How To Double Glaze Existing Windows

If you are searching for how to double glaze existing windows, you are probably living with cold draughts in winter, hot rooms in summer and more outside noise than you would like. The good news is you do not always need full window replacements to get the benefits of double glazed windows. There are several ways to upgrade existing windows and doors so they perform more like modern energy efficient double glazing. In this guide, we walk through the main options for double glazing existing windows, how they work, what you can DIY and when to hire a professional. You will also see how retrofitting double glazing can improve energy efficiency, cut energy bills and reduce noise pollution while keeping your existing window frames where possible.

Before You Start

Before you jump into retrofit double glazing or secondary glazing systems, it helps to understand your current window setup and what you want to change. That way you can match the right solution to your home and budget.

Check The Condition Of Your Existing Windows

Start with a slow walk around the house and look closely at each existing window and door:

• Inspect timber windows and sash windows for rot, peeling paint and swollen or warped sections
• Check aluminium frames for corrosion, bent sections and loose joints
• Look for cracked or loose glass panes in each single glazed window
• Run your hand around the edges to feel for draughts and air leakage
• Note any condensation, moisture buildup, mould or staining around frames and sills

Retrofitting double glazed windows depends on existing window frames being strong and reasonably square. If current windows are badly damaged, rotten or very out of alignment, complete window replacement with new double glazed windows will usually give better long term performance.

Work Out Your Goals (Heat, Noise, Condensation, Security)

Double glazing existing windows is not just about energy efficiency. It can solve different comfort and performance issues, so be clear about your priorities:

• Heat and energy: reduce heat loss through glass in winter, limit heat gain in summer and lower ongoing energy costs
• Comfort: keep indoor temperature more stable and avoid cold glass and draughts
• Noise: reduce noise pollution from traffic, neighbours or nearby businesses
• Condensation: cut down on fogged glass and moisture that damages plaster and timber windows
• Security: make windows harder to break and frames harder to force

Both retrofit double glazing and secondary glazing create an extra insulating layer and air gap that improves thermal performance. The best choice will depend on whether heat, noise reduction or preserving original window frames matters most to you.

Decide On Budget And How Much DIY You Want To Do

Your budget and your appetite for DIY will shape which double glazing option makes sense:

• Secondary glazing costs less than full double glazed units and can often be fitted as a DIY solution
• Retrofit double glazing in existing window frames usually costs less than complete window replacement but more than simple secondary glazing
• High performance systems like acrylic panel secondary glazing are often around one third of the cost of new double glazed windows per square metre

If you are confident with tools, you may choose a DIY kit for some windows and professional installation for trickier openings. If you want guaranteed performance and acoustic insulation, a professional installation is normally the safest path.

Option 1: Retrofit Insulated Glass Units (IGUs) Into Existing Frames

Retrofit double glazing is the closest approach to having a brand new double glazed window, while still keeping your current window frames in place.

What Retrofit Double Glazing Is

Retrofit double glazing replaces the single glaze inside an existing window with an insulated glass unit. An IGU or double glazed unit usually includes:

• Two glass panes separated by a spacer bar
• An air gap or argon gas filled gap between the panes
• Desiccant inside the spacer to manage moisture
• Seals around the edges to create an airtight seal

These double glazed units are then fitted into your existing window frames. Double glazing improves energy efficiency by slowing heat transfer through conduction, convection and radiation and can reduce heat loss or heat gain through windows by close to 30 percent compared to single glazing in many homes.

When Your Frames Are Suitable (Aluminium, Timber, UPVC)

Not every existing window can accept an IGU. Retrofit double glazed window units work best where:

• Aluminium frames have enough depth to accept a thicker double glazed unit
• Timber windows are solid, with no major rot and with sashes that can be rebated a little deeper
• The frame and sash joints are strong enough to support the extra weight of two panes of glass

If original window frames are badly warped, have very poor insulation or leak air through gaps and cracks, retrofit double glazing may not solve the underlying issue. In that case, new double glazing with thermally broken frames or high performance uPVC frames may be more effective.

