G-value is a metric that measures how much solar heat passes through a material – in other words, how easy is it for the sun’s rays to penetrate through your window? Find out more about G-Value and what it means when you are choosing a window in our guide below.
G-value is measured on a scale of 0-1 where the higher the value, the more sunlight is able to come in.
Sunlight or solar radiation can flood your home with natural light and offer free heating to warm up your room, and cut your energy usage and costs. But on the other hand, too much solar gain can make your room unbearably hot and stuffy.
This article explains the ideal window G-value and why it’s important.
G-Value Explained
In basic terms, G-value measures the temperature increase caused by sunlight through your window and surrounding space. Depending on where you live, the G-value of your windows can impact how energy efficient and comfortable your home is.
In warmer climates a high G-value is not ideal because it means a lot of heat is passing through your windows. Your room could become sweltering and you’d have to rely on air conditioning or fans to keep it cool. A low G-value would be better because your windows would effectively be blocking out the heat to prevent it from getting stuffy.
It’s the opposite in colder climates where you’d want windows to have a higher G-value to make use of heat from the sun. This can reduce your need for heating, lowering your bills.
Roof windows let in three times more natural light compared to vertical windows, so they’re an excellent way to make use of the sunlight for natural heat.
Is a low G-value better?
The G-value scale starts from 0 to 1.
- 0 means no solar heat can penetrate your windows (imagine a shield blocking it)
- 1 means complete sunlight can come through (imagine there being no window)
A low G-value isn’t always better because it means you can’t make use of the sun for heating. It’s best to aim for the middle at 0.5, so you get a good balance. The ideal window G-value, however, depends on your climate and window’s location.
South-facing windows let in plenty of light, so it’s best to opt for a window with a higher G-value to make use of it. But west-facing windows let strong sunlight in in the afternoon, so choose windows with a lower G-value to counteract this.
Roof windows also play an important role because of their location. If you're battling cold weather, high G-value roof windows can help to make use of the sun for heating. Opening roof windows would work particularly well because you can let sunlight in directly.
How do you calculate it?
To calculate the G-value of your windows, use the formula below:
Total solar gain + incident solar radiation = G-value
You’ll need to add together the heat transferred through the window with the amount absorbed and emitted into your home.
What is solar gain?
Solar gain, or solar heat gain, is the amount an object increases in temperature because of solar radiation (where sunlight penetrates an object and heats up the enclosed space).
Your G-value is essentially a measure of solar gain.
So, if your windows’ G-value is 0.8, they’ll have really high solar gain. If it’s 0.4, your windows will be doing a better job of blocking out the heat.
Solar gain is both good and bad for homeowners. On the one hand, it means you can heat up the room more efficiently, but on the other hand, it can become uncomfortable if it gets too hot.
Why does G-value and solar gain matter for windows?
Knowing your windows’ G-value is important because it helps you choose the right type for your home to ensure it’s comfortable all year round and energy efficient.
G-value also has a direct impact on your bills. Low solar gain, for example, could mean you use less air conditioning and high solar gain means you could cut heating costs, especially with high quality roof windows that let in sunlight.
However, windows with a high G-value could also cause glare and sun damage to your interiors. Take a conservatory as an example, it’s typically fully or partially glazed but if the G-value of the glazed area is too high, it could cause your furniture or artwork to fade/discolour.
It might be best to opt for non-opening roof windows in this case to prevent direct sunlight from hitting your interiors. Fakro roof windows are also protected with a UV layer.
Difference between G-value and U-value
U-value and G-value both measure the performance of your windows.
But a U-value measures how much heat is lost through a window (how well it insulates), while G-value examines how much solar heat passes through it.
With U-value, the lower the better because it means your windows are doing a better job of insulating your room.
In June 2022, changes were made to Building Regulations Part L to improve the energy efficiency of homes with any new windows and doors installed. The maximum U-value for new windows and doors is 1.4W/m2K. But anything below 1.6W/m2K for windows in the UK is deemed good.
Fakro pivot roof windows have a U-value of 1.3 and flat roof windows, 1.1, well below the maximum, helping homeowners retain heat in their homes and cut down energy use.
Benefits of a good G-value
Finding the ideal balance based on your location and climate can offer many benefits:
- Reduced heating and cooling costs
- Improved energy efficiency
- Comfortable temperature all year round
- Natural light
- Reduced glare
Why is G-value important?
To recap, G-value is important because it measures how much free heating homeowners can get from the sun. Strategically choosing your windows and installing roof windows can also improve energy efficiency and heating/cooling costs.
Eurocell supplies top quality roof windows that ensure plenty of sunlight floods your home. Choose from high-quality pivoting roof windows, and opening and non-opening flat rooflights.
Browse our range of roof window products here.
