Is It Worth Replacing My Double Glazed Units? (The 30-Year Upgrade Guide)

At Williams Window Fabrication, we’ve been manufacturing and installing glass in Clydebank for exactly 30 years. One of the most common questions we get asked in our Advice Hub is: “My frames look fine, but is the glass still doing its job?”

Many homeowners don’t realize that while a uPVC frame can last for decades, the glass technology inside it has undergone a quiet revolution. If your double glazing was installed 20 or 25 years ago, you aren’t just losing heat—you are essentially living with an “obsolete” level of insulation.

The “Silver Strip” Test: Is Your Glass ‘Old and Cold’?

You can check the age and efficiency of your windows in five seconds without any tools. Look at the spacer bar—that thin strip that separates the two panes of glass.

• Is it Silver or Gold Metal? If your spacer is a shiny metallic material (usually aluminium), your windows are “Old and Cold.” Metal is a high-conductor of heat, meaning these spacers act like a thermal bridge, literally sucking the warmth out of your room and creating that familiar “cold spot” and condensation around the edge of your glass.

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  Warm edge black spacer

  Is it Black or Grey Foam? This is a Modern Warm-Edge Spacer. These are made from non-metallic, low-conductivity materials that keep the edge of the glass warm, reducing heat loss by up to 65% at the perimeter compared to the old metal versions.

The Science of Savings: What’s Inside Modern Glass?

Replacing a “blown” or misted unit is a no-brainer, but even clear old glass is often worth replacing because of three massive technological leaps:

1. Low-E (Low Emissivity) Coatings: Modern glass has a microscopically thin, transparent coating of metal oxide. This acts like a thermal mirror, reflecting the heat from your radiators back into the room rather than letting it escape through the glass.

2. Argon Gas Filling: 25 years ago, the gap between your panes was just plain air. Today, we fill that cavity with Argon gas. Argon is denser than air, which significantly slows down the movement of heat (convection) between the panes.

3. Black Warm-Edge Spacers: As mentioned above, these eliminate the “cold bridge” at the edge of the unit, significantly reducing the risk of internal condensation and mould.

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The Numbers: 25-Year-Old Glass vs. Modern High-Performance Glass

To put this into perspective, let’s look at a standard 1m x 1m window—roughly the size of a standard kitchen or bedroom window.

The “U-Value” is the measure of heat loss (the lower the number, the better the window is at keeping heat in).

What does this mean for a 1m x 1m window?

By simply swapping the glass units (keeping your existing frames), you cut the heat escaping through that specific opening by more than half. Across a whole house, this can lead to a drastic reduction in heating bills and a noticeable increase in comfort—no more sitting next to a “chilly” window in the winter.

Is It Time to Upgrade?

If your windows are reaching their 20th or 30th birthday, they have served you well, but they are likely the weakest link in your home’s insulation.

Replacing just the glass units is:

• Faster: Most houses can be completed in a single day.

• Cleaner: No mess, no scaffolding, and no re-plastering required.

• Cheaper: It can cost up to 50% less than a full frame replacement while giving you nearly the same energy-saving benefits.

Ready to see the difference? As we celebrate 30 years of fabricating in Clydebank, let the experts at Williams Windows & Doors give your home a 2026 upgrade.

Contact us today for a free quote on replacement glass units.