Step By Step Overview

Even when you use professional installation, it helps to understand the basic process of retrofitting double glazed units into existing frames.

Measure And Assess Existing Sashes And Frames

The installer measures each existing window and checks that the current window frame can support the extra thickness and weight. They will look at frame depth, drainage channels, frame condition and how the sashes move.

Order New Double Glazed Units To Suit

Based on those measurements, new double glazed units are ordered. You will choose glass types such as clear glass, toned glass, laminated glass, low E glass or combinations depending on whether noise reduction, solar control or maximum insulation is the priority. The gap between panes and whether the unit is filled with air or argon gas also affects thermal performance.

Remove Old Single Glazing And Prepare Frames

The old single glazing is removed from the sash or frame, along with any old putty or beads. The installer cleans the rebate and makes small changes where necessary so the new double glazed unit sits correctly and can drain.

Fit And Seal The New Double Glazed Units

The double glazed unit is placed into the frame on packers so glass is separated from the bottom of the frame and can drain. Beads and seals are installed around the edges and a continuous bead of sealant is applied to create an airtight seal and protect the IGU edges from moisture.

Check Operation, Drainage And Sealing

Finally, the window is tested to make sure it opens, closes and locks properly, drainage slots are clear and no obvious drafts are present. Correctly installed retrofit double glazing will help keep warm air inside in winter and reduce external heat entering in summer, while still allowing natural light into the room.

Pros And Cons Of Retrofit IGUs

Retrofit double glazing existing windows has several advantages:

• Improves energy efficiency and can cut heating costs noticeably
• Reduces external noise and improves acoustic insulation
• Makes glass tougher to break, improving security
• Preserves the look of traditional windows and older facades
• Is less disruptive than complete window replacements

The main drawbacks are:

• Dependent on the condition and design of existing frames
• Generally requires professional installation
• May not fully solve draughts if the window frame itself leaks air
• Costs more than basic secondary glazing, although less than full replacement windows in most cases

Option 2: Add Secondary Glazing On The Inside

Secondary glazing is often the most cost effective alternative to traditional double glazing, especially when you want to keep perfectly good windows in place.

What Secondary Glazing Is

Secondary glazing adds a second pane of glass or an acrylic panel on the inside of a single glazed window, creating an insulating air gap between the two panes. It works in a similar way to double glazed windows by:

• Trapping a layer of still air that reduces heat transfer
• Reducing heat loss in winter and limiting heat gain in summer
• Cutting external noise by trapping sound between panes

Secondary glazing is popular for older windows, heritage facades and situations where complete window replacement is not allowed or would be very costly.

Types Of Secondary Glazing Systems (Fixed, Sliding, Magnetic, Clip On)

There are different secondary glazing systems for different window types and budgets:

• Fixed acrylic or glass panes for non opening windows
• Sliding or lift out panels for double hung windows and sliding windows
• Hinged internal panels for casement and awning windows
• DIY acrylic panel kits with magnetic seals or clip on trims

Some professionally installed systems use clear acrylic panels mounted on timber or aluminium frames with magnets to hold them in place. These can achieve similar thermal performance to new double glazing while keeping current windows and doors intact.

Step By Step Overview

Secondary glazing is often quicker and less invasive than retrofitting double glazed units.

Choose System And Measure Openings

First, you choose a system that matches your window types and how often you need to open them. You then carefully measure each opening so the secondary panels fit snugly and create an effective insulating layer. Accurate measurements are essential, particularly around older windows with slightly uneven frames.

Prepare Reveals, Trims Or Tracks

The installer fixes small trims, tracks or magnetic strips to the existing window frames or reveals. These provide support for the second pane and set the width of the air gap between glass panes.

Install Secondary Panels And Seals

The acrylic panel or glass sheet is installed onto the trims or magnets, forming the secondary glazing layer. Perimeter seals are added so air flow between the two layers is limited, which helps with both thermal performance and noise reduction.

Test Opening, Closing And Ventilation

Once in place, windows are checked to ensure they still open for ventilation. Sliding panels are tested and hinged panels are opened and closed to confirm smooth operation and a tight seal when closed.

