Retrofit double glazing or full replacement?
Keeping your frames and swapping only the glass costs about half as much and delivers most of the comfort. It also does not suit every frame, and on the thin builder-spec aluminium in a lot of Casey estates it does not suit them at all. Here is the honest comparison, including where we tell people not to bother with us.
Same glass, old frame, half the price.
A retrofit takes the single pane out of your existing window and fits an insulated glass unit into the same frame. The frame stays, the sash stays, the hardware stays, the reveal and the architrave are never touched. Nobody pulls anything out of your wall, there is no perimeter to reseal, and no painter has to come back afterwards.
That is where the money goes. In 2026 a medium retrofit into a sound frame runs about $700 to $1,200 per opening. A new double-glazed aluminium window installed in the same opening is $1,600 to $2,400. The difference is not the glass, because it is the same insulated glass unit in both cases. The difference is the frame, the removal, the disposal, the perimeter sealing and the $180 to $480 of architrave make-good that a full replacement drags behind it. Skip all of that and you are at roughly 40 to 60 per cent of the replacement cost.
The performance you buy is real. Casey is Climate Zone 6 under the National Construction Code, and a single-glazed window is by a wide margin the worst part of the envelope on a 2001 brick veneer. Centre-of-glass U-value goes from roughly 5.5 to 6.5 down to about 2.7 to 3.5. That is the number that stops water running down your panes on a 5 degree July morning, which is the number that ends the mould on the architraves.
Which Berwick frames take a retrofit, and which do not.
An insulated glass unit weighs a bit over twice what the single pane it replaces weighed, and it is thicker. Both of those facts decide whether your frame is a candidate, and neither of them can be assessed from a photo.
- Timber sash: usually yes. The old Berwick cottages around High Street are often the best candidates. Timber sections are deep, the pocket can be routed, and slimline IGUs now fit profiles that would not have taken a standard unit a decade ago. It is also frequently the only path, because a heritage overlay will not let you change the frame anyway. See our Casey permits page for what the overlay allows.
- Heavier residential and commercial aluminium: usually yes. Deeper sections, adequate pocket, hardware with headroom.
- Fixed panes: almost always yes. No hinges, no rollers, no stays. The extra weight goes straight into the frame and sits there.
- Thin builder-spec aluminium sliding sashes: usually no. This is the big one for Casey, because it is what went into the 1990s and 2000s estates by the thousand. The glazing pocket is too shallow to take the unit, and the sash and rollers were specified for a single pane. Force it and the sash bows, the roller housing gives out, and you have spent $900 to make the window worse.
- Anything with hardware already at the end of its life: no. If the rollers are screeching and the gaskets are hard, you are about to load a failing mechanism with double the weight.
So the assessment is about the moving parts, not the glass. We measure the pocket depth, check the sash section, and test the hinges, stays and rollers. That is a site visit, not a phone call, and it is free either way.
What retrofit cannot fix, at any price.
This is the part the retrofit-only operators skip. The frame stays. If it is chalked, it is still chalked. If the gaskets are hard, you still have draughts. If the rollers scream, they still scream, only now with more weight on them. And most importantly for a Berwick winter, a standard aluminium frame is a thermal bridge that conducts cold straight through from outside to inside, and retrofitting the glass does nothing about it. You can go from water running down the whole pane to a light beading on the frame edge. Better, and for most people enough, but not the same as a thermally broken frame with an IGU, which is what actually ends it. That premium is $250 to $400 per window over standard aluminium and it is covered on our aluminium windows page.
A worked example, both ways.
Worked example, 2026. Brick veneer in Endeavour Hills, built 1998, ten openings. Six are medium aluminium awnings and fixed panes in reasonable condition with sound powder coat and working hardware. Four are thin-section sliding windows with chalked frames, hard gaskets and rollers on the way out.
Retrofit everything: not possible. The four sliders will not take it. Any operator quoting all ten as a retrofit has not looked at the sashes.
Replace everything: ten double-glazed aluminium openings at about $1,900 each is $19,000, plus $380 each in make-good, so roughly $22,800. Painter afterwards on top. Best outcome, highest number.
