Understanding Window Energy Ratings
David ReynoldsIn this article
1Introduction
Did you know that up to 25% of your home's heat can be lost through poorly performing windows? In the UK's often challenging climate, especially in the North East, choosing the right energy-efficient windows is not just about comfort—it's about substantial cost savings and reducing your environmental footprint.
At Window Warriors, we've helped thousands of Newcastle homeowners transform their properties with energy-efficient window solutions. Yet we find that while many people know they want "energy-efficient windows," relatively few understand the rating system that helps compare different options and make informed decisions.
Why Window Energy Ratings Matter
Window Energy Ratings (WERs) provide a standardized way to compare the energy performance of different windows. Understanding these ratings can help you make better choices for your home, potentially saving thousands of pounds in energy costs over the lifetime of your windows while creating a more comfortable living environment.
2What are Window Energy Ratings?
Window Energy Ratings (WERs) are a standardized system used in the UK to measure and compare the energy efficiency of windows. Introduced by the British Fenestration Rating Council (BFRC), these ratings allow homeowners to make informed decisions when selecting new windows.
The Window Energy Rating Label
Each rated window displays a rainbow-colored label similar to those found on household appliances. The rating scale runs from A++ (most efficient) to E (least efficient), with each grade representing the window's overall energy performance.
The label provides a simple visual way to compare different windows at a glance, making it easier for homeowners to choose the most energy-efficient options for their property.
How Ratings Are Calculated
Window energy ratings aren't arbitrary—they're based on a scientific calculation that considers three key factors:
- Solar heat gain (g-value): How much heat from the sun passes through the window
- Heat loss (U-value): How much heat escapes through the window
- Air leakage (L-value): How much air can pass through the window joints
The Rating Formula
The overall energy rating is determined by the following formula:
Energy Index = Solar Heat Gain – Heat Loss – Air Leakage
(g-value × 0.9) – (U-value × 1.0) – (L-value × 0.3)
This calculation produces a single energy index number, which is then mapped to the A++ to E rating scale. The higher the number, the better the window's overall energy performance.
Window Warriors Expert Tip
While the overall rating is useful for quick comparisons, understanding the individual components (U-value, g-value, and L-value) can help you choose windows that are optimized for your specific property's orientation and exposure. For north-facing windows, prioritize low U-values, while south-facing windows might benefit from optimized g-values to balance solar gain with heat retention.
3Understanding the Key Factors: U-value, G-value, and L-value
To truly understand window energy efficiency, it's essential to delve deeper into the three primary factors that determine a window's performance: the U-value, G-value, and L-value. Each of these measurements provides valuable insight into different aspects of a window's thermal efficiency.
U-value: Heat Loss Through the Window
The U-value measures how easily heat can pass through a window. It quantifies the rate of heat transfer through a structure divided by the difference in temperature across that structure.
U-value is measured in W/m²K (Watts per square meter per Kelvin)
Lower U-values indicate better insulation. For example, a single-glazed window might have a U-value of 5.0 W/m²K, while a high-performance triple-glazed window could have a U-value as low as 0.8 W/m²K. This means the triple-glazed window allows approximately 6 times less heat to escape.
Typical U-values for Different Window Types
| Window Type | Typical U-value (W/m²K) | Relative Performance |
|---|---|---|
| Single glazing | 4.8 - 5.8 | Poor |
| Double glazing (standard) | 2.0 - 3.0 | Average |
| Double glazing (low-E) | 1.2 - 1.8 | Good |
| Triple glazing | 0.8 - 1.2 | Excellent |
G-value: Solar Heat Gain
The G-value (also known as the Solar Heat Gain Coefficient or SHGC) measures how much solar radiation passes through a window. It represents the percentage of solar heat that enters through the window compared to the total solar radiation falling on it.
G-value is expressed as a number between 0 and 1 (or as a percentage)
Higher G-values indicate more solar heat gain. For example, a G-value of 0.7 means 70% of the available solar heat passes through the window. This can be beneficial in winter (providing free heating) but potentially problematic in summer (causing overheating).
Balancing U-value and G-value
The ideal balance between U-value and G-value depends on your climate and the orientation of your windows. In cold climates, you might want windows with low U-values and higher G-values on south-facing elevations to maximize heat gain in winter. In warmer climates or for windows with high sun exposure, you might prefer lower G-values to prevent overheating.
L-value: Air Leakage
The L-value measures the amount of air that can pass through the window frame joints. It quantifies how airtight a window is, which affects both energy efficiency and comfort by preventing drafts.
L-value is measured in m³/h/m² (cubic meters per hour per square meter)
Lower L-values indicate less air leakage. Modern, high-quality windows typically have L-values below 1.0 m³/h/m². Windows with poor seals or aging frames can have much higher values, leading to significant heat loss and uncomfortable drafts.
Window Warriors Expert Tip
Even the most thermally efficient glazing will underperform if the window isn't properly installed or has poor airtightness. When replacing windows, ensure that installation is carried out by qualified professionals who use proper sealing techniques around the frame to minimize air leakage.
4How to Read and Compare Window Energy Labels
When shopping for new windows, understanding how to interpret energy labels is crucial for making informed decisions. The standardized Window Energy Rating (WER) label provides a wealth of information that can help you compare different options.