Pros And Cons Of Secondary Glazing

Secondary glazing offers several benefits:

• A cost effective alternative to full double glazing
• Lower secondary glazing costs compared with replacement windows
• Can reduce heat loss through windows by up to half in some homes
• Helps reduce noise and create a quieter living space
• Preserves original frames and is ideal for heritage or strata properties

Potential downsides include:

• Performance depends heavily on installation quality and gap width
• Slightly more complex cleaning in some setups
• Basic DIY systems may not match the same performance as high end double glazed units

Option 3: Replace Sashes Or Complete Windows

In some homes, the most sensible way to double glaze existing windows is to replace parts of the window or the whole unit.

Sash Replacement With Double Glazed Units

On some timber windows and sash windows, it is possible to keep the original frame and only replace the sashes with new sashes designed for double glazing. These sashes are deeper, accept two panes of glass and a spacer bar, and deliver similar thermal performance to new windows while maintaining the original frame line and external look.

Full Frame Replacement With New Double Glazed Windows

Full frame replacement involves removing the entire window and installing new double glazing windows, often in uPVC or thermally broken aluminium frames. This option:

• Delivers high thermal performance and energy efficient windows
• Fixes long standing problems with poor insulation and warped frames
• Allows you to choose new window types and ventilation patterns

It is more expensive but also gives the most predictable improvement in energy consumption, external noise reduction and comfort and energy efficiency, especially in very old houses with failing frames.

When Replacement Is Better Than Retrofitting

Complete replacement is usually better when:

• Frames are rotten, corroded or structurally weak
• Windows leak badly around the frame, not just through glass
• You want different window types, such as moving from single glazed double hung windows to modern awning windows
• You are already planning major renovations or façade changes

While the initial investment is higher, replacing single glazing with new double glazed windows can increase property value and provide long lasting performance improvements.

DIY Or Professional Installation?

There is a big difference between fitting a simple acrylic panel and handling heavy double glazed units. Knowing what you can safely DIY will help you choose the right approach.

What You Can Realistically DIY

Most homeowners can handle:

• Installing DIY secondary glazing kits with acrylic panels and magnetic seals
• Applying DIY window film as a temporary insulating layer
• Fitting simple clip on or screw fixed acrylic panels for fixed windows

These options create an insulating layer and an air gap, reduce heat loss, help keep the house warm and cut external noise. They are also a cost effective way to reduce energy bills without altering existing frames.

Tools, Skills And Safety Requirements

Even basic DIY solutions still need:

• Accurate measuring and careful cutting or ordering to size
• Basic tools such as a tape measure, drill, screwdriver and saw
• Careful handling of acrylic and attention to achieving an airtight seal

When it comes to retrofitting double glazed units with two panes of glass in each sash, safety glass regulations, weight and risk of breakage make this work best suited to trained glaziers and window specialists.

When To Hire A Professional Installer

Professional installation is strongly recommended when:

• Retrofitting double glazed units into existing window frames
• Undertaking complete window replacements
• Working at height or with very large panes of glass
• You want guaranteed performance and clear u value and r value data

A professional will also assess whether your current window frames can safely carry the weight of double glazing and will ensure an airtight seal and proper drainage, which are critical for long term thermal performance and to avoid moisture problems inside frames.

Practical Tips For A Successful Double Glazing Upgrade

A little planning can make your double glazing existing windows project smoother, faster and more effective.

Getting Accurate Measurements

Accurate measurements are essential for both retrofit double glazing and secondary glazing. Measure visible glass sizes and frame sizes separately and note which windows are out of square or difficult to open. Mis-measured panels lead to gaps, poor acoustic insulation and reduced thermal performance.

Choosing Glass, Gaps And Coatings For Your Climate

When choosing new double glazing or secondary glazing, consider:

• Gap width between panes; at least 12 mm is recommended for good insulation
• Whether you want low emissivity glass to reflect radiant heat back into the room
• If toned glass or solar control coatings are helpful on north or west facing windows to reduce solar heat gain

In colder climates, clear double glazing with internal low E coatings helps keep warm air inside. In hotter climates, solar control options can reduce external heat entering the house while still allowing visible light in.

