It’s easy to get distracted by the appearance of a window when you’re choosing one for your home or office. The colour of the frame, the finish, or the design takes the front seat while picking up the appropriate door or window.
But there’s one more thing that quietly affects how comfortable and energy-efficient your home will be: how the window handles heat.
That’s where U-values and G-values come into play.
These two numbers indicate how well your windows perform by showing how much heat they retain and how much heat they allow through from the sun. By understanding these values, you can pick windows that feel comfortable all year long, not too hot in the summer and not too cold during the winter.
Let’s understand U-values and G-values and why you should pay attention to them:
What is a U-Value?
The U-value (also known as the U-factor) indicates how well a window retains heat.
It is measured in W/m²·K (watts per square meter per degree of temperature difference), but you don’t need to worry about the math. This is what matters:
1. The insulation is better if the U-value is lower.
2. A window with a low U-value is like a thick winter coat. It keeps the heat in your home, so you don’t have to turn up the heat to stay warm.
3. Older windows with only one pane of glass often have U-values higher than 4.0 W/m²K, which means they let heat escape easily.
4. The current UK Building Regulation requirement for replacement windows is that they have a U-value of 1.6 W/m²K. This is a mandatory requirement for both new installations and replacements.
5. Triple glazing can lower that even more, to about 1.0 W/m²K, which is great for insulation.
So, when you compare windows, a lower U-value means the windows will let less heat out, which will lower your energy bills and make your home warmer.
What is a G-Value?
G-values tell you how much sunlight comes in, while U-values tell you how much heat leaves.
The G-value, which is also called the solar factor or solar heat gain coefficient, tells you how much of the sun’s energy gets into your home through the glass. It can be shown as a number between 0 and 1 or as a percentage.
The more solar heat your window lets in, the higher the G-value.
This is how it works in real life:
1. A G-value of 0.7 (70%) means that 70% of the heat from the sun gets into your house.
2. A G-value of 0.4 (40%) means that most of that heat is stopped.
Depending on your house’s direction, a higher G-value could be both good or bad.
A higher G-value is useful in colder spots or rooms that face north, as it keeps the spaces warm naturally on sunny days.
If your room faces south or west, too much solar light can make it too hot. In that case, a lower G-value (solar control glass) might be more comfortable.
Your G-value helps you find the right level of comfort. If it’s too low, your rooms might feel cold, and if it’s too high, you’ll be fighting the heat in the summer.
U-Values vs. G-Values—The Balance
This is where things get interesting: U-values and G-values work together, but they don’t always go hand in hand.
A low U-value means less heat loss, which is good for insulation.
A high G-value means that more solar heat comes in, which is great for passive warmth.
The hard part is finding the right windows that will keep your home warm in the winter but not make it feel like a greenhouse in the summer.
For instance:
To keep a south-facing room from getting too hot, it might need windows with a low U-value and a moderate G-value.
A window that faces north might need a higher G-value to let in as much light and warmth as possible.
In the UK, where the weather can go from cold winters to surprisingly warm summers, this balance is especially important.
A balanced window design can lower your heating and cooling bills by letting the sun in when you need it and keeping the heat in when you don’t.
Things Homeowners Should Consider
Before you buy new windows, there are a few simple things to keep in mind:
1. Look at both the U-value and the G-value: Don’t stop at one number; they work together. Good manufacturers will always give you both.
2. Think about window orientation: Windows that face south or west get more sun. To keep things from getting too hot, look for a lower G-value. Windows that face north and east get more natural warmth when they have a higher G-value.
3. Check how well the whole window works: Check to see if the U-value applies to the whole window unit, not just the glass. Bad frames or seals can make windows less efficient.
4. Make sure you follow the building rules: Windows that are being replaced in the UK must have a U-value of 1.4 W/m²K or higher. Even though G-values aren’t strictly controlled, they are important for comfort and energy performance.
5. Ask about different types of coatings and glazing: Low-E coatings, gas fills, and warm-edge spacers can all make your U-value or G-value better, which will make you more comfortable overall.
In conclusion
Knowing what U-values and G-values are will help you make smart choices that will keep your home comfortable, save energy, and stay ready for all seasons.
A window that is just right will keep you warm in the winter, cool in the summer, and lower your energy bills all year long.
So before you decide on your next window upgrade, take a quick look at those numbers — because behind every stylish frame, it’s the science that makes your home truly comfortable.
If you’re unsure about which window would be the most suitable for your home, Bowalker’s Free Home Consultation can guide you in the right direction! So know your windows completely before you invest in them with Bowalker.
Book a Free Home Consultation today!
