What is a U Value?
The U value measures the rate at which heat passes through a material such as glass or an aluminium frame. It is expressed in watts per square metre per kelvin (W/m²K).
- Single glazed glass has high U values, which means rapid heat loss.
- Double glazing lowers this substantially, keeping rooms warmer.
- Advanced glazing and thermally broken frames can achieve excellent U values, making modern front doors, bifold doors and sliding doors suitable even for energy-conscious new builds.
It’s worth noting that here at Express when we talk about U values in the context of building regulations, we mean the overall U value of the complete product (glass and frame combined). Some companies may only quote the U value of the glass alone, which is usually lower and sounds more impressive, but that doesn’t give the full picture. The number that really matters, and the one used for compliance, is always the overall U value.
All our products come with high-performance double glazing as standard, meeting or exceeding the latest building regulation requirements. Combined with thermally broken aluminium frames, our premium doors and windows deliver outstanding insulation and long-lasting efficiency.
Why U Values Matter for Your Home
A lower U value means your home is easier and more affordable to heat. Choosing doors with good U values offers:
- Confidence that your property meets current regulations
- A more comfortable environment throughout the year
- Lower heating costs
- Added kerb appeal and increased ROI when selling your home
At Express Direct, we know that efficiency matters to every customer, whether you’re a DIY homeowner looking for pre-made bifold doors or a tradesperson working to tight timescales. That’s why we keep a wide range of stock bifold doors with tested U values available for delivery within five working days.
“Customers often ask us what is a U value and whether it really makes a difference. My answer is simple: it makes a huge difference. Every aluminium door and window we sell is designed with energy efficiency in mind. With U values that meet or exceed current regulations, and stock products ready to go within days, our customers know they’re getting excellent performance without a long wait.”
— Steve Bromberg, Managing Director, Express Bifolds Direct
How to Calculate a U Value
While many people search for how to calculate U value or how to work out U values, the process itself is highly technical. In simple terms, the U value is calculated by dividing the thermal conductivity (λ value) of each material by its thickness, then combining the results across the full product. This is why elements such as glazing type, gas fills, coatings, and thermal breaks in aluminium frames all affect the final figure.
It’s important to remember that the figure which counts is the overall U value of the window or door (frame + glazing combined), not just the glass alone. Less reputable suppliers may only quote the glass U value, but this does not reflect the performance of the full product.
Rather than working this out yourself, it’s best to rely on certified figures from trusted suppliers. At Express Direct, all our U values are independently tested, giving you accurate and reliable performance data.
Every product in our range features thermally broken aluminium frames and high-performance double glazing, achieving an overall U value of 1.1 as standard. To put this into perspective, UK building regulations require windows to achieve a maximum U value of 1.6 W/m²K, and doors 1.8 W/m²K. Our doors never exceed 1.6, and our windows achieve 1.5 or lower, with many products performing even better.
FAQs
A U value of around 1.2 W/m²K or lower is considered good for double glazing. At Express Direct, our double glazing exceeds this with a U value of 1.1 as standard.
Not at all. Thanks to thermally broken aluminium frames and high-performance glazing, our front doors, bifold doors and sliding doors offer competitive U values comparable to, and often better than, many standard windows.
• U value measures how much heat passes through a material (lower is better).
• R value is the inverse as it measures resistance to heat flow (higher is better).
• G value (or solar factor) measures how much solar heat passes through glazing.
Yes, triple glazing usually achieves a lower U value than double glazing because of the extra pane and gas-filled cavity. However, the overall efficiency also depends on the frame design and installation quality.
U values tell you how well windows and doors keep heat in, and the figure that matters most is the overall U value of the full product, not just the glass. Understanding U values helps you choose products that make your home more comfortable, reduce energy bills, and meet the latest regulations.
At Express Direct, every door and window is designed with performance in mind. With tested U values, high-performance double glazing, and thermally broken aluminium frames, you can be confident in both efficiency and quality. And with a wide range of stock bifold doors ready for delivery within five working days, upgrading your home has never been simpler.
Explore our range online or visit one of our showrooms today, and let our expert team help you find the right products for a warmer, more efficient home.