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U-Values vs. G-Values vs. WERs: Decoding the Science of Energy Efficient Windows
U-Values vs. G-Values vs. WERs: Decoding the Science of Energy Efficient Windows
If you have ever tried to compare high-performance windows, you have likely encountered a wall of acronyms. Installers talk about WERs, architects specify U-values, and glass manufacturers mention G-values. It can be incredibly confusing.
- 📉 U-Value (Insulation): Measures heat loss. Lower is better (0.8 is superb, 1.4 is standard). See Part L Regs Gov.uk.
- ☀️ G-Value (Solar Gain): Measures free heat from the sun. High is good for winter heating; Low is good for preventing overheating.
- 🌈 WER (Energy Rating): The consumer “Rainbow” label (A-G) managed by the BFRC External.
Are they measuring the same thing? Is a higher number better, or a lower one? The truth is, these metrics often pull in opposite directions. A window that is excellent at keeping heat in might be terrible at letting heat out in summer.
Table of Contents
1. The Big 3 Metrics Compared
Use this visual matrix to understand exactly what each number is telling you and which goal you should aim for.
Window Performance Matrix
What it Measures
Heat Loss. How fast heat escapes.
The Goal
LOWER is Better. Aim for 1.2 W/m²K or less.
Best For
Extensions, North-facing rooms, and reducing bills.
What it Measures
Solar Gain. Sun heat entering room.
The Goal
Balance. High for winter heating, Low for summer cooling.
Best For
Preventing overheating (Low G).
What it Measures
Overall efficiency balance (A-G).
The Goal
HIGHER is Better. Aim for ‘A’ or ‘A+’.
Best For
Standard replacement windows.
2. U-Values Explained (The Critical Detail)
The U-value is the most scientific measure of insulation. It tells you: “If it is 1 degree colder outside than inside, how much heat will escape through one square metre of this window?”
The lower the number, the better the insulator. Single glazing is approx 5.8; old double glazing is 2.8; modern KJM double glazing is 1.2; and triple glazing can be as low as 0.8.
Be careful when comparing quotes. There are two types of U-value:
- Ug (Centre Pane): Measures the glass only. Usually a very low number (e.g., 1.0).
- Uw (Whole Window): Measures the entire unit including the frame. This is usually higher because frames lose more heat.
3. G-Values: Free Heat vs. Overheating
The G-value is expressed as a number between 0 and 1. A G-value of 0.75 means 75% of the sun’s heat enters the room.
The Conflict: In the UK winter, a high G-value is fantastic (free heating). However, in a highly insulated modern home with large south-facing glass, a high G-value can be disastrous in summer.
If you are building a south-facing extension, ask for Solar Control Glass (low G-value) to prevent overheating.
4. Window Energy Ratings (WER)
The WER is the colourful “rainbow” sticker managed by the BFRC. It puts the U-value, G-value, and air leakage into a formula to give a consumer-friendly letter.
5. The Science: How Low-E Glass Works
How can a piece of glass stop heat? The secret is Low Emissivity (Low-E) Coating.
This is a microscopically thin layer of metal oxide applied to the inner surface of the glass. It acts as a selective mirror:
- Short-Wave Radiation (Sunlight): Passes straight through, warming your room.
- Long-Wave Radiation (Body Heat): Bounces off the coating and reflects back into the room.
6. The Future Homes Standard 2025
Building Regulations are tightening. The upcoming Future Homes Standard Gov.uk aims to ensure new homes produce 75-80% less carbon emissions. To achieve this, U-value targets are expected to drop drastically, likely towards 0.80 W/m²K for new builds.
This standard is difficult for double glazing to meet, paving the way for Triple Glazing to become the new norm.
7. Frequently Asked Questions
Surprisingly, not always. Triple glazing has a fantastic (low) U-value, which is good. However, the extra pane of glass blocks some free solar heat (lower G-value). Because the WER formula rewards solar gain, some triple glazed units achieve a similar WER rating to high-spec double glazing, even though the triple glazing keeps the heat in much better.
Low-E (Low Emissivity) coatings reflect heat back into the room to lower the U-value. Modern ‘Soft-Coat’ Low-E glass offers the best balance: it is invisible to the naked eye, offers excellent insulation, and maintains a neutral G-value for natural light.
You need to check the G-value. If you have a south-facing room or a conservatory that overheats, you want a window with a lower G-value (often called Solar Control glass). This will block more of the sun’s energy from entering the room while still insulating against the cold in winter.
📚 Explore Our Glass Knowledge Hub
This article is part of our comprehensive series on high-performance windows. To understand the science, costs, and comparisons in more detail, start here:
Mark Pearce is a highly experienced specialist in the double glazing industry, having built his knowledge from the ground up through hands-on experience in installation, manufacturing, sales, and business management. Beyond his role at KJM, Mark is a keen blogger, writer, and traveller, often sharing insights on the latest innovations in the home improvement industry.
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