What U-Value Should I Be Looking For in New Windows and Doors?

If you’re comparing new windows and doors, you’ll quickly come across the term U-value.

But what does it actually mean — and what U-value should you realistically be looking for?

Here’s the simple answer:

For modern UK homes, you should typically be looking for a U-value of 1.4 W/m²K or lower for windows and doors — ideally closer to 1.2 W/m²K for higher performance systems.

Let’s break down why that matters.

What Is a U-Value?

A U-value measures how much heat passes through a window or door.

The lower the number, the better the insulation.

• High U-value = more heat escapes

• Low U-value = better thermal efficiency

It’s measured in W/m²K (Watts per square metre Kelvin).

In simple terms:
Lower U-value = warmer home = lower energy loss.

What U-Value Do Building Regulations Require?

Under current UK Building Regulations (Part L):

• Replacement windows typically must achieve 1.4 W/m²K or better

• New-build specifications may require 1.2 W/m²K or better

Most quality modern aluminium and uPVC systems meet or exceed this — but not all are equal.

What’s Considered a Good U-Value for Aluminium Windows?

Modern aluminium windows with thermal breaks and double glazing usually achieve:

• Around 1.4 W/m²K as standard

• Around 1.2–1.3 W/m²K for higher-spec systems

• Even lower with triple glazing options

Premium aluminium systems are designed with advanced thermal barriers inside the frame to prevent heat transfer — something older aluminium systems didn’t have.

That’s why modern aluminium performs very differently to what people remember from 20+ years ago.

What About Aluminium Bifold and Sliding Doors?

Because doors contain more glass, performance depends heavily on the glazing specification.

Typical U-values:

• Standard double-glazed aluminium bifolds: 1.4–1.5 W/m²K

• Higher-performance systems: 1.2–1.3 W/m²K

• Triple-glazed options: Can improve further

It’s important to compare like-for-like when reviewing specifications.

Is Lower Always Better?

In theory, yes — but there’s a point of diminishing returns.

For most UK homes:

• Anything around 1.2–1.4 W/m²K performs very well

• Extremely low U-values may increase cost significantly

• The overall installation quality matters just as much

Air tightness, correct fitting, and proper sealing all impact real-world performance.

Aluminium vs uPVC U-Values

uPVC often achieves slightly lower U-values more easily due to the insulating nature of the material.

However, modern aluminium systems close that gap significantly thanks to:

• Polyamide thermal breaks

• Advanced gasket systems

• High-performance glazing

In practical terms, both materials can meet current regulations comfortably when properly specified.

So What Should You Choose?

If you're replacing windows or doors in an existing property:

Aim for 1.4 W/m²K or lower.

If you're working on a new build or want enhanced performance:

Look closer to 1.2 W/m²K.

More important than chasing the lowest number is choosing a well-engineered system from a reputable manufacturer and ensuring correct installation.

Final Thoughts

When comparing windows and doors, U-value is one of the most important performance indicators — but it shouldn’t be viewed in isolation.

Strength, security, longevity, glazing specification and installation quality all play a role in how your home performs.

If you’re unsure which specification suits your property, the Home Build Doors team can help you understand the differences before you place an order.