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What is Secondary Glazing? Types, Benefits & Costs Explained
What is Secondary Glazing? Types, Benefits & Costs Explained
Category: Windows / Heritage
Reading Time: 6 Minutes
📌 What You Need to Know
- Definition: A discreet, internal window installed behind your existing primary window.
- The Benefit: It provides modern thermal and acoustic performance without altering the external character of the building.
- The Usage: Ideal for Listed Buildings, conservation areas, or where noise pollution is extreme.
If you live in a Listed Building in Winchester or a noisy flat in Andover, you might feel stuck. You can’t change your old single-glazed windows because of planning rules, or perhaps you just don’t want to rip out beautiful original timber frames.
This is where Secondary Glazing comes in. It is the unsung hero of the glazing world—a high-performance solution that sits quietly on the inside, keeping the heat in and the noise out.
Table of Contents
1. What Exactly is Secondary Glazing?
Unlike double glazing, which involves replacing the entire window unit (frame and glass), secondary glazing is an independent window system installed on the room side of your existing window.
It creates a “double window” effect. The original external window remains untouched, preserving the kerb appeal and historic fabric of the property. The new internal frame is usually made of slimline aluminium, which is durable, lightweight, and can be colour-matched to blend in with your interior decor.
2. The 4 Main Styles (And Where to Use Them)
One size does not fit all. To ensure your windows still open and function correctly, you need to choose the right style of secondary glazing. Here are the four main types we install at KJM Group.
Opening Styles
Best For: Traditional Sash Windows.
These slide up and down, matching the movement of your original sash window. The meeting rails are aligned so the secondary frame is virtually invisible from the outside.
Best For: Casement windows or wide openings.
Two or more panels slide left and right on tracks. This is the most versatile option for general windows and allows easy access for cleaning.
Best For: Fire escapes or large clear openings.
The unit opens like a door on side or top hinges. This provides 100% clear access to the outer window, making it essential for fire egress routes.
Best For: Windows you rarely open.
A simple panel that lifts out of the frame completely. It offers the tightest seal (great for economy) but is heavier to remove for cleaning.
3. Why Choose It Over Replacement?
🏛️ Heritage & Planning
For Grade I and Grade II listed buildings, replacing original timber windows is often forbidden. Secondary glazing is classed as a “reversible alteration,” meaning it rarely requires Listed Building Consent. Historic England External provides specific guidance confirming it as a preferred method for improving energy efficiency in historic homes.
🔇 Superior Soundproofing
Ideally, for soundproofing, you want a large gap between the panes of glass. Standard double glazing has a gap of 16mm-20mm. With secondary glazing, we can install it 100mm+ away from the primary window. This large air gap effectively “decouples” the panes, stopping sound waves from traveling through.
🔥 Thermal Efficiency
It eliminates the draughts that rattle through old sash windows. By creating a sealed pocket of air, it can reduce heat loss through the window by up to 65%, significantly lowering your energy bills.
4. Glass Options: Thermal vs. Acoustic
Just like primary windows, you can choose the glass to suit your needs:
- Standard Glass: Good for general draft proofing.
- Stadip Silence (Acoustic): A special laminated glass with an acoustic interlayer. Essential if you live on a main road.
- Low-E Glass: Features a coating to reflect heat back into the room, maximizing thermal retention.
5. How Much Does it Cost?
Because secondary glazing uses a slimline aluminium frame rather than a heavy-duty structural uPVC frame, it is generally more affordable than full window replacement.
However, the price varies significantly based on the opening style you choose. Here is a general hierarchy of costs to help you budget:
| Style | Relative Cost | Why? |
|---|---|---|
| Lift-Out Unit | £ (Lowest) | Simple design with no moving mechanical parts. Ideal for budget projects. |
| Horizontal Slider | ££ (Medium) | Requires tracks and rollers but is a standard manufacturing process. |
| Hinged Unit | ££ (Medium) | Requires compression seals and hinges but typically single pane. |
| Vertical Slider | £££ (Highest) | Complex spiral balance mechanism needed to hold the window open safely (counter-weight). |
6. Frequently Asked Questions
Yes, it is very effective. By creating an internal seal, it prevents the warm, moist air inside your home from hitting the cold single-glazed outer window. This stops the water droplets forming on the glass.
Absolutely. We design the secondary system to match your primary window. If you have a sliding sash, we fit a sliding secondary unit. You simply slide the inner pane up, then the outer pane up, to get fresh air.
If installed with a 100mm+ air gap and acoustic glass, secondary glazing can reduce noise by over 50 decibels. To put that in perspective, standard double glazing reduces noise by about 30-35 decibels.
📚 Explore Our Secondary Glazing Hub
Now you know the basics, dive deeper into the specific benefits relevant to your home:
Mark Pearce is a highly experienced specialist in the double glazing industry, having built his knowledge from the ground up through hands-on experience in installation, manufacturing, sales, and business management. Beyond his role at KJM, Mark is a keen blogger, writer, and traveller, often sharing insights on the latest innovations in the home improvement industry.
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