Understanding double glazing U-Values: How energy efficient are your windows?

If you’ve ever found yourself on a window installation website, you’ve likely encountered the term ‘U-Values,’ followed by a string of numbers and letters. Who thought windows would be so confusing!

Although U-Values actually serve a really important purpose, they measure the thermal efficiency of a window, which is arguably the most important performance indicator for new glazing in 2026.

The lower the U-value, the better the window is at reducing heat transfer.

Current UK building regulations require replacement windows to achieve a U-value of 1.4 W/m²K or lower, but if your windows are more than 15-20 years old, there’s a strong chance they’re falling well short of that standard. This means that your home will be colder in the winter and warmer in the summer, and crucially, your energy bills will be higher.

What is a U-value?

A U-value measures the rate of heat transfer through a building element, whether that’s a wall, roof or window. In plain terms, it tells you how quickly heat passes from one side of the product to the other. In summer, this could mean a menacing heatwave turning your home into a sauna, and in winter you could lose any precious heat generated from the radiator.

U-values are measured in watts per square metre kelvin (W/m²K). So, a window with a U-value of 1.4 W/m²K allows 1.4 watts of heat to escape for every square metre of glass, for every degree of temperature difference between inside and out.

The lower the number, the less heat escapes. Old single glazing typically sits at around 5.0-5.8 W/m²K, compare that to today’s best-performing triple glazing at 0.74 W/m²K, and the difference is extraordinary.

How are U-values calculated?

The U-value of a whole window considers several factors working together:

• The glass panes: How many, and the gap between them
• The gas fill: Argon or krypton gas between panes for better insulation
• Low emissivity (Low-E) coatings: A near-invisible layer that reflects heat back into the room
• The frame material: uPVC is our recommended frame material if thermal performance is your top priority

It’s a holistic approach to measuring thermal performance, which is why two windows that look identical can perform very differently.

It’s also worth noting that U-values differ from window energy ratings (WERs), which factor in solar gain alongside heat loss. U-values remain the most precise measure used in current building regulations.

Why are good U-values of 1.4 W/m²K or lower so important?

Lower energy bills

The Energy Saving Trust have regularly reported that upgrading from single glazing to A-rated double glazing can save a typical semi-detached home up to £110 a year on energy bills. Better thermal efficiency means your boiler works less hard, and you spend less keeping warm.

A more comfortable home

Poor thermal performance creates cold spots near frames and persistent draughts, even with the heating on. Windows with a low U-value keep interior glass surfaces warmer, reducing condensation and making your home noticeably more comfortable.

A lower carbon footprint

Less heat loss means less energy demand, and lower carbon emissions. Improving your windows is one of the most practical steps you can take to tighten your home’s thermal envelope and reduce your environmental impact.

What constitutes a good U-value?

These are estimates; the actual figure can vary depending on your location, the property itself, and several other factors.

The gap between old single glazing and today’s best-performing systems is remarkable, and upgrading can transform how your home feels and functions day to day.

What type of window offers the best thermal performance?

A quick guide to the energy efficient windows we offer at Leamore Windows.

uPVC casement windows

Casement windows are the most popular style across UK homes, largely because of how simple they are. Due to this simplicity, they are cost-effective, they just open in or out with a handle.

With double glazing they achieve around 1.3 W/m²K, dropping to approximately 1.0 W/m²K with triple glazing.

Flush sash windows

Our VEKA flush sash system sits flat within the outer frame, replicating the look of traditional timber. With 44mm triple glazing, it can achieve U-values as low as 0.79 W/m²K, an outstanding result for a window with such a classic aesthetic.

Tilt & turn windows

Tilt the handle 90° for secure ventilation, or 180° to open the whole sash inward. Practical, draught-proof and thermally impressive, achieving down to 0.81 W/m²K with triple glazing.

Vertical sliding windows

A perfect match for period properties, vertical sliding windows have been popular for centuries with their up-down movement. They’ve now evolved for the needs of the modern market, and modern versions are incredibly competitive when it comes to thermal performance, you won’t notice much difference with competitors.

Residence Collection windows

Whilst looking beautiful in the process, The Residence Collection can achieve U-values as low as 0.74 W/m²K with triple glazing. The frames are made using uPVC, but are designed to replicate the appearance of timber. That means realistic graining, wood effects, timber welded corners, but no maintenance and elite thermal performance.

Energy efficient windows from Leamore Windows

We’ve been manufacturing our own double-glazed units in the West Midlands since the mid-1980s, giving us direct control over quality. Our full range covers casement, flush sash, tilt and turn, vertical sliding and Residence Collection windows, all available with high-performance double or triple glazing, Low-E coatings and argon or krypton gas fills. Frame colours range from classic white and golden oak to anthracite grey and dual-colour combinations.

Whether you want to cut energy bills, reduce cold spots or increase your property value, we’ll help you find the right window for your home.

Who are Leamore Windows?

Established in 1983 in Walsall, we’ve spent over four decades earning a reputation as one of the West Midlands’ most trusted double glazing companies. We manufacture our own units, employ no-pressure salespeople and have won the ‘Windows and Conservatories’ Business of the Year award eight times.

We’re FENSA-registered and have featured on BBC One’s DIY SOS. From our showroom in Bloxwich, we serve homeowners across Walsall, Wolverhampton, Dudley, Lichfield, Stafford, Stourbridge and beyond.

Come and see our range in person, or get in touch today.

01922 473 737

Reaymer Close, Bloxwich, WS2 7QZ

Mon-Thu 7:00-17:30 | Fri 7:00-16:30 | Sat 8:30-15:00