The mixed answer, which is what we would actually quote: retrofit the six sound openings at about $950 each, $5,700, and fully replace the four failing sliders in double-glazed aluminium at about $1,900 each plus make-good, so roughly $9,120. Total about $14,820. That is $8,000 under the full replacement, the whole house is double glazed, and the only compromise is that six of the frames stay non-thermally-broken and twenty-eight years old. In eight or ten years those six frames will need doing, and the IGUs, which carry 10 to 20 year seal warranties under AS/NZS 4666, can usually be carried across into the new frames if the sizes work.
That mixed answer is the one most Casey houses land on, and it is the reason we quote both ways at every measure instead of leading with the bigger number. Two caveats worth stating plainly. First, retrofit does not carry an AS 2047 window label, because that label belongs to a complete tested unit, not to a frame you have modified: if your job is permit-triggered or in a BAL-rated bushfire area up around Harkaway or Narre Warren North, full replacement is generally the cleaner compliance path. Second, if the frames are twenty-five years old and chalking across the board, spending $9,000 on retrofit glass into frames you will replace in six years is false economy. Our warning signs page helps you work out which house you have, the pricing guide has the per-opening bands, and we assess across the whole City of Casey.
Retrofit questions we get asked.
Can you retrofit double glazing into existing Berwick window frames?
Sometimes, and the frame decides, not you. Retrofit means cutting out the single pane and fitting an insulated glass unit into the existing frame, keeping the sash, the hardware and the reveal. It works well on timber, on heavier commercial-grade aluminium and on the thicker residential sections. It works poorly on the thin builder-spec aluminium sliding sashes that went into a lot of 1990s Casey estates, because the glazing pocket is too shallow to take the unit and the sash was never designed for the weight. The only honest answer comes from measuring the pocket depth and checking the sash section on site.
How much does retrofit double glazing cost compared to new windows in Berwick?
Roughly 40 to 60 per cent of full replacement, which is the whole argument for it. In 2026 a medium retrofit IGU into a sound existing frame runs about $700 to $1,200 per opening against $1,600 to $2,400 for a new double-glazed aluminium window installed. Across a ten-opening Berwick house that is about $8,000 to $11,000 versus $16,000 to $25,000. The saving comes from what you are not paying for: no frame, no removal, no perimeter reseal, no architrave make-good and no painter afterwards.
Does retrofit double glazing perform as well as new double-glazed windows?
On the glass, yes. On the frame, no. The insulated glass unit is the same unit either way, so the U-value at the centre of the glass drops from around 5.5 to 6.5 down to roughly 2.7 to 3.5, and that is most of the benefit. What retrofit cannot fix is the frame, which stays a thermal bridge conducting cold straight through, and it cannot fix hardened gaskets or worn rollers either. A new thermally broken frame with an IGU outperforms a retrofit into an old frame, meaningfully. It also costs about twice as much.
Will retrofitting double glazing stop condensation in my Berwick house?
It will stop it on the glass, which is where you are seeing it, and that is usually enough to end the mould problem on the sills and architraves. What it may not stop is condensation forming on the aluminium frame itself, because a standard non-thermally-broken frame will still sit close to outside temperature on a 5 degree Berwick morning. On a bad exposure you can go from water running down the whole pane to a light beading on the frame edge. Better, not perfect. Thermally broken frames are what fix the last of it.
How do I know if my frames can take the extra weight?
An insulated glass unit weighs a bit over twice what the single pane it replaces weighed. On a fixed pane that extra load is carried by the frame and it is rarely an issue. On an operable sash it goes through the hinges, friction stays and rollers, which were specified for the original weight. So the assessment is about the moving parts, not the glass. We check the glazing pocket depth, the sash section, and the condition and rating of the hardware. Where the hardware cannot carry it, the honest answer is a new window, and we will say so.
Free assessments across Casey.
We quote retrofit and replacement side by side.
Free measure, both numbers, and an honest answer on which of your frames can take a retrofit.