Ensuring Good Sealing, Drainage And Ventilation

Good double glazing is about more than two glass panes. Make sure that:

• Seals are continuous around every double glazed unit or secondary panel
• Drainage holes in aluminium frames remain clear so water does not pool around the glass
• You keep some windows able to open easily for ventilation so indoor air quality stays healthy

Well fitted curtains, blinds and pelmets can add another insulating layer and further improve thermal performance and comfort.

Minimising Mess And Protecting Finishes During Installation

Before any work, move furniture away from windows, take down window furnishings and protect floors and sills with drop sheets. This is especially important for older windows and doors where paint may be fragile and you want to preserve existing finishes.

Questions To Ask Suppliers And Installers

Good questions help you compare retrofit double glazing, secondary glazing and new windows on the same basis.

Compatibility With Your Existing Frames

Ask installers to explain:

• Whether your existing window frames are suitable for retrofit double glazed units
• Whether secondary glazing is recommended for particular problem rooms
• What they would do with any older windows that are not in good condition

A specialist who offers both retrofitting and replacement will give more balanced advice than someone who only sells one system.

Energy Ratings And Expected Performance

Ask for clear information on:

• U value and r value of any proposed systems
• Window Energy Rating Scheme data where it is available
• Expected impact on heating costs and cooling costs based on similar projects

Remember that retrofitting double glazed windows and secondary glazing can reduce heat loss and energy consumption substantially, even if they do not always match the very best performance of a full new window system.

Warranties, Compliance And Standards

Check:

• How long glass, seals and frames are covered
• What happens if a sealed double glazed unit fogs or leaks
• How the installation complies with Australian standards and local building rules

This applies both to new double glazing and to secondary glazing systems that use acrylic panels and magnetic seals.

Timeframes And Total Installed Cost

Get quotes that clearly outline:

• Supply and installation timeframes
• Cost per square metre and total cost per window
• What is included, including disposal of old glass and making good trims and plaster

Retrofitting or secondary glazing is usually less invasive and considerably cheaper than full window replacement, but it is still important to understand all costs up front.

Aftercare And Maintenance

Looking after your upgraded windows will help protect your investment and keep performance high.

Cleaning And Caring For New Glazed Units

Use mild cleaners on glass and soft cloths on acrylic panels. Avoid harsh chemicals or abrasive pads, especially on low E coatings or acrylic surfaces. Wipe down frames regularly to remove dust and check drainage slots are free of debris.

Checking Seals, Hardware And Operation Over Time

At least once a year:

• Check that all windows open, close and lock smoothly
• Inspect seals, beads and magnetic strips for gaps or damage
• Look for signs of condensation between panes in any double glazed unit

Minor issues caught early are usually simple to fix and prevent bigger thermal performance problems later.

When To Call Your Installer Back For Adjustments

Call your installer if you notice:

• Fogging between panes of glass in a double glazed unit
• Secondary glazing panels that no longer sit tight against their frames
• Frames that have shifted so windows stick or do not close properly

Most reputable installers expect to make small adjustments after installation, especially in older homes where frames can move slightly over time.

Ready To Upgrade Your Existing Windows With Double Glazing?

Upgrading older windows with double glazing does not always mean ripping everything out and starting again. You can double glaze existing windows with retrofit double glazing, add secondary glazing for a cost effective insulating layer, or replace selected windows with brand new double glazed windows where frames have reached the end of their life. All three methods can improve energy efficiency, reduce heat loss, cut external noise and make your home more comfortable in every season. If you’re ready to explore the best option for your home, the next step is to speak with experienced local glaziers who understand retrofitting, secondary glazing and full window replacements. Trade Heroes helps you compare glazing specialists, request quotes and choose solutions that suit your climate, budget and existing window frames. You can find glaziers through Trade Heroes in Perth, Sydney, Adelaide, Melbourne, Hobart or Brisbane.