it's a good idea to invest in windows with a high g-value. This way, you can utilise the heat from the sun's energy to keep your energy consumption down and save money. It is also a more sustainable choice that can improve the indoor climate.

Getting the right windows for your home is important to provide you with the most comfortable temperature. A great option is triple-glazed windows, which trap the cold air between the three glass panes and make it harder for cold air to come in. You can contribute to a better indoor climate by choosing a window with a high g-value in the glass pane.

If you're looking for windows that fulfil these requirements, explore Klar's range of three-layer windows here.

What should I do if it gets too hot inside?

If you find that it gets unbearably hot during the summer with windows that have a high g-value, you may want to opt for sun protective glass, which we offer as a glass type on all our windows.

Choosing this has the following benefits:

  • Reduce your energy consumption for cooling and heating.

  • Enjoy a better and more comfortable indoor climate.

  • Avoid overheating during the hot summer months.

For a more temporary solution for the summer months, you can always buy an awning to cover the window. Another example is to use a pair of light-coloured curtains to keep the heat out in the summer.