Anatomy of a Window Energy Rating Label
- 1
Energy Rating Band
The letter grade from A++ (most efficient) to E (least efficient) that gives you an at-a-glance indication of the window's overall energy performance.
- 2
Energy Index
The numerical value that determines the rating band. A positive value indicates that the window provides net energy gain.
- 3
U-value
The rate of heat loss through the window (lower is better).
- 4
G-value
The percentage of solar heat gain through the window.
- 5
L-value
The air leakage rating, indicating how airtight the window is.
- 6
Verification & Certification
Look for the BFRC logo and license number to ensure the ratings are independently verified.
Understanding the Rating Bands
| Rating | Energy Index (kWh/m²/year) | Efficiency Level | Description |
|---|---|---|---|
| A++ | 20 or more | Exceptional | Net energy gain; contributes more energy to the home than it loses |
| A+ | 10 to 19 | Excellent | Very high energy performance; likely to provide net energy gain in ideal conditions |
| A | 0 to 9 | Very Good | High energy performance; minimal heat loss |
| B | -10 to -1 | Good | Good thermal performance; meets current building regulations |
| C | -20 to -11 | Average | Standard thermal performance; acceptable for many applications |
| D | -30 to -21 | Below Average | Moderate heat loss; typically older double glazing or basic products |
| E | -70 to -31 | Poor | High heat loss; typically single glazing or very old double glazing |
Building Regulations Requirements
Current UK Building Regulations typically require new windows to achieve a minimum WER of Band C or a U-value of 1.6 W/m²K for replacements in existing dwellings. For new builds, the requirements are more stringent, typically requiring Band B ratings or better. Always check the current regulations as they are periodically updated to improve energy efficiency standards.
Tips for Comparing Window Energy Labels
Look Beyond the Letter Rating
While the overall letter rating (A++, A, B, etc.) provides a useful snapshot, don't forget to examine the specific U-value, G-value, and L-value. Two windows with the same overall rating might have different performance characteristics that could be more suitable for your specific needs.
Consider Window Orientation
For south-facing windows, you might prioritize a balance of U-value and G-value to control solar gain. For north-facing windows, a lower U-value becomes more important as there's less direct sunlight to offset heat loss.
Verify the Certification
Ensure the windows you're considering have been rated by an accredited body like the BFRC. Look for the certification logo and registration number on the label to confirm the ratings are legitimate and independently verified.
Calculate Potential Energy Savings
Use the energy index value to estimate potential energy savings compared to your existing windows. While this won't give you an exact figure, it can help you assess the potential return on investment for different window options.
Window Warriors Expert Tip
When comparing prices between different window providers, ensure you're comparing products with similar energy ratings. A significantly cheaper window with a lower energy rating may cost you more in the long run through increased heating bills. Always ask for the window energy label details if they're not prominently displayed.
5Conclusion & Recommendations
Window energy ratings provide a valuable framework for comparing the thermal performance of different windows and making informed decisions for your home. As energy costs continue to rise and environmental concerns become increasingly important, investing in energy-efficient windows is both economically sensible and environmentally responsible.
Key Takeaways
Higher ratings mean better efficiency: Windows with ratings of A+ or A++ offer the best thermal performance and can contribute to significant energy savings.
Balance is key: Consider all three performance factors (U-value, G-value, and L-value) rather than focusing solely on the overall rating.
Context matters: The ideal window specifications vary based on your property's orientation, local climate, and specific requirements.
Installation matters: Even the most energy-efficient windows will underperform if poorly installed. Professional installation is crucial for achieving optimal performance.
Think long-term: While energy-efficient windows may have a higher upfront cost, the energy savings over their lifetime typically justify the investment.
Our Recommendations
1For Maximum Energy Efficiency
Choose windows with an A++ or A+ rating, featuring triple glazing and low-E glass. These provide the best insulation and will minimize heat loss in all conditions.
Best for: Homes in cold climates, north-facing elevations, or properties where minimizing heat loss is the primary concern.
2For Balanced Performance
Select windows with an A rating that balance good U-values with moderate G-values. This provides excellent thermal performance while managing solar gain.
Best for: Most UK homes, mixed orientations, and properties where a good balance of heat retention and solar management is desired.
3For Budget Considerations
If budget is a constraint, focus on windows with a B rating that still offer good thermal performance while being more cost-effective than top-rated models.
Best for: Rental properties, phased renovation projects, or homeowners working with limited budgets who still want good energy performance.
Remember that window energy ratings, while important, are just one factor to consider alongside aesthetics, security features, ventilation options, and durability. The optimal choice for your home will depend on balancing all these factors within your budget and architectural requirements.
By understanding window energy ratings, you'll be well-equipped to make informed decisions that can enhance your home's comfort, reduce your energy bills, and minimize your environmental footprint for years to come.
Need Help Choosing Energy-Efficient Windows?
Our window specialists can guide you through the options and help you select the perfect energy-efficient windows for your home. Book a free consultation today!
James W.
Thermal Performance Specialist | Window Warriors
James has over 15 years of experience in the fenestration industry, specializing in thermal performance testing and window energy certifications. He regularly contributes to industry publications and has helped numerous homeowners optimize their window choices for maximum energy efficiency.
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