Secondary Glazing vs Double Glazing: Which Is Better for Noise and Heat?

One of the most common questions we hear from London homeowners is: "Should I get secondary glazing or double glazing?" It's a fair question, and the answer isn't as straightforward as many glazing companies would have you believe.

The truth is, both solutions have their place. But for noise reduction, listed buildings, and many period properties, secondary glazing consistently outperforms double glazing. Let's dive into the science and practicalities to help you make the right choice.

The Fundamental Difference

Double glazing replaces your existing windows entirely with sealed units containing two panes of glass separated by a small gap (typically 16-20mm) filled with air or argon gas.

Secondary glazing adds a completely separate window on the inside of your existing window, creating a much larger air gap (typically 100-200mm) between the original glass and the new pane.

This distinction is crucial because the size of the air gap determines acoustic performance, while the quality of the seal determines thermal performance.

Noise Reduction: Secondary Glazing Wins

If your primary concern is blocking traffic noise, aircraft noise, or urban sound pollution, secondary glazing is the clear winner. Here's why:

The Physics of Sound

Sound waves lose energy as they travel through air. The larger the air gap between two barriers, the more energy is lost. Double glazing's 16-20mm gap simply isn't enough to significantly attenuate low-frequency sounds like traffic rumble, bus engines, or aircraft noise.

Secondary glazing with a 100mm+ air gap can achieve noise reductions of 45-54dB when fitted with acoustic laminate glass. Standard double glazing typically achieves only 25-35dB.

Real-World Performance

Glazing Type	Air Gap	Typical Noise Reduction
Single glazing	N/A	20-25dB
Standard double glazing	16-20mm	25-32dB
Acoustic double glazing	16-20mm	32-38dB
Secondary glazing (standard)	100mm+	35-42dB
Secondary glazing (10.8mm acoustic)	100mm+	45-54dB

For homes near busy roads, flight paths, or railway lines, this difference is transformative. A 10dB reduction is perceived as halving the volume, so the gap between double and secondary glazing represents a genuinely life-changing improvement.

Thermal Performance: It's Closer Than You Think

Double glazing is often marketed as the gold standard for thermal insulation, but secondary glazing performs remarkably well:

• Double glazing U-value: 1.2-1.6 W/m²K
• Secondary glazing U-value: 1.8-2.2 W/m²K (when combined with existing single glazing)

Yes, double glazing has a slight edge in pure thermal terms. However, secondary glazing also eliminates draughts by creating an airtight seal on the inside of your window reveal. For many period properties with poorly fitting original windows, this draught elimination provides comfort improvements that rival or exceed double glazing.

Secondary glazing can reduce heat loss through windows by up to 65% and cut heating bills significantly.

Listed Buildings: Secondary Glazing Is Often Your Only Option

If you own a Grade I, Grade II, or Grade II* listed building, replacing original windows with double glazing typically requires Listed Building Consent—and it's frequently refused.

Conservation officers protect the "historic fabric" of listed buildings, and original windows are usually considered an essential part of that fabric. Even "heritage-style" double glazing often fails because:

• The glass reflection looks different to original single glazing
• The glazing bars are thicker to accommodate the sealed units
• The sash weights may need modification for the heavier glass

Secondary glazing, by contrast, is installed on the inside of your existing windows. It doesn't alter the external appearance and is fully reversible—two factors that make it acceptable to most conservation officers without formal consent.

For listed building owners in Kensington, Chelsea, and Westminster, secondary glazing represents the only viable path to modern comfort.

Cost Comparison

Costs vary significantly based on window size, access, and specification, but here's a general guide:

Double Glazing (per window)

• Standard UPVC: £400-£600
• Timber (heritage style): £800-£1,500
• Aluminium (slim profile): £1,000-£2,000

Secondary Glazing (per window)

• Standard 6mm glass: £250-£450
• Acoustic laminate glass: £400-£700
• High-spec heritage finish: £600-£1,000

Secondary glazing is typically 30-50% cheaper than equivalent-quality double glazing, with the added benefit of preserving your original windows.

Installation Disruption

Double glazing installation:

• Requires scaffolding for upper floors
• Original windows removed and disposed of
• Significant mess and debris
• 1-2 days per room
• House exposed to elements during installation

Secondary glazing installation:

• No scaffolding required (internal installation)
• Original windows preserved
• Minimal mess
• 2-4 hours per window
• No exposure to weather

When Should You Choose Double Glazing?

Double glazing makes sense when:

• Your existing windows are beyond repair
• You're in a new-build or unlisted property with no heritage constraints
• Noise isn't a significant concern
• You prioritise marginal thermal gains over acoustic performance

When Should You Choose Secondary Glazing?

Secondary glazing is the better choice when:

• Noise reduction is a priority
• You live in a listed building or conservation area
• You want to preserve original windows
• You need a cost-effective solution
• You want minimal installation disruption
• You're renting and can't modify the building

The Verdict

For most London period properties, secondary glazing offers the best balance of noise reduction, thermal performance, heritage preservation, and value. It's not a compromise—it's often the superior solution.

If you're unsure which option suits your property, request a free survey from our heritage glazing specialists. We'll assess your specific situation and provide honest advice—even if that means recommending double glazing for your particular circumstances.

Call 020 7060 1572 to discuss your options with an expert.