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U Values Explained

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The Basics

What You Need to Know About U-Values

What is a U-Value?

A U-value measures how much heat passes through a material. It's expressed in watts per square metre per degree Kelvin (W/m²K). The lower the number, the better the insulation — and the warmer your home stays.

Why Lower is Better

A single-glazed window has a U-value of around 5.0 — meaning it loses heat rapidly. A modern double-glazed unit with Low-E coating achieves 1.3, keeping your home warmer and cutting energy bills significantly.

What Affects the U-Value?

Three factors determine a sealed unit's U-value: the glass coating (Low-E reduces radiated heat loss), the gas fill (argon or krypton slow conduction), and the spacer bar (warm-edge spacers reduce thermal bridging at the glass edge).

Building Regulations in England require replacement windows to achieve a maximum U-value of 1.4 W/m²K. Our Optitherm S1 specification comfortably beats this at 1.3 W/m²K.

At a Glance

U-Value Comparison by Glass Type

Glass Type U-Value (W/m²K) Energy Rating Best For
Single Glazed (4mm) 5.0 G Period properties (conservation areas)
Old Double Glazed (6-12-6) 2.8 E Replacement candidate
Standard Double (4-16-4) 2.0 C Budget-friendly upgrade
Low-E K Glass (4K-16-4) 1.8 B Good all-round performance
Optitherm S1 (4-16Ar-4S1) 1.3 A Exceeds Building Regulations
Triple Glazed (4-12-4-12-4) 0.8 A+ Maximum thermal performance
Compliance

Building Regulations & U-Values

Under Part L of the Building Regulations, replacement windows in England must achieve a maximum whole-window U-value of 1.4 W/m²K, or a minimum Window Energy Rating (WER) of Band C. These requirements apply when you are replacing an entire window unit — frame and glass together.

However, if you are only replacing the sealed glass unit within an existing frame, this is classified as a repair rather than a replacement. Building Regulations do not apply to repairs, so you are free to choose any glass specification. That said, we strongly recommend upgrading to Low-E glass whenever possible — the cost difference is minimal and the long-term energy savings are substantial.

All our replacement window installations are registered through FENSA (Fenestration Self-Assessment Scheme), which provides Building Control sign-off and a certificate for your records. This certificate is required when selling your property.

Replacing sealed units within existing frames is classified as a repair — Building Regulations do not apply. We still recommend upgrading to Low-E glass for long-term energy savings.

Compare Your Options

Use our free U-value comparison calculator to see exactly how much heat loss you could reduce by upgrading your glazing.

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Common Questions About U-Values

Frequently Asked Questions

Everything you need to know about our double glazing repair and replacement services across the South West.

  • For replacement windows in England, the maximum permitted whole-window U-value is 1.4 W/m²K. Alternatively, windows can meet the requirement with a minimum Window Energy Rating of Band C. Scotland, Wales, and Northern Ireland have slightly different requirements — contact us for specific guidance.

  • For insulation, yes — a lower U-value means less heat loss. However, there are diminishing returns. The jump from single glazing (5.0) to standard double (2.0) saves far more energy than going from 1.3 to 0.8. Triple glazing offers the best performance but is heavier, more expensive, and not always necessary. We help you find the right balance of performance and cost.

  • A glass U-value (Ug) measures the thermal performance of the glass pane itself — measured at the centre of the pane. A window U-value (Uw) includes the frame, spacer bars, and edge effects. The whole-window Uw is always higher (worse) than the centre-pane Ug because frames and edges conduct more heat. Building Regulations refer to the whole-window Uw value.

  • Yes — argon gas fill improves a sealed unit's U-value by approximately 0.3 W/m²K compared to dry air. Argon is denser than air, which slows heat transfer by convection between the glass panes. It's odourless, non-toxic, and makes up about 1% of the atmosphere. The cost difference is small, so we recommend argon fill as standard on all Low-E units.

  • If you don't have documentation, you can estimate based on the age and type of your windows. Pre-2002 double glazing typically has a U-value of 2.8–3.0. Post-2002 units are usually 1.6–2.0. Modern Low-E units (post-2010) are typically 1.2–1.4. If you send us a photo of your current windows, we can provide a more accurate estimate and recommend the best upgrade path.

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