Making Sense of Window Performance: U-values & BFRC Energy Ratings Explained

If you’re a homeowner thinking about upgrading to Liniar’s highly energy-efficient PVCu windows and doors, you’ve probably come across terms like U-values and BFRC Energy Ratings. 

It’s easy to get confused by this jargon, especially when different suppliers use different energy performance measures. Understanding what these terms really mean will give you the clearest picture of how your new Liniar windows and doors will perform, helping you make a well-informed purchasing decision.

What Is a U-value?

A U-value is a technical measure of heat transfer passing through a material – in this case a window or door. From the warm inside of your home to the cooler outside (in winter) or hot outside to cooler inside (in summer). It’s expressed in watts per square metre per degree (W/m²K). In simple terms, the lower the U-value, the slower heat transfers and the better the product helps keep your home at a year-round comfortable temperature.

Homeowners will see U-values used widely because they’re a core part of UK Building Regulations for energy performance. However, a U-value alone doesn’t tell the whole story. It measures only one aspect - heat transfer - and doesn’t account for how much warmth sunlight might let into a room or how draught-free a unit is.

What is a BFRC Energy Rating?

A BFRC Energy Rating takes a broader, more consumer-friendly view. Developed by the British Fenestration Rating Council, the UK’s most recognised and independent rating body for fenestration products, the system assigns ratings from A++ (most efficient) down to E.

Instead of focusing just on heat transfer, the BFRC rating considers three important factors:

• Thermal transfer (U-value) – how effectively the combined window frame and glass prevent heat transfer in/out.
• Solar gain (G-value) – The amount of solar heat passing through the glass, naturally warming your home
• Air leakage (L-value) – The amount of air passing through gaps or poor seals around frames

By weighing all three, the BFRC label gives you a more holistic perspective - not just how much heat might escape, but how a product performs in real-world conditions throughout the year. This makes it easier to compare products side-by-side and choose the one that offers the best overall performance for your home.

Why This Matters for Your Home

Understanding both U-values and BFRC ratings helps you make smarter choices about energy-efficient PVCu windows and doors, with real benefits which include:

Lower energy bills: A better thermal performance means less heat loss in the cold season and less heat gain in during hotter weather, which reduces reliance on expensive heating and cooling systems.

Improved comfort: Energy efficient windows and doors help maintain more consistent indoor temperatures - fewer cold spots in winter and less overheating in summer - contributing to better home thermal comfort all year round.

Reduced carbon emissions: More energy efficient windows play a role in creating low carbon homes and supporting the UK’s Net Zero future.

Final Thought

When comparing quotes, look for both U-values and BFRC Energy Ratings. A low U-value is important, but the BFRC rating puts it into context with solar gain and air leakage, giving you a clearer picture of performance in your everyday life. Knowing this empowers you to make choices that are not just technically sound, but genuinely beneficial for comfort, cost and sustainability.

You can see U-value capabilities for each Liniar product on our individual product pages and discuss you specific needs with your chosen supplier, who you can find at: https://www.liniar.co.uk/find-a